What should main return in C and C

Question

What is the correct  most efficient  way to define the main   function in C and C       int main   or void main       and why  And how about the arguments  If int main   then return 1 or return 0   There are numerous duplicates of this question  including   What are the valid signatures for C s main   function  The return type of main   function Difference between void main   and int main    main   s signature in C   What is the proper declaration of main        For C    with a very good answer indeed  Styles of main   functions in C Return type of main   method in C int main   vs void main   in C  Related   C       int main int argc  char   argv  C       int main int argc  char  argv    Is char  envp   as a third argument to main   portable  Must the int main   function return a value in all compilers  Why is the type of the main   function in C and C   left to the user to define  Why does int main     compile  Legal definitions of main   in C  14

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The return value of main   shows how the program exited  If the return value is zero it means that the execution was successful while any non-zero value will represent that something went bad in the execution

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If you really have issues related to efficiency of returning an integer from a process  you should probably avoid to call that process so many times that this return value becomes an issue   If you are doing this  call a process so many times   you should find a way to put your logic directly inside the caller  or in a DLL file  without allocate a specific process for each call  the multiple process allocations bring you the relevant efficiency problem in this case   In detail  if you only want to know if returning 0 is more or less efficient than returning 1  it could depend from the compiler in some cases  but generically  assuming they are read from the same source  local  field  constant  embedded in the code  function result  etc   it requires exactly the same number of clock cycles

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The accepted answer appears to be targetted for C    so I thought I d add an answer that pertains to C  and this differs in a few ways  There were also some changes made between ISO IEC 9899 1989  C90  and ISO IEC 9899 1999  C99   main   should be declared as either  int main void  int main int argc  char   argv   Or equivalent  For example  int main int argc  char  argv    is equivalent to the second one  In C90  the int return type can be omitted as it is a default  but in C99 and newer  the int return type may not be omitted  If an implementation permits it  main   can be declared in other ways  e g   int main int argc  char  argv    char  envp      but this makes the program implementation defined  and no longer strictly conforming  The standard defines 3 values for returning that are strictly conforming  that is  does not rely on implementation defined behaviour   0 and EXIT SUCCESS for a successful termination  and EXIT FAILURE for an unsuccessful termination  Any other values are non-standard and implementation defined  In C90  main   must have an explicit return statement at the end to avoid undefined behaviour  In C99 and newer  you may omit the return statement from main    If you do  and main   finished  there is an implicit return 0  Finally  there is nothing wrong from a standards point of view with calling main   recursively from a C program

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Return 0 on success and non-zero for error  This is the standard used by UNIX and DOS scripting to find out what happened with your program

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The return value can be used by the operating system to check how the program was closed   Return value 0 usually means OK in most operating systems  the ones I can think of anyway    It also can be checked when you call a process yourself  and see if the program exited and finished properly   It s NOT just a programming convention

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Omit return 0  When a C or C   program reaches the end of main the compiler will automatically generate code to return 0  so there is no need to put return 0  explicitly at the end of main     Note  when I make this suggestion  it s almost invariably followed by one of two kinds of comments    I didn t know that   or  That s bad advice    My rationale is that it s safe and useful to rely on compiler behavior explicitly supported by the standard   For C  since C99  see ISO IEC 9899 1999 section 5 1 2 2 3            a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument  reaching the   that terminates the main function returns a value of 0    For C    since the first standard in 1998  see ISO IEC 14882 1998 section 3 6 1      If control reaches the end of main without encountering a return statement  the effect is that of executing return 0    All versions of both standards since then  C99 and C  98  have maintained the same idea   We rely on automatically generated member functions in C    and few people write explicit return  statements at the end of a void function   Reasons against omitting seem to boil down to  it looks weird    If  like me  you re curious about the rationale for the change to the C standard read this question   Also note that in the early 1990s this was considered  sloppy practice  because it was undefined behavior  although widely supported  at the time     Additionally  the C   Core Guidelines contains multiple instances of omitting return 0  at the end of main and no instances in which an explicit return is written   Although there is not yet a specific guideline on this particular topic in that document  that seems at least a tacit endorsement of the practice   So I advocate omitting it  others disagree  often vehemently    In any case  if you encounter code that omits it  you ll know that it s explicitly supported by the standard and you ll know what it means

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The return value for main indicates how the program exited  Normal exit is represented by a 0 return value from main  Abnormal exit is signaled by a non-zero return  but there is no standard for how non-zero codes are interpreted  As noted by others  void main   is prohibited by the C   standard and should not be used  The valid C   main signatures are   int main     and  int main int argc  char  argv      which is equivalent to  int main int argc  char   argv    It is also worth noting that in C    int main   can be left without a return-statement  at which point it defaults to returning 0  This is also true with a C99 program  Whether return 0  should be omitted or not is open to debate  The range of valid C program main signatures is much greater     Efficiency is not an issue with the main function  It can only be entered and left once  marking the program s start and termination  according to the C   standard  For C  re-entering main   is allowed  but should be avoided

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I was under the impression that standard specifies that main doesn t need a return value as a successful return was OS based  zero in one could be either a success or a failure in another   therefore the absence of return was a cue for the compiler to insert the successful return itself   However I usually return 0

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Keep in mind that even though you re returning an int  some OSes  Windows  truncate the returned value to a single byte  0-255

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What to return depends on what you want to do with the executable  For example if you are using your program with a command line shell  then you need to return 0 for a success and a non zero for failure  Then you would be able to use the program in shells with conditional processing depending on the outcome of your code  Also you can assign any nonzero value as per your interpretation  for example for critical errors different program exit points could terminate a program with different exit values   and which is available to the calling shell which can decide what to do by inspecting the value returned  If the code is not intended for use with shells and the returned value does not bother anybody then it might be omitted  I personally use the signature int main  void       return 0

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Returning 0 should tell the programmer that the program has successfully finished the job

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If you really have issues related to efficiency of returning an integer from a process  you should probably avoid to call that process so many times that this return value becomes an issue   If you are doing this  call a process so many times   you should find a way to put your logic directly inside the caller  or in a DLL file  without allocate a specific process for each call  the multiple process allocations bring you the relevant efficiency problem in this case   In detail  if you only want to know if returning 0 is more or less efficient than returning 1  it could depend from the compiler in some cases  but generically  assuming they are read from the same source  local  field  constant  embedded in the code  function result  etc   it requires exactly the same number of clock cycles

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Standard C     Hosted Environment  For a hosted environment  that s the normal one   the C11 standard  ISO IEC 9899 2011  says      5 1 2 2 1 Program startup      The function called at program startup is named main  The implementation declares no   prototype for this function  It shall be defined with a return type of int and with no   parameters   int main void                      or with two parameters  referred to here as argc and argv  though any names may be   used  as they are local to the function in which they are declared    int main int argc  char  argv                        or equivalent 10  or in some other implementation-defined manner       If they are declared  the parameters to the main function shall obey the following   constraints          The value of argc shall be nonnegative    argv argc  shall be a null pointer    If the value of argc is greater than zero  the array members argv 0  through   argv argc-1  inclusive shall contain pointers to strings  which are given   implementation-defined values by the host environment prior to program startup  The   intent is to supply to the program information determined prior to program startup   from elsewhere in the hosted environment  If the host environment is not capable of   supplying strings with letters in both uppercase and lowercase  the implementation   shall ensure that the strings are received in lowercase    If the value of argc is greater than zero  the string pointed to by argv 0    represents the program name  argv 0  0  shall be the null character if the   program name is not available from the host environment  If the value of argc is   greater than one  the strings pointed to by argv 1  through argv argc-1    represent the program parameters    The parameters argc and argv and the strings pointed to by the argv array shall   be modifiable by the program  and retain their last-stored values between program   startup and program termination          10  Thus  int can be replaced by a typedef name defined as int  or the type of argv can be written as   char   argv  and so on    Program termination in C99 or C11  The value returned from main   is transmitted to the  environment  in an implementation-defined way      5 1 2 2 3 Program termination      1 If the return type of the main function is a type compatible with int  a return from the   initial call to the main function is equivalent to calling the exit function with the value   returned by the main function as its argument 11  reaching the   that terminates the   main function returns a value of 0  If the return type is not compatible with int  the   termination status returned to the host environment is unspecified       11  In accordance with 6 2 4  the lifetimes of objects with automatic storage duration declared in main   will have ended in the former case  even where they would not have in the latter    Note that 0 is mandated as  success   You can use EXIT FAILURE and EXIT SUCCESS from  lt stdlib h gt  if you prefer  but 0 is well established  and so is 1  See also Exit codes greater than 255     possible    In C89  and hence in Microsoft C   there is no statement about what happens if the main   function returns but does not specify a return value  it therefore leads to undefined behaviour      7 22 4 4 The exit function        5 Finally  control is returned to the host environment  If the value of status is zero or EXIT SUCCESS  an implementation-defined form of the status successful termination is returned  If the value of status is EXIT FAILURE  an implementation-defined form of the status unsuccessful termination is returned  Otherwise the status returned is implementation-defined    Standard C       Hosted Environment  The C  11 standard  ISO IEC 14882 2011  says      3 6 1 Main function  basic start main         1 A program shall contain a global function called main  which is the designated start of the program               2 An implementation shall not predefine the main function  This function shall not be overloaded  It shall   have a return type of type int  but otherwise its type is implementation defined    All implementations   shall allow both of the following definitions of main   int main                       and  int main int argc  char  argv                        In the latter form argc shall be the number of arguments passed to the program from the environment   in which the program is run  If argc is nonzero these arguments shall be supplied in argv 0    through argv argc-1  as pointers to the initial characters of null-terminated multibyte strings  NTMBSs   17 5 2 1 4 2  and argv 0  shall be the pointer to the initial character of a NTMBS that represents the   name used to invoke the program or     The value of argc shall be non-negative  The value of argv argc    shall be 0    Note  It is recommended that any further  optional  parameters be added after argv     end   note          3 The function main shall not be used within a program  The linkage  3 5  of main is implementation-defined               5 A return statement in main has the effect of leaving the main function  destroying any objects with automatic   storage duration  and calling std  exit with the return value as the argument  If control reaches the end   of main without encountering a return statement  the effect is that of executing  return 0     The C   standard explicitly says  It  the main function  shall have a return type of type int  but otherwise its type is implementation defined   and requires the same two signatures as the C standard to be supported as options  So a  void main    is directly not allowed by the C   standard  though there s nothing it can do to stop a non-standard implementation allowing alternatives   Note that C   forbids the user from calling main  but the C standard does not    There s a paragraph of   18 5 Start and termination in the C  11 standard that is identical to the paragraph from   7 22 4 4 The exit function in the C11 standard  quoted above   apart from a footnote  which simply documents that EXIT SUCCESS and EXIT FAILURE are defined in  lt cstdlib gt     Standard C     Common Extension  Classically  Unix systems support a third variant   int main int argc  char   argv  char   envp            The third argument is a null-terminated list of pointers to strings  each of which is an environment variable which has a name  an equals sign  and a value  possibly empty    If you do not use this  you can still get at the environment via  extern char   environ     This global variable is unique among those in POSIX in that it does not have a header that declares it   This is recognized by the C standard as a common extension  documented in Annex J      J 5 1 Environment arguments        1 In a hosted environment  the main function receives a third argument  char  envp      that points to a null-terminated array of pointers to char  each of which points to a string   that provides information about the environment for this execution of the program  5 1 2 2 1     Microsoft C  The Microsoft VS 2010 compiler is interesting  The web site says      The declaration syntax for main is   int main          or  optionally   int main int argc  char  argv    char  envp           Alternatively  the main and wmain functions can be declared as returning void  no return value   If you declare main or wmain as returning void  you cannot return an exit code to the parent process or operating system by using a return statement  To return an exit code when main or wmain is declared as void  you must use the exit function    It is not clear to me what happens  what exit code is returned to the parent or OS  when a program with void main   does exit     and the MS web site is silent too   Interestingly  MS does not prescribe the two-argument version of main   that the C and C   standards require  It only prescribes a three argument form where the third argument is char   envp  a pointer to a list of environment variables   The Microsoft page also lists some other alternatives     wmain   which takes wide character strings  and some more   The Microsoft Visual nbsp Studio nbsp 2005 version of this page does not list void main   as an alternative   The versions from Microsoft Visual nbsp Studio nbsp 2008 onwards do   Standard C     Freestanding Environment  As noted early on  the requirements above apply to hosted environments  If you are working with a freestanding environment  which is the alternative to a hosted environment   then the standard has much less to say  For a freestanding environment  the function called at program startup need not be called main and there are no constraints on its return type   The standard says      5 1 2 Execution environments      Two execution environments are defined  freestanding and hosted  In both cases    program startup occurs when a designated C function is called by the execution   environment  All objects with static storage duration shall be initialized  set to their initial values  before program startup  The manner and timing of such initialization are otherwise unspecified  Program termination returns control to the execution environment       5 1 2 1 Freestanding environment      In a freestanding environment  in which C program execution may take place without any benefit of an operating system   the name and type of the function called at program startup are implementation-defined  Any library facilities available to a freestanding program  other than the minimal set required by clause 4  are implementation-defined       The effect of program termination in a freestanding environment is implementation-defined    The cross-reference to clause 4 Conformance refers to this        5 A strictly conforming program shall use only those features of the language and library specified in this International Standard 3  It shall not produce output dependent on any unspecified  undefined  or implementation-defined behavior  and shall not exceed any minimum implementation limit         6 The two forms of conforming implementation are hosted and freestanding  A conforming hosted implementation shall accept any strictly conforming program  A conforming freestanding implementation shall accept any strictly conforming program in which the use of the features specified in the library clause  clause 7  is confined to the contents of the standard headers  lt float h gt    lt iso646 h gt    lt limits h gt    lt stdalign h gt      lt stdarg h gt    lt stdbool h gt    lt stddef h gt    lt stdint h gt   and    lt stdnoreturn h gt   A conforming implementation may have extensions  including   additional library functions   provided they do not alter the behavior of any strictly conforming program 4         7 A conforming program is one that is acceptable to a conforming implementation 5       3  A strictly conforming program can use conditional features  see 6 10 8 3  provided the use is guarded by an appropriate conditional inclusion preprocessing directive using the related macro  For example    ifdef   STDC IEC 559      FE UPWARD defined                      fesetround FE UPWARD                  endif       4  This implies that a conforming implementation reserves no identifiers other than those explicitly reserved in this International Standard       5  Strictly conforming programs are intended to be maximally portable among conforming implementations  Conforming programs may depend upon non-portable features of a conforming implementation    It is noticeable that the only header required of a freestanding environment that actually defines any functions is  lt stdarg h gt   and even those may be     and often are     just macros    Standard C       Freestanding Environment  Just as the C standard recognizes both hosted and freestanding environment  so too does the C   standard   Quotes from ISO IEC 14882 2011       1 4 Implementation compliance  intro compliance         7 Two kinds of implementations are defined  a hosted implementation and a freestanding implementation  For a hosted implementation  this International Standard defines the set of available libraries  A freestanding   implementation is one in which execution may take place without the benefit of an operating system  and has an implementation-defined set of libraries that includes certain language-support libraries  17 6 1 3          8 A conforming implementation may have extensions  including additional library functions   provided they do not alter the behavior of any well-formed program  Implementations are required to diagnose programs that   use such extensions that are ill-formed according to this International Standard  Having done so  however  they can compile and execute such programs         9 Each implementation shall include documentation that identifies all conditionally-supported constructs that it does not support and defines all locale-specific characteristics 3      3  This documentation also defines implementation-defined behavior  see 1 9       17 6 1 3 Freestanding implementations  compliance       Two kinds of implementations are defined  hosted and freestanding  1 4   For a hosted implementation  this International Standard describes the set of available headers       A freestanding implementation has an implementation-defined set of headers  This set shall include at least the headers shown in Table 16       The supplied version of the header  lt cstdlib gt  shall declare at least the functions abort  atexit  at quick exit  exit  and quick exit  18 5   The other headers listed in this table shall meet the same requirements as for a hosted implementation       Table 16     C   headers for freestanding implementations  Subclause                           Header s                                       lt ciso646 gt  18 2  Types                          lt cstddef gt  18 3  Implementation properties      lt cfloat gt   lt limits gt   lt climits gt  18 4  Integer types                  lt cstdint gt  18 5  Start and termination          lt cstdlib gt  18 6  Dynamic memory management      lt new gt  18 7  Type identification            lt typeinfo gt  18 8  Exception handling             lt exception gt  18 9  Initializer lists              lt initializer list gt  18 10 Other runtime support          lt cstdalign gt   lt cstdarg gt   lt cstdbool gt  20 9  Type traits                    lt type traits gt  29    Atomics                        lt atomic gt     What about using int main   in C   The standard   5 1 2 2 1 of the C11 standard shows the preferred notation      int main void      but there are also two examples in the standard which show int main      6 5 3 4   8 and   6 7 6 3   20   Now  it is important to note that examples are not  normative   they are only illustrative   If there are bugs in the examples  they do not directly affect the main text of the standard   That said  they are strongly indicative of expected behaviour  so if the standard includes int main   in an example  it suggests that int main   is not forbidden  even if it is not the preferred notation      6 5 3 4 The sizeof and  Alignof operators                 8 EXAMPLE 3 In this example  the size of a variable length array is computed and returned from a function    include  lt stddef h gt   size t fsize3 int n        char b n 3      variable length array     return sizeof b     execution time sizeof   int main         size t size      size   fsize3 10      fsize3 returns 13     return 0

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In C  the Section 5 1 2 2 1 of the C11 standard  emphasis mine    It shall be defined with a return type of int and with no parameters  int main void                 or with two parameters  referred to here as argc and argv  though any names may be used  as they are local to the function in which they are declared   int main int argc  char  argv                    However for some beginners like me  an abstract example would allow me to get a grasp on it  When you write a method in your program  e g  int read file char filename LEN     then you want  as the caller of this method to know if everything went well  because failures can happen  e g  file could not be found   By checking the return value of the method you can know if everything went well or not  it s a mechanism for the method to signal you about its successful execution  or not   and let the caller  you  e g  in your main method  decide how to handle an unexpected failure  So now imagine I write a C program for a micro-mechanism which is used in a more complex system  When the system calls the micro-mechanism  it wants to know if everything went as expected  so that it can handle any potential error  If the C program s main method would return void  then how would the calling-system know about the execution of its subsystem  the micro-mechanism   It cannot  that s why main   returns int  in order to communicate to its caller a successful  or not  execution  In other words  The rational is that the host environment  i e  Operating System  OS   needs to know if the program finished correctly  Without an int-compatible type as a return type  eg  void   the  quot status returned to the host environment is unspecified quot   i e  undefined behavior on most OS

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Return 0 on success and non-zero for error  This is the standard used by UNIX and DOS scripting to find out what happened with your program

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Keep in mind that even though you re returning an int  some OSes  Windows  truncate the returned value to a single byte  0-255

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Note that the C and C   standards define two kinds of implementations  freestanding and hosted   C90 hosted environment  Allowed forms 1      int main  void      int main  int argc  char  argv         main  void      main  int argc  char  argv              etc  similar forms with implicit int     Comments  The former two are explicitly stated as the allowed forms  the others are implicitly allowed because C90 allowed  quot implicit int quot  for return type and function parameters  No other form is allowed   C90 freestanding environment  Any form or name of main is allowed 2   C99 hosted environment  Allowed forms 3      int main  void      int main  int argc  char  argv           or in some other implementation-defined manner      Comments  C99 removed  quot implicit int quot  so main   is no longer valid  A strange  ambiguous sentence  quot or in some other implementation-defined manner quot  has been introduced  This can either be interpreted as  quot the parameters to int main   may vary quot  or as  quot main can have any implementation-defined form quot   Some compilers have chosen to interpret the standard in the latter way  Arguably  one cannot easily state that they are not conforming by citing the standard in itself  since it is is ambiguous  However  to allow completely wild forms of main   was probably    not the intention of this new sentence  The C99 rationale  not normative  implies that the sentence refers to additional parameters to int main 4  Yet the section for hosted environment program termination then goes on arguing about the case where main does not return int 5  Although that section is not normative for how main should be declared  it definitely implies that main might be declared in a completely implementation-defined way even on hosted systems   C99 freestanding environment  Any form or name of main is allowed 6   C11 hosted environment  Allowed forms 7      int main  void      int main  int argc  char  argv           or in some other implementation-defined manner       C11 freestanding environment  Any form or name of main is allowed 8   Note that int main   was never listed as a valid form for any hosted implementation of C in any of the above versions  In C  unlike C       and  void  have different meanings  The former is an obsolescent feature which may be removed from the language  See C11 future language directions   6 11 6 Function declarators The use of function declarators with empty parentheses  not prototype-format parameter type declarators  is an obsolescent feature     C  03 hosted environment  Allowed forms 9      int main        int main  int argc  char  argv     Comments  Note the empty parenthesis in the first form  C   and C are different in this case  because in C   this means that the function takes no parameters  But in C it means that it may take any parameter   C  03 freestanding environment  The name of the function called at startup is implementation-defined  If it is named main   it must follow the stated forms 10         implementation-defined name  or      int main        int main  int argc  char  argv      C  11 hosted environment  Allowed forms 11      int main        int main  int argc  char  argv     Comments  The text of the standard has been changed but it has the same meaning   C  11 freestanding environment  The name of the function called at startup is implementation-defined  If it is named main   it must follow the stated forms 12         implementation-defined name  or      int main        int main  int argc  char  argv      References  ANSI X3 159-1989 2 1 2 2 Hosted environment   quot Program startup quot    The function called at program startup is named main  The implementation declares no prototype for this function  It shall be defined with a return type of int and with no parameters       int main void                   or with two parameters  referred to here as argc and argv  though any names may be used  as they are local to the function in which they are declared        int main int argc  char  argv                    ANSI X3 159-1989 2 1 2 1 Freestanding environment    In a freestanding environment  in which C program execution may take place without any benefit of an operating system   the name and type of the function called at program startup are implementation-defined    ISO 9899 1999 5 1 2 2 Hosted environment - gt  5 1 2 2 1 Program startup   The function called at program startup is named main  The implementation declares no prototype for this function  It shall be defined with a return type of int and with no parameters       int main void                   or with two parameters  referred to here as argc and argv  though any names may be used  as they are local to the function in which they are declared        int main int argc  char  argv                    or equivalent 9  or in some other implementation-defined manner    Rationale for International Standard     Programming Languages     C  Revision 5 10  5 1 2 2 Hosted environment -- gt  5 1 2 2 1 Program startup   The behavior of the arguments to main  and of the interaction of exit  main and atexit  see   7 20 4 2  has been codified to curb some unwanted variety in the representation of argv strings  and in the meaning of values returned by main    The specification of argc and argv as arguments to main recognizes extensive prior practice  argv argc  is required to be a null pointer to provide a redundant check for the end of the list  also on the basis of common practice    main is the only function that may portably be declared either with zero or two arguments   The number of other functions    arguments must match exactly between invocation and definition   This special case simply recognizes the widespread practice of leaving off the arguments to main when the program does not access the program argument strings  While many implementations support more than two arguments to main  such practice is neither blessed nor forbidden by the Standard  a program that defines main with three arguments is not strictly conforming  see   J 5 1      ISO 9899 1999 5 1 2 2 Hosted environment -- gt  5 1 2 2 3 Program termination   If the return type of the main function is a type compatible with int  a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument 11  reaching the   that terminates the main function returns a value of 0  If the return type is not compatible with int  the termination status returned to the host environment is unspecified    ISO 9899 1999 5 1 2 1 Freestanding environment   In a freestanding environment  in which C program execution may take place without any benefit of an operating system   the name and type of the function called at program startup are implementation-defined    ISO 9899 2011 5 1 2 2 Hosted environment - gt  5 1 2 2 1 Program startup  This section is identical to the C99 one cited above   ISO 9899 1999 5 1 2 1 Freestanding environment  This section is identical to the C99 one cited above   ISO 14882 2003 3 6 1 Main function   An implementation shall not predefine the main function  This function shall not be overloaded  It shall have a return type of type int  but otherwise its type is implementation-defined  All implementations shall allow both of the following definitions of main       int main                   and      int main int argc  char  argv                    ISO 14882 2003 3 6 1 Main function   It is implementation-defined whether a program in a freestanding environment is required to define a main function    ISO 14882 2011 3 6 1 Main function   An implementation shall not predefine the main function  This function shall not be overloaded  It shall have a return type of type int  but otherwise its type is implementation-defined  All implementations shall allow both     a function of    returning int and     a function of  int  pointer to pointer to char  returning int as the type of main  8 3 5     ISO 14882 2011 3 6 1 Main function  This section is identical to the C  03 one cited above

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Here is a small demonstration of the usage of return codes     When using the various tools that the Linux terminal provides one can use the return code for example for error handling after the process has been completed  Imagine that the following text file myfile is present      This is some example in order to check how grep works    When you execute the grep command a process is created  Once it is through  and didn t break  it returns some code between 0 and 255  For example     grep order myfile   If you do    echo      0   you will get a 0  Why  Because grep found a match and returned an exit code 0  which is the usual value for exiting with a success  Let s check it out again but with something that is not inside our text file and thus no match will be found     grep foo myfile   echo      1   Since grep failed to match the token  foo  with the content of our file the return code is 1  this is the usual case when a failure occurs but as stated above you have plenty of values to choose from    Now the following bash script  simply type it in a Linux terminal  although very basic should give some idea of error handling     grep foo myfile   CHECK         CHECK -eq 0   amp  amp  echo  Match found       CHECK -ne 0   amp  amp  echo  No match was found    No match was found   After the second line nothing is printed to the terminal since  foo  made grep return 1 and we check if the return code of grep was equal to 0  The second conditional statement echoes its message in the last line since it is true due to CHECK    1   As you can see if you are calling this and that process it is sometimes essential to see what it has returned  by the return value of main

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The return value for main indicates how the program exited  Normal exit is represented by a 0 return value from main  Abnormal exit is signaled by a non-zero return  but there is no standard for how non-zero codes are interpreted  As noted by others  void main   is prohibited by the C   standard and should not be used  The valid C   main signatures are   int main     and  int main int argc  char  argv      which is equivalent to  int main int argc  char   argv    It is also worth noting that in C    int main   can be left without a return-statement  at which point it defaults to returning 0  This is also true with a C99 program  Whether return 0  should be omitted or not is open to debate  The range of valid C program main signatures is much greater     Efficiency is not an issue with the main function  It can only be entered and left once  marking the program s start and termination  according to the C   standard  For C  re-entering main   is allowed  but should be avoided

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I was under the impression that standard specifies that main doesn t need a return value as a successful return was OS based  zero in one could be either a success or a failure in another   therefore the absence of return was a cue for the compiler to insert the successful return itself   However I usually return 0

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Returning 0 should tell the programmer that the program has successfully finished the job

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The return value can be used by the operating system to check how the program was closed   Return value 0 usually means OK in most operating systems  the ones I can think of anyway    It also can be checked when you call a process yourself  and see if the program exited and finished properly   It s NOT just a programming convention

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main   in C89 and K amp R C unspecified return types default to    int    return 1  return 0     If you do not write a return statement in int main    the closing   will return 0 by default  return 0 or return 1 will be received by the parent process  In a shell it goes into a shell variable  and if you are running your program form a shell and not using that variable then you need not worry about the return value of main      See How can I get what my main function has returned        a out   echo      This way you can see that it is the variable    which receives the least significant byte of the return value of main     In Unix and DOS scripting  return 0 on success and non-zero for error are usually returned  This is the standard used by Unix and DOS scripting to find out what happened with your program and controlling the whole flow

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Return 0 on success and non-zero for error  This is the standard used by UNIX and DOS scripting to find out what happened with your program

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What is the correct  most efficient  way to define the main   function in C and C       int main   or void main       and why    Those words   most efficient   don t change the question   Unless you re in a freestanding environment  there is one universally correct way to declare main    and that s as returning int      What should main   return in C and C      It s not what should main   return  it s what does main   return   main   is  of course  a function that someone else calls   You don t have any control over the code that calls main     Therefore  you must declare main   with a type-correct signature to match its caller   You simply don t have any choice in the matter   You don t have to ask yourself what s more or less efficient  or what s better or worse style  or anything like that  because the answer is already perfectly well defined  for you  by the C and C  standards   Just follow them      If int main   then return 1 or return 0    0 for success  nonzero for failure   Again  not something you need to  or get to  pick  it s defined by the interface you re supposed to be conforming to

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Keep in mind that even though you re returning an int  some OSes  Windows  truncate the returned value to a single byte  0-255

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I believe that main   should return either EXIT SUCCESS or EXIT FAILURE  They are defined in stdlib h

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What is the correct  most efficient  way to define the main   function in C and C       int main   or void main       and why    Those words   most efficient   don t change the question   Unless you re in a freestanding environment  there is one universally correct way to declare main    and that s as returning int      What should main   return in C and C      It s not what should main   return  it s what does main   return   main   is  of course  a function that someone else calls   You don t have any control over the code that calls main     Therefore  you must declare main   with a type-correct signature to match its caller   You simply don t have any choice in the matter   You don t have to ask yourself what s more or less efficient  or what s better or worse style  or anything like that  because the answer is already perfectly well defined  for you  by the C and C  standards   Just follow them      If int main   then return 1 or return 0    0 for success  nonzero for failure   Again  not something you need to  or get to  pick  it s defined by the interface you re supposed to be conforming to

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Here is a small demonstration of the usage of return codes     When using the various tools that the Linux terminal provides one can use the return code for example for error handling after the process has been completed  Imagine that the following text file myfile is present      This is some example in order to check how grep works    When you execute the grep command a process is created  Once it is through  and didn t break  it returns some code between 0 and 255  For example     grep order myfile   If you do    echo      0   you will get a 0  Why  Because grep found a match and returned an exit code 0  which is the usual value for exiting with a success  Let s check it out again but with something that is not inside our text file and thus no match will be found     grep foo myfile   echo      1   Since grep failed to match the token  foo  with the content of our file the return code is 1  this is the usual case when a failure occurs but as stated above you have plenty of values to choose from    Now the following bash script  simply type it in a Linux terminal  although very basic should give some idea of error handling     grep foo myfile   CHECK         CHECK -eq 0   amp  amp  echo  Match found       CHECK -ne 0   amp  amp  echo  No match was found    No match was found   After the second line nothing is printed to the terminal since  foo  made grep return 1 and we check if the return code of grep was equal to 0  The second conditional statement echoes its message in the last line since it is true due to CHECK    1   As you can see if you are calling this and that process it is sometimes essential to see what it has returned  by the return value of main

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I was under the impression that standard specifies that main doesn t need a return value as a successful return was OS based  zero in one could be either a success or a failure in another   therefore the absence of return was a cue for the compiler to insert the successful return itself   However I usually return 0

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Standard C     Hosted Environment  For a hosted environment  that s the normal one   the C11 standard  ISO IEC 9899 2011  says      5 1 2 2 1 Program startup      The function called at program startup is named main  The implementation declares no   prototype for this function  It shall be defined with a return type of int and with no   parameters   int main void                      or with two parameters  referred to here as argc and argv  though any names may be   used  as they are local to the function in which they are declared    int main int argc  char  argv                        or equivalent 10  or in some other implementation-defined manner       If they are declared  the parameters to the main function shall obey the following   constraints          The value of argc shall be nonnegative    argv argc  shall be a null pointer    If the value of argc is greater than zero  the array members argv 0  through   argv argc-1  inclusive shall contain pointers to strings  which are given   implementation-defined values by the host environment prior to program startup  The   intent is to supply to the program information determined prior to program startup   from elsewhere in the hosted environment  If the host environment is not capable of   supplying strings with letters in both uppercase and lowercase  the implementation   shall ensure that the strings are received in lowercase    If the value of argc is greater than zero  the string pointed to by argv 0    represents the program name  argv 0  0  shall be the null character if the   program name is not available from the host environment  If the value of argc is   greater than one  the strings pointed to by argv 1  through argv argc-1    represent the program parameters    The parameters argc and argv and the strings pointed to by the argv array shall   be modifiable by the program  and retain their last-stored values between program   startup and program termination          10  Thus  int can be replaced by a typedef name defined as int  or the type of argv can be written as   char   argv  and so on    Program termination in C99 or C11  The value returned from main   is transmitted to the  environment  in an implementation-defined way      5 1 2 2 3 Program termination      1 If the return type of the main function is a type compatible with int  a return from the   initial call to the main function is equivalent to calling the exit function with the value   returned by the main function as its argument 11  reaching the   that terminates the   main function returns a value of 0  If the return type is not compatible with int  the   termination status returned to the host environment is unspecified       11  In accordance with 6 2 4  the lifetimes of objects with automatic storage duration declared in main   will have ended in the former case  even where they would not have in the latter    Note that 0 is mandated as  success   You can use EXIT FAILURE and EXIT SUCCESS from  lt stdlib h gt  if you prefer  but 0 is well established  and so is 1  See also Exit codes greater than 255     possible    In C89  and hence in Microsoft C   there is no statement about what happens if the main   function returns but does not specify a return value  it therefore leads to undefined behaviour      7 22 4 4 The exit function        5 Finally  control is returned to the host environment  If the value of status is zero or EXIT SUCCESS  an implementation-defined form of the status successful termination is returned  If the value of status is EXIT FAILURE  an implementation-defined form of the status unsuccessful termination is returned  Otherwise the status returned is implementation-defined    Standard C       Hosted Environment  The C  11 standard  ISO IEC 14882 2011  says      3 6 1 Main function  basic start main         1 A program shall contain a global function called main  which is the designated start of the program               2 An implementation shall not predefine the main function  This function shall not be overloaded  It shall   have a return type of type int  but otherwise its type is implementation defined    All implementations   shall allow both of the following definitions of main   int main                       and  int main int argc  char  argv                        In the latter form argc shall be the number of arguments passed to the program from the environment   in which the program is run  If argc is nonzero these arguments shall be supplied in argv 0    through argv argc-1  as pointers to the initial characters of null-terminated multibyte strings  NTMBSs   17 5 2 1 4 2  and argv 0  shall be the pointer to the initial character of a NTMBS that represents the   name used to invoke the program or     The value of argc shall be non-negative  The value of argv argc    shall be 0    Note  It is recommended that any further  optional  parameters be added after argv     end   note          3 The function main shall not be used within a program  The linkage  3 5  of main is implementation-defined               5 A return statement in main has the effect of leaving the main function  destroying any objects with automatic   storage duration  and calling std  exit with the return value as the argument  If control reaches the end   of main without encountering a return statement  the effect is that of executing  return 0     The C   standard explicitly says  It  the main function  shall have a return type of type int  but otherwise its type is implementation defined   and requires the same two signatures as the C standard to be supported as options  So a  void main    is directly not allowed by the C   standard  though there s nothing it can do to stop a non-standard implementation allowing alternatives   Note that C   forbids the user from calling main  but the C standard does not    There s a paragraph of   18 5 Start and termination in the C  11 standard that is identical to the paragraph from   7 22 4 4 The exit function in the C11 standard  quoted above   apart from a footnote  which simply documents that EXIT SUCCESS and EXIT FAILURE are defined in  lt cstdlib gt     Standard C     Common Extension  Classically  Unix systems support a third variant   int main int argc  char   argv  char   envp            The third argument is a null-terminated list of pointers to strings  each of which is an environment variable which has a name  an equals sign  and a value  possibly empty    If you do not use this  you can still get at the environment via  extern char   environ     This global variable is unique among those in POSIX in that it does not have a header that declares it   This is recognized by the C standard as a common extension  documented in Annex J      J 5 1 Environment arguments        1 In a hosted environment  the main function receives a third argument  char  envp      that points to a null-terminated array of pointers to char  each of which points to a string   that provides information about the environment for this execution of the program  5 1 2 2 1     Microsoft C  The Microsoft VS 2010 compiler is interesting  The web site says      The declaration syntax for main is   int main          or  optionally   int main int argc  char  argv    char  envp           Alternatively  the main and wmain functions can be declared as returning void  no return value   If you declare main or wmain as returning void  you cannot return an exit code to the parent process or operating system by using a return statement  To return an exit code when main or wmain is declared as void  you must use the exit function    It is not clear to me what happens  what exit code is returned to the parent or OS  when a program with void main   does exit     and the MS web site is silent too   Interestingly  MS does not prescribe the two-argument version of main   that the C and C   standards require  It only prescribes a three argument form where the third argument is char   envp  a pointer to a list of environment variables   The Microsoft page also lists some other alternatives     wmain   which takes wide character strings  and some more   The Microsoft Visual nbsp Studio nbsp 2005 version of this page does not list void main   as an alternative   The versions from Microsoft Visual nbsp Studio nbsp 2008 onwards do   Standard C     Freestanding Environment  As noted early on  the requirements above apply to hosted environments  If you are working with a freestanding environment  which is the alternative to a hosted environment   then the standard has much less to say  For a freestanding environment  the function called at program startup need not be called main and there are no constraints on its return type   The standard says      5 1 2 Execution environments      Two execution environments are defined  freestanding and hosted  In both cases    program startup occurs when a designated C function is called by the execution   environment  All objects with static storage duration shall be initialized  set to their initial values  before program startup  The manner and timing of such initialization are otherwise unspecified  Program termination returns control to the execution environment       5 1 2 1 Freestanding environment      In a freestanding environment  in which C program execution may take place without any benefit of an operating system   the name and type of the function called at program startup are implementation-defined  Any library facilities available to a freestanding program  other than the minimal set required by clause 4  are implementation-defined       The effect of program termination in a freestanding environment is implementation-defined    The cross-reference to clause 4 Conformance refers to this        5 A strictly conforming program shall use only those features of the language and library specified in this International Standard 3  It shall not produce output dependent on any unspecified  undefined  or implementation-defined behavior  and shall not exceed any minimum implementation limit         6 The two forms of conforming implementation are hosted and freestanding  A conforming hosted implementation shall accept any strictly conforming program  A conforming freestanding implementation shall accept any strictly conforming program in which the use of the features specified in the library clause  clause 7  is confined to the contents of the standard headers  lt float h gt    lt iso646 h gt    lt limits h gt    lt stdalign h gt      lt stdarg h gt    lt stdbool h gt    lt stddef h gt    lt stdint h gt   and    lt stdnoreturn h gt   A conforming implementation may have extensions  including   additional library functions   provided they do not alter the behavior of any strictly conforming program 4         7 A conforming program is one that is acceptable to a conforming implementation 5       3  A strictly conforming program can use conditional features  see 6 10 8 3  provided the use is guarded by an appropriate conditional inclusion preprocessing directive using the related macro  For example    ifdef   STDC IEC 559      FE UPWARD defined                      fesetround FE UPWARD                  endif       4  This implies that a conforming implementation reserves no identifiers other than those explicitly reserved in this International Standard       5  Strictly conforming programs are intended to be maximally portable among conforming implementations  Conforming programs may depend upon non-portable features of a conforming implementation    It is noticeable that the only header required of a freestanding environment that actually defines any functions is  lt stdarg h gt   and even those may be     and often are     just macros    Standard C       Freestanding Environment  Just as the C standard recognizes both hosted and freestanding environment  so too does the C   standard   Quotes from ISO IEC 14882 2011       1 4 Implementation compliance  intro compliance         7 Two kinds of implementations are defined  a hosted implementation and a freestanding implementation  For a hosted implementation  this International Standard defines the set of available libraries  A freestanding   implementation is one in which execution may take place without the benefit of an operating system  and has an implementation-defined set of libraries that includes certain language-support libraries  17 6 1 3          8 A conforming implementation may have extensions  including additional library functions   provided they do not alter the behavior of any well-formed program  Implementations are required to diagnose programs that   use such extensions that are ill-formed according to this International Standard  Having done so  however  they can compile and execute such programs         9 Each implementation shall include documentation that identifies all conditionally-supported constructs that it does not support and defines all locale-specific characteristics 3      3  This documentation also defines implementation-defined behavior  see 1 9       17 6 1 3 Freestanding implementations  compliance       Two kinds of implementations are defined  hosted and freestanding  1 4   For a hosted implementation  this International Standard describes the set of available headers       A freestanding implementation has an implementation-defined set of headers  This set shall include at least the headers shown in Table 16       The supplied version of the header  lt cstdlib gt  shall declare at least the functions abort  atexit  at quick exit  exit  and quick exit  18 5   The other headers listed in this table shall meet the same requirements as for a hosted implementation       Table 16     C   headers for freestanding implementations  Subclause                           Header s                                       lt ciso646 gt  18 2  Types                          lt cstddef gt  18 3  Implementation properties      lt cfloat gt   lt limits gt   lt climits gt  18 4  Integer types                  lt cstdint gt  18 5  Start and termination          lt cstdlib gt  18 6  Dynamic memory management      lt new gt  18 7  Type identification            lt typeinfo gt  18 8  Exception handling             lt exception gt  18 9  Initializer lists              lt initializer list gt  18 10 Other runtime support          lt cstdalign gt   lt cstdarg gt   lt cstdbool gt  20 9  Type traits                    lt type traits gt  29    Atomics                        lt atomic gt     What about using int main   in C   The standard   5 1 2 2 1 of the C11 standard shows the preferred notation      int main void      but there are also two examples in the standard which show int main      6 5 3 4   8 and   6 7 6 3   20   Now  it is important to note that examples are not  normative   they are only illustrative   If there are bugs in the examples  they do not directly affect the main text of the standard   That said  they are strongly indicative of expected behaviour  so if the standard includes int main   in an example  it suggests that int main   is not forbidden  even if it is not the preferred notation      6 5 3 4 The sizeof and  Alignof operators                 8 EXAMPLE 3 In this example  the size of a variable length array is computed and returned from a function    include  lt stddef h gt   size t fsize3 int n        char b n 3      variable length array     return sizeof b     execution time sizeof   int main         size t size      size   fsize3 10      fsize3 returns 13     return 0

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Return 0 on success and non-zero for error  This is the standard used by UNIX and DOS scripting to find out what happened with your program

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The return value of main   shows how the program exited  If the return value is zero it means that the execution was successful while any non-zero value will represent that something went bad in the execution

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The return value for main indicates how the program exited  Normal exit is represented by a 0 return value from main  Abnormal exit is signaled by a non-zero return  but there is no standard for how non-zero codes are interpreted  As noted by others  void main   is prohibited by the C   standard and should not be used  The valid C   main signatures are   int main     and  int main int argc  char  argv      which is equivalent to  int main int argc  char   argv    It is also worth noting that in C    int main   can be left without a return-statement  at which point it defaults to returning 0  This is also true with a C99 program  Whether return 0  should be omitted or not is open to debate  The range of valid C program main signatures is much greater     Efficiency is not an issue with the main function  It can only be entered and left once  marking the program s start and termination  according to the C   standard  For C  re-entering main   is allowed  but should be avoided

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Omit return 0  When a C or C   program reaches the end of main the compiler will automatically generate code to return 0  so there is no need to put return 0  explicitly at the end of main     Note  when I make this suggestion  it s almost invariably followed by one of two kinds of comments    I didn t know that   or  That s bad advice    My rationale is that it s safe and useful to rely on compiler behavior explicitly supported by the standard   For C  since C99  see ISO IEC 9899 1999 section 5 1 2 2 3            a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument  reaching the   that terminates the main function returns a value of 0    For C    since the first standard in 1998  see ISO IEC 14882 1998 section 3 6 1      If control reaches the end of main without encountering a return statement  the effect is that of executing return 0    All versions of both standards since then  C99 and C  98  have maintained the same idea   We rely on automatically generated member functions in C    and few people write explicit return  statements at the end of a void function   Reasons against omitting seem to boil down to  it looks weird    If  like me  you re curious about the rationale for the change to the C standard read this question   Also note that in the early 1990s this was considered  sloppy practice  because it was undefined behavior  although widely supported  at the time     Additionally  the C   Core Guidelines contains multiple instances of omitting return 0  at the end of main and no instances in which an explicit return is written   Although there is not yet a specific guideline on this particular topic in that document  that seems at least a tacit endorsement of the practice   So I advocate omitting it  others disagree  often vehemently    In any case  if you encounter code that omits it  you ll know that it s explicitly supported by the standard and you ll know what it means

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I believe that main   should return either EXIT SUCCESS or EXIT FAILURE  They are defined in stdlib h

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In C  the Section 5 1 2 2 1 of the C11 standard  emphasis mine    It shall be defined with a return type of int and with no parameters  int main void                 or with two parameters  referred to here as argc and argv  though any names may be used  as they are local to the function in which they are declared   int main int argc  char  argv                    However for some beginners like me  an abstract example would allow me to get a grasp on it  When you write a method in your program  e g  int read file char filename LEN     then you want  as the caller of this method to know if everything went well  because failures can happen  e g  file could not be found   By checking the return value of the method you can know if everything went well or not  it s a mechanism for the method to signal you about its successful execution  or not   and let the caller  you  e g  in your main method  decide how to handle an unexpected failure  So now imagine I write a C program for a micro-mechanism which is used in a more complex system  When the system calls the micro-mechanism  it wants to know if everything went as expected  so that it can handle any potential error  If the C program s main method would return void  then how would the calling-system know about the execution of its subsystem  the micro-mechanism   It cannot  that s why main   returns int  in order to communicate to its caller a successful  or not  execution  In other words  The rational is that the host environment  i e  Operating System  OS   needs to know if the program finished correctly  Without an int-compatible type as a return type  eg  void   the  quot status returned to the host environment is unspecified quot   i e  undefined behavior on most OS

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What to return depends on what you want to do with the executable  For example if you are using your program with a command line shell  then you need to return 0 for a success and a non zero for failure  Then you would be able to use the program in shells with conditional processing depending on the outcome of your code  Also you can assign any nonzero value as per your interpretation  for example for critical errors different program exit points could terminate a program with different exit values   and which is available to the calling shell which can decide what to do by inspecting the value returned  If the code is not intended for use with shells and the returned value does not bother anybody then it might be omitted  I personally use the signature int main  void       return 0

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The accepted answer appears to be targetted for C    so I thought I d add an answer that pertains to C  and this differs in a few ways  There were also some changes made between ISO IEC 9899 1989  C90  and ISO IEC 9899 1999  C99   main   should be declared as either  int main void  int main int argc  char   argv   Or equivalent  For example  int main int argc  char  argv    is equivalent to the second one  In C90  the int return type can be omitted as it is a default  but in C99 and newer  the int return type may not be omitted  If an implementation permits it  main   can be declared in other ways  e g   int main int argc  char  argv    char  envp      but this makes the program implementation defined  and no longer strictly conforming  The standard defines 3 values for returning that are strictly conforming  that is  does not rely on implementation defined behaviour   0 and EXIT SUCCESS for a successful termination  and EXIT FAILURE for an unsuccessful termination  Any other values are non-standard and implementation defined  In C90  main   must have an explicit return statement at the end to avoid undefined behaviour  In C99 and newer  you may omit the return statement from main    If you do  and main   finished  there is an implicit return 0  Finally  there is nothing wrong from a standards point of view with calling main   recursively from a C program

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The accepted answer appears to be targetted for C    so I thought I d add an answer that pertains to C  and this differs in a few ways  There were also some changes made between ISO IEC 9899 1989  C90  and ISO IEC 9899 1999  C99   main   should be declared as either  int main void  int main int argc  char   argv   Or equivalent  For example  int main int argc  char  argv    is equivalent to the second one  In C90  the int return type can be omitted as it is a default  but in C99 and newer  the int return type may not be omitted  If an implementation permits it  main   can be declared in other ways  e g   int main int argc  char  argv    char  envp      but this makes the program implementation defined  and no longer strictly conforming  The standard defines 3 values for returning that are strictly conforming  that is  does not rely on implementation defined behaviour   0 and EXIT SUCCESS for a successful termination  and EXIT FAILURE for an unsuccessful termination  Any other values are non-standard and implementation defined  In C90  main   must have an explicit return statement at the end to avoid undefined behaviour  In C99 and newer  you may omit the return statement from main    If you do  and main   finished  there is an implicit return 0  Finally  there is nothing wrong from a standards point of view with calling main   recursively from a C program

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The accepted answer appears to be targetted for C    so I thought I d add an answer that pertains to C  and this differs in a few ways  There were also some changes made between ISO IEC 9899 1989  C90  and ISO IEC 9899 1999  C99   main   should be declared as either  int main void  int main int argc  char   argv   Or equivalent  For example  int main int argc  char  argv    is equivalent to the second one  In C90  the int return type can be omitted as it is a default  but in C99 and newer  the int return type may not be omitted  If an implementation permits it  main   can be declared in other ways  e g   int main int argc  char  argv    char  envp      but this makes the program implementation defined  and no longer strictly conforming  The standard defines 3 values for returning that are strictly conforming  that is  does not rely on implementation defined behaviour   0 and EXIT SUCCESS for a successful termination  and EXIT FAILURE for an unsuccessful termination  Any other values are non-standard and implementation defined  In C90  main   must have an explicit return statement at the end to avoid undefined behaviour  In C99 and newer  you may omit the return statement from main    If you do  and main   finished  there is an implicit return 0  Finally  there is nothing wrong from a standards point of view with calling main   recursively from a C program

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I believe that main   should return either EXIT SUCCESS or EXIT FAILURE  They are defined in stdlib h

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I was under the impression that standard specifies that main doesn t need a return value as a successful return was OS based  zero in one could be either a success or a failure in another   therefore the absence of return was a cue for the compiler to insert the successful return itself   However I usually return 0

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main   in C89 and K amp R C unspecified return types default to    int    return 1  return 0     If you do not write a return statement in int main    the closing   will return 0 by default  return 0 or return 1 will be received by the parent process  In a shell it goes into a shell variable  and if you are running your program form a shell and not using that variable then you need not worry about the return value of main      See How can I get what my main function has returned        a out   echo      This way you can see that it is the variable    which receives the least significant byte of the return value of main     In Unix and DOS scripting  return 0 on success and non-zero for error are usually returned  This is the standard used by Unix and DOS scripting to find out what happened with your program and controlling the whole flow

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The return value for main indicates how the program exited  Normal exit is represented by a 0 return value from main  Abnormal exit is signaled by a non-zero return  but there is no standard for how non-zero codes are interpreted  As noted by others  void main   is prohibited by the C   standard and should not be used  The valid C   main signatures are   int main     and  int main int argc  char  argv      which is equivalent to  int main int argc  char   argv    It is also worth noting that in C    int main   can be left without a return-statement  at which point it defaults to returning 0  This is also true with a C99 program  Whether return 0  should be omitted or not is open to debate  The range of valid C program main signatures is much greater     Efficiency is not an issue with the main function  It can only be entered and left once  marking the program s start and termination  according to the C   standard  For C  re-entering main   is allowed  but should be avoided

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Note that the C and C   standards define two kinds of implementations  freestanding and hosted   C90 hosted environment  Allowed forms 1      int main  void      int main  int argc  char  argv         main  void      main  int argc  char  argv              etc  similar forms with implicit int     Comments  The former two are explicitly stated as the allowed forms  the others are implicitly allowed because C90 allowed  quot implicit int quot  for return type and function parameters  No other form is allowed   C90 freestanding environment  Any form or name of main is allowed 2   C99 hosted environment  Allowed forms 3      int main  void      int main  int argc  char  argv           or in some other implementation-defined manner      Comments  C99 removed  quot implicit int quot  so main   is no longer valid  A strange  ambiguous sentence  quot or in some other implementation-defined manner quot  has been introduced  This can either be interpreted as  quot the parameters to int main   may vary quot  or as  quot main can have any implementation-defined form quot   Some compilers have chosen to interpret the standard in the latter way  Arguably  one cannot easily state that they are not conforming by citing the standard in itself  since it is is ambiguous  However  to allow completely wild forms of main   was probably    not the intention of this new sentence  The C99 rationale  not normative  implies that the sentence refers to additional parameters to int main 4  Yet the section for hosted environment program termination then goes on arguing about the case where main does not return int 5  Although that section is not normative for how main should be declared  it definitely implies that main might be declared in a completely implementation-defined way even on hosted systems   C99 freestanding environment  Any form or name of main is allowed 6   C11 hosted environment  Allowed forms 7      int main  void      int main  int argc  char  argv           or in some other implementation-defined manner       C11 freestanding environment  Any form or name of main is allowed 8   Note that int main   was never listed as a valid form for any hosted implementation of C in any of the above versions  In C  unlike C       and  void  have different meanings  The former is an obsolescent feature which may be removed from the language  See C11 future language directions   6 11 6 Function declarators The use of function declarators with empty parentheses  not prototype-format parameter type declarators  is an obsolescent feature     C  03 hosted environment  Allowed forms 9      int main        int main  int argc  char  argv     Comments  Note the empty parenthesis in the first form  C   and C are different in this case  because in C   this means that the function takes no parameters  But in C it means that it may take any parameter   C  03 freestanding environment  The name of the function called at startup is implementation-defined  If it is named main   it must follow the stated forms 10         implementation-defined name  or      int main        int main  int argc  char  argv      C  11 hosted environment  Allowed forms 11      int main        int main  int argc  char  argv     Comments  The text of the standard has been changed but it has the same meaning   C  11 freestanding environment  The name of the function called at startup is implementation-defined  If it is named main   it must follow the stated forms 12         implementation-defined name  or      int main        int main  int argc  char  argv      References  ANSI X3 159-1989 2 1 2 2 Hosted environment   quot Program startup quot    The function called at program startup is named main  The implementation declares no prototype for this function  It shall be defined with a return type of int and with no parameters       int main void                   or with two parameters  referred to here as argc and argv  though any names may be used  as they are local to the function in which they are declared        int main int argc  char  argv                    ANSI X3 159-1989 2 1 2 1 Freestanding environment    In a freestanding environment  in which C program execution may take place without any benefit of an operating system   the name and type of the function called at program startup are implementation-defined    ISO 9899 1999 5 1 2 2 Hosted environment - gt  5 1 2 2 1 Program startup   The function called at program startup is named main  The implementation declares no prototype for this function  It shall be defined with a return type of int and with no parameters       int main void                   or with two parameters  referred to here as argc and argv  though any names may be used  as they are local to the function in which they are declared        int main int argc  char  argv                    or equivalent 9  or in some other implementation-defined manner    Rationale for International Standard     Programming Languages     C  Revision 5 10  5 1 2 2 Hosted environment -- gt  5 1 2 2 1 Program startup   The behavior of the arguments to main  and of the interaction of exit  main and atexit  see   7 20 4 2  has been codified to curb some unwanted variety in the representation of argv strings  and in the meaning of values returned by main    The specification of argc and argv as arguments to main recognizes extensive prior practice  argv argc  is required to be a null pointer to provide a redundant check for the end of the list  also on the basis of common practice    main is the only function that may portably be declared either with zero or two arguments   The number of other functions    arguments must match exactly between invocation and definition   This special case simply recognizes the widespread practice of leaving off the arguments to main when the program does not access the program argument strings  While many implementations support more than two arguments to main  such practice is neither blessed nor forbidden by the Standard  a program that defines main with three arguments is not strictly conforming  see   J 5 1      ISO 9899 1999 5 1 2 2 Hosted environment -- gt  5 1 2 2 3 Program termination   If the return type of the main function is a type compatible with int  a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument 11  reaching the   that terminates the main function returns a value of 0  If the return type is not compatible with int  the termination status returned to the host environment is unspecified    ISO 9899 1999 5 1 2 1 Freestanding environment   In a freestanding environment  in which C program execution may take place without any benefit of an operating system   the name and type of the function called at program startup are implementation-defined    ISO 9899 2011 5 1 2 2 Hosted environment - gt  5 1 2 2 1 Program startup  This section is identical to the C99 one cited above   ISO 9899 1999 5 1 2 1 Freestanding environment  This section is identical to the C99 one cited above   ISO 14882 2003 3 6 1 Main function   An implementation shall not predefine the main function  This function shall not be overloaded  It shall have a return type of type int  but otherwise its type is implementation-defined  All implementations shall allow both of the following definitions of main       int main                   and      int main int argc  char  argv                    ISO 14882 2003 3 6 1 Main function   It is implementation-defined whether a program in a freestanding environment is required to define a main function    ISO 14882 2011 3 6 1 Main function   An implementation shall not predefine the main function  This function shall not be overloaded  It shall have a return type of type int  but otherwise its type is implementation-defined  All implementations shall allow both     a function of    returning int and     a function of  int  pointer to pointer to char  returning int as the type of main  8 3 5     ISO 14882 2011 3 6 1 Main function  This section is identical to the C  03 one cited above

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Keep in mind that even though you re returning an int  some OSes  Windows  truncate the returned value to a single byte  0-255

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I believe that main   should return either EXIT SUCCESS or EXIT FAILURE  They are defined in stdlib h

[c++] What should main() return in C and C++?

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