Do I cast the result of malloc

Question

In this question  someone suggested in a comment that I should not cast the result of malloc  i e   int  sieve   malloc sizeof int    length     rather than   int  sieve    int    malloc sizeof int    length     Why would this be the case

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As others stated  it is not needed for C  but necessary for C    If you think you are going to compile your C code with a C   compiler  for whatever reasons  you can use a macro instead  like    ifdef   cplusplus   define NEW type  count    type   calloc count  sizeof type     else   define NEW type  count   calloc count  sizeof type     endif   That way you can still write it in a very compact way   int  sieve   NEW int  1     and it will compile for C and C

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This question is subject of opinion-based abuse  Sometimes I notice comments like that   Don t cast the result of malloc  or  Why you don t cast the result of malloc  on questions where OP uses casting  The comments itself contain a hyperlink to this question  That is in any possible manner inappropriate and incorrect as well  There is no right and no wrong when it is truly a matter of one s own coding-style   Why is this happening  It s based upon two reasons   This question is indeed opinion-based  Technically  the question should have been closed as opinion-based years ago  A  quot Do I quot  or  quot Don t I quot  or equivalent  quot Should I quot  or  quot Shouldn t I quot  question  you just can t answer focused without an attitude of one s own opinion  One of the reason to close a question is because it  quot might lead to opinion-based answers quot  as it is well shown here   Many answers  including the most apparent and accepted answer of  unwind  are either completely or almost entirely opinion-based  f e  a mysterious  quot clutter quot  that would be added to your code if you do casting or repeating yourself would be bad  and show a clear and focused tendency to omit the cast  They argue about the redundancy of the cast on one side but also and even worse argue to solve a bug caused by a bug failure of programming itself - to not  include  lt stdlib h gt  if one want to use malloc       I want to bring a true view of some points discussed  with less of my personal opinion  A few points need to be noted especially   Such a very susceptible question to fall into one s own opinion needs an answer with neutral pros and cons  Not only cons or pros  A good overview of pros and cons is listed in this answer  https   stackoverflow com a 33047365 12139179  I personally consider this because of that reason the best answer  so far       One reason which is encountered at most to reason the omission of the cast is that the cast might hide a bug  If someone uses an implicit declared malloc   that returns int  implicit functions are gone from the standard since C99  and sizeof int     sizeof int    as shown in this question Why does this code segfault on 64-bit architecture but work fine on 32-bit  the cast would hide a bug  While this is true  it only shows half of the story as the omission of the cast would only be a forward-bringing solution to an even bigger bug - not including stdlib h when using malloc    This will never be a serious issue  If you   Use a compiler compliant to C99 or above  which is recommended and should be mandatory   and  Aren t so absent to forgot to include stdlib h  when you want to use malloc   in your code  which is a huge bug itself        Some people argue about C   compliance of C code  as the cast is obliged in C    First of all to say in general  Compiling C code with a C   compiler is not a good practice  C and C   are in fact two completely different languages with different semantics  But If you really want need to make C code compliant to C   and vice versa use compiler switches instead of any cast  Since the cast is with tendency declared as redundant or even harmful  I want to take a focus on these questions  which give good reasons why casting can be useful or even necessary   https   stackoverflow com a 34094068 12139179  https   stackoverflow com a 36297486 12139179  https   stackoverflow com a 33044300 12139179       The cast can be non-beneficial when your code  respectively the type of the assigned pointer  and with that the type of the cast   changes  although this is in most cases unlikely  Then you would need to maintain change all casts too and if you have a few thousand calls to memory-management functions in your code  this can really summarizing up and decrease the maintenance efficiency    Summary  Fact is  that the cast is redundant per the C standard  already since ANSI-C  C89 C90   if the assigned pointer point to an object of fundamental alignment requirement  which includes the most of all objects   You don t need to do the cast as the pointer is automatically aligned in this case    quot The order and contiguity of storage allocated by successive calls to the aligned alloc  calloc  malloc  and realloc functions is unspecified  The pointer returned if the allocation succeeds is suitably aligned so that it may be assigned to a pointer to any type of object with a fundamental alignment requirement and then used to access such an object or an array of such objects in the space allocated  until the space is explicitly deallocated   quot  Source  C18    7 22 3 1     quot A fundamental alignment is a valid alignment less than or equal to  Alignof  max align t   Fundamental alignments shall be supported by the implementation for objects of all storage durations  The alignment requirements of the following types shall be fundamental alignments       all atomic  qualified  or unqualified basic types       all atomic  qualified  or unqualified enumerated types       all atomic  qualified  or unqualified pointer types       all array types whose element type has a fundamental alignment requirement 57       all types specified in Clause 7 as complete object types       all structure or union types all of whose elements have types with fundamental alignment requirements and none of whose elements have an alignment specifier specifying an alignment that is not a fundamental alignment   As specified in 6 2 1  the later declaration might hide the prior declaration  quot   Source  C18    6 2 8 2  However  if you allocate memory for an implementation-defined object of extended alignment requirement  the cast would be needed   An extended alignment is represented by an alignment greater than  Alignof  max align t   It is implementation-defined whether any extended alignments are supported and the storage durations for which they are supported  A type having an extended alignment requirement is an over-aligned type 58  Source  C18    6 2 8 3  Everything else is a matter of the specific use case and one s own opinion  Please be careful how you educate yourself  I recommend you to read all of the answers made so far carefully first  as well as their comments which may point at a failure  and then build your own opinion if you or if you not cast the result of malloc   at a specific case  Please note  There is no right and wrong answer to that question  It is a matter of style and you yourself decide which way you choose  if you aren t forced to by education or job of course   Please be aware of that and don t let trick you   Last note  I voted to lately close this question as opinion-based  which is indeed needed since years  If you got the close reopen privilege I would like to invite you to do so  too

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The best thing to do when programming in C whenever it is possible    Make your program compile through a C compiler with all warnings turned on -Wall and fix all errors and warnings Make sure there are no variables declared as auto Then compile it using a C   compiler with -Wall and -std c  11  Fix all errors and warnings  Now compile using the C compiler again  Your program should now compile without any warning and contain fewer bugs    This procedure lets you take advantage of C   strict type checking  thus reducing the number of bugs  In particular  this procedure forces you to include stdlib hor you will get     malloc was not declared within this scope   and also forces you to cast the result of malloc or you will get     invalid conversion from void  to T    or what ever your target type is   The only benefits from writing in C instead of C   I can find are   C has a well specified ABI C   may generate more code  exceptions  RTTI  templates  runtime polymorphism    Notice that the second cons should in the ideal case disappear when using the subset common to C together with the static polymorphic feature   For those that finds C   strict rules inconvenient  we can use the C  11 feature with inferred  type  auto memblock static cast lt T  gt  malloc n sizeof T       Mult may overflow

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The returned type is void   which can be cast to the desired type of data pointer in order to be dereferenceable

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In C  you don t need to cast the return value of malloc  The pointer to void returned by malloc is automagically converted to the correct type  However  if you want your code to compile with a C   compiler  a cast is needed  A preferred alternative among the community is to use the following   int  sieve   malloc sizeof  sieve   length     which additionally frees you from having to worry about changing the right-hand side of the expression if ever you change the type of sieve   Casts are bad  as people have pointed out  Especially pointer casts

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From the Wikipedia      Advantages to casting         Including the cast may allow a C program or function to compile as C      The cast allows for pre-1989 versions of malloc that originally returned a char      Casting can help the developer identify inconsistencies in type sizing should the destination pointer type change  particularly if the pointer is declared far from the malloc   call  although modern compilers and static analyzers can warn on such behaviour without requiring the cast           Disadvantages to casting         Under the ANSI C standard  the cast is redundant    Adding the cast may mask failure to include the header stdlib h  in   which the prototype for malloc is found  In the absence of a   prototype for malloc  the standard requires that the C compiler   assume malloc returns an int  If there is no cast  a warning is   issued when this integer is assigned to the pointer  however  with   the cast  this warning is not produced  hiding a bug  On certain   architectures and data models  such as LP64 on 64-bit systems  where   long and pointers are 64-bit and int is 32-bit   this error can   actually result in undefined behaviour  as the implicitly declared   malloc returns a 32-bit value whereas the actually defined function   returns a 64-bit value  Depending on calling conventions and memory   layout  this may result in stack smashing  This issue is less likely   to go unnoticed in modern compilers  as they uniformly produce   warnings that an undeclared function has been used  so a warning will   still appear  For example  GCC s default behaviour is to show a   warning that reads  incompatible implicit declaration of built-in   function  regardless of whether the cast is present or not    If the type of the pointer is changed at its declaration  one may   also  need to change all lines where malloc is called and cast       Although malloc without casting is preferred method and most experienced programmers choose it  you should use whichever you like having aware of the issues   i e  If you need to compile C program as C    Although it is a separate language  you must cast the result of use malloc

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A void pointer is a generic pointer and C supports implicit conversion from a void pointer type to other types  so there is no need of explicitly typecasting it   However  if you want the same code work perfectly compatible on a C   platform  which does not support implicit conversion  you need to do the typecasting  so it all depends on usability

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You can  but don t need to cast in C  You have to cast if that code is compiled as C

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I prefer to do the cast  but not manually  My favorite is using g new and g new0 macros from glib  If glib is not used  I would add similar macros  Those macros reduce code duplication without compromising type safety  If you get the type wrong  you would get an implicit cast between non-void pointers  which would cause a warning  error in C     If you forget to include the header that defines g new and g new0  you would get an error  g new and g new0 both take the same arguments  unlike malloc that takes fewer arguments than calloc  Just add 0 to get zero-initialized memory  The code can be compiled with a C   compiler without changes

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This is what The GNU C Library Reference manual says       You can store the result of malloc into any pointer variable without a   cast  because ISO C automatically converts the type void   to another   type of pointer when necessary  But the cast is necessary in contexts   other than assignment operators or if you might want your code to run   in traditional C    And indeed the ISO C11 standard  p347  says so       The pointer returned if the allocation succeeds is suitably aligned so   that it may be assigned to a pointer to any type of object with a   fundamental alignment requirement and then used to access such an   object or an array of such objects in the space allocated  until the   space is explicitly deallocated

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In the C language  a void pointer can be assigned to any pointer  which is why you should not use a type cast  If you want  type safe  allocation  I can recommend the following macro functions  which I always use in my C projects    include  lt stdlib h gt   define NEW ARRAY ptr  n   ptr    malloc  n    sizeof   ptr    define NEW ptr  NEW ARRAY  ptr   1    With these in place you can simply say  NEW ARRAY sieve  length     For non-dynamic arrays  the third must-have function macro is   define LEN arr   sizeof  arr    sizeof  arr  0     which makes array loops safer and more convenient   int i  a 100    for  i   0  i  lt  LEN a   i

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You do cast  because    It makes your code more portable between C and C    and as SO experience shows  a great many programmers claim they are writing in C when they are really writing in C    or C plus local compiler extensions   Failing to do so can hide an error  note all the SO examples of confusing when to write type   versus type     The idea that it keeps you from noticing you failed to  include an appropriate header file misses the forest for the trees  It s the same as saying  don t worry about the fact you failed to ask the compiler to complain about not seeing prototypes -- that pesky stdlib h is the REAL important thing to remember   It forces an extra cognitive cross-check  It puts the  alleged  desired type right next to the arithmetic you re doing for the raw size of that variable  I bet you could do an SO study that shows that malloc   bugs are caught much faster when there s a cast  As with assertions  annotations that reveal intent decrease bugs   Repeating yourself in a way that the machine can check is often a great idea  In fact  that s what an assertion is  and this use of cast is an assertion  Assertions are still the most general technique we have for getting code correct  since Turing came up with the idea so many years ago

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In C you get an implicit conversion from void   to any other  data  pointer

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A void pointer is a generic object pointer and C supports implicit conversion from a void pointer type to other types  so there is no need of explicitly typecasting it   However  if you want the same code work perfectly compatible on a C   platform  which does not support implicit conversion  you need to do the typecasting  so it all depends on usability

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People used to GCC and Clang are spoiled   It s not all that good out there   I have been pretty horrified over the years by the staggeringly aged compilers I ve been required to use   Often companies and managers adopt an ultra-conservative approach to changing compilers and will not even test if a new compiler   with better standards compliance and code optimization   will work in their system   The practical reality for working developers is that when you re coding you need to cover your bases and  unfortunately  casting mallocs is a good habit if you cannot control what compiler may be applied to your code   I would also suggest that many organizations apply a coding standard of their own and that that should be the method people follow if it is defined   In the absence of explicit guidance I tend to go for most likely to compile everywhere  rather than slavish adherence to a standard   The argument that it s not necessary under current standards is quite valid   But that argument omits the practicalities of the real world   We do not code in a world ruled exclusively by the standard of the day  but by the practicalities of what I like to call  local management s reality field    And that s bent and twisted more than space time ever was   -   YMMV   I tend to think of casting malloc as a defensive operation   Not pretty  not perfect  but generally safe     Honestly  if you ve not included stdlib h then you ve way more problems than casting malloc

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It is not mandatory to cast the results of malloc  since it returns void    and a void  can be pointed to any datatype

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In C you can implicitly convert a void pointer to any other kind of pointer  so a cast is not necessary  Using one may suggest to the casual observer that there is some reason why one is needed  which may be misleading

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TL DR int  sieve    int    malloc sizeof int    length    has two problems  The cast and that you re using the type instead of variable as argument for sizeof  Instead  do like this  int  sieve   malloc sizeof  sieve   length    Long version No  you don t cast the result  since   It is unnecessary  as void   is automatically and safely promoted to any other pointer type in this case  It adds clutter to the code  casts are not very easy to read  especially if the pointer type is long   It makes you repeat yourself  which is generally bad  It can hide an error if you forgot to include  lt stdlib h gt   This can cause crashes  or  worse  not cause a crash until way later in some totally different part of the code   Consider what happens if pointers and integers are differently sized  then you re hiding a warning by casting and might lose bits of your returned address  Note  as of C99 implicit functions are gone from C  and this point is no longer relevant since there s no automatic assumption that undeclared functions return int   As a clarification  note that I said  quot you don t cast quot   not  quot you don t need to cast quot   In my opinion  it s a failure to include the cast  even if you got it right  There are simply no benefits to doing it  but a bunch of potential risks  and including the cast indicates that you don t know about the risks  Also note  as commentators point out  that the above talks about straight C  not C    I very firmly believe in C and C   as separate languages  To add further  your code needlessly repeats the type information  int  which can cause errors  It s better to de-reference the pointer being used to store the return value  to  quot lock quot  the two together  int  sieve   malloc length   sizeof  sieve    This also moves the length to the front for increased visibility  and drops the redundant parentheses with sizeof  they are only needed when the argument is a type name  Many people seem to not know  or ignore  this  which makes their code more verbose  Remember  sizeof is not a function      While moving length to the front may increase visibility in some rare cases  one should also pay attention that in the general case  it should be better to write the expression as  int  sieve   malloc sizeof  sieve   length    Since keeping the sizeof first  in this case  ensures multiplication is done with at least size t math  Compare  malloc sizeof  sieve   length   width  vs  malloc length   width   sizeof  sieve  the second may overflow the length   width when width and length are smaller types than size t

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The casting of malloc is unnecessary in C but mandatory in C     Casting is unnecessary in C because of    void   is automatically and safely promoted to any other pointer type in the case of C  It can hide an error if you forgot to include  lt stdlib h gt   This can cause crashes  If pointers and integers are differently sized  then you re hiding a warning by casting and might lose bits of your returned address  If the type of the pointer is changed at its declaration  one may also need to change all lines where malloc is called and cast    On the other hand  casting may increase the portability of your program  i e  it allows a C program or function to compile as C

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I put in the cast simply to show disapproval of the ugly hole in the type system  which allows code such as the following snippet to compile without diagnostics  even though no casts are used to bring about the bad conversion   double d  void  p    amp d  int  q   p    I wish that didn t exist  and it doesn t in C    and so I cast  It represents my taste  and my programming politics  I m not only casting a pointer  but effectively  casting a ballot  and casting out demons of stupidity  If I can t actually cast out stupidity  then at least let me express the wish to do so with a gesture of protest   In fact  a good practice is to wrap malloc  and friends  with functions that return unsigned char    and basically never to use void   in your code   If you need a generic pointer-to-any-object  use a char   or unsigned char    and have casts in both directions  The one relaxation that can be indulged  perhaps  is using functions like memset and memcpy without casts   On the topic of casting and C   compatibility  if you write your code so that it compiles as both C and C    in which case you have to cast the return value of malloc when assigning it to something other than void     you can do a very helpful thing for yourself  you can use macros for casting which translate to C   style casts when compiling as C    but reduce to a C cast when compiling as C      In a header somewhere     ifdef   cplusplus  define strip qual TYPE  EXPR   const cast lt TYPE gt  EXPR    define convert TYPE  EXPR   static cast lt TYPE gt  EXPR    define coerce TYPE  EXPR   reinterpret cast lt TYPE gt  EXPR    else  define strip qual TYPE  EXPR    TYPE   EXPR    define convert TYPE  EXPR    TYPE   EXPR    define coerce TYPE  EXPR    TYPE   EXPR    endif   If you adhere to these macros  then a simple grep search of your code base for these identifiers will show you where all your casts are  so you can review whether any of them are incorrect   Then  going forward  if you regularly compile the code with C    it will enforce the use of an appropriate cast  For instance  if you use strip qual just to remove a const or volatile  but the program changes in such a way that a type conversion is now involved  you will get a diagnostic  and you will have to use a combination of casts to get the desired conversion   To help you adhere to these macros  the the GNU C    not C   compiler has a beautiful feature  an optional diagnostic which is produced for all occurrences of C style casts         -Wold-style-cast  C   and Objective-C   only           Warn if an old-style  C-style  cast to a non-void type is used          within a C   program   The new-style casts  dynamic cast           static cast  reinterpret cast  and const cast  are less vulnerable          to unintended effects and much easier to search for    If your C code compiles as C    you can use this -Wold-style-cast option to find out all occurrences of the  type  casting syntax that may creep into the code  and follow up on these diagnostics by replacing it with an appropriate choice from among the above macros  or a combination  if necessary    This treatment of conversions is the single largest standalone technical justification for working in a  Clean C   the combined C and C   dialect  which in turn technically justifies casting the return value of malloc

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Casting the value returned by malloc   is not necessary now  but I d like to add one point that seems no one has pointed out   In the ancient days  that is  before ANSI C provides the void   as the generic type of pointers  char   is the type for such usage  In that case  the cast can shut down the compiler warnings   Reference  C FAQ

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As other stated  it is not needed for C  but for C    Including the cast may allow a C program or function to compile as C    In C it is unnecessary  as void   is automatically and safely promoted to any other pointer type  But if you cast then  it can hide an error if you forgot to include stdlib h  This can cause crashes  or  worse  not cause a crash until way later in some totally different part of the code    Because stdlib h contains the prototype for malloc is found  In the absence of a prototype for malloc  the standard requires that the C compiler assumes malloc returns an int  If there is no cast  a warning is issued when this integer is assigned to the pointer  however  with the cast  this warning is not produced  hiding a bug

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The concept behind void pointer is that it can be casted to any data type that is why malloc returns void  Also you must be aware of automatic typecasting  So it is not mandatory to cast the pointer though you must do it  It helps in keeping the code clean and helps debugging

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For me  the take home and conclusion here is that casting malloc in C is totally NOT necessary but if you however cast  it wont affect malloc as malloc will still allocate to you your requested blessed memory space  Another take home is the reason or one of the reasons people do casting and this is to enable them compile same program either in C or C     There may be other reasons but other reasons  almost certainly  would land you in serious trouble sooner or later

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It depends on the programming language and compiler  If you use malloc in C  there is no need to type cast it  as it will automatically type cast  However  if you are using C    then you should type cast because malloc will return a void  type

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No  you don t cast the result of malloc     In general  you don t cast to or from void     A typical reason given for not doing so is that failure to  include  lt stdlib h gt  could go unnoticed  This isn t an issue anymore for a long time now as C99 made implicit function declarations illegal  so if your compiler conforms to at least C99  you will get a diagnostic message   But there s a much stronger reason not to introduce unnecessary pointer casts   In C  a pointer cast is almost always an error  This is because of the following rule    6 5 p7 in N1570  the latest draft for C11       An object shall have its stored value accessed only by an lvalue expression that has one of   the following types        a type compatible with the effective type of the object        a qualified version of a type compatible with the effective type of the object        a type that is the signed or unsigned type corresponding to the effective type of the   object        a type that is the signed or unsigned type corresponding to a qualified version of the   effective type of the object        an aggregate or union type that includes one of the aforementioned types among its   members  including  recursively  a member of a subaggregate or contained union   or       a character type    This is also known as the strict aliasing rule  So the following code is undefined behavior   long x   5  double  p    double    amp x  double y    p    And  sometimes surprisingly  the following is as well   struct foo   int x     struct bar   int x  int y     struct bar b     1  2   struct foo  p    struct foo    amp b  int z   p- gt x    Sometimes  you do need to cast pointers  but given the strict aliasing rule  you have to be very careful with it  So  any occurrence of a pointer cast in your code is a place you have to double-check for its validity  Therefore  you never write an unnecessary pointer cast   tl dr  In a nutshell  Because in C  any occurrence of a pointer cast should raise a red flag for code requiring special attention  you should never write unnecessary pointer casts     Side notes    There are cases where you actually need a cast to void    e g  if you want to print a pointer   int x   5  printf   p n    void    amp x     The cast is necessary here  because printf   is a variadic function  so implicit conversions don t work  In C    the situation is different  Casting pointer types is somewhat common  and correct  when dealing with objects of derived classes  Therefore  it makes sense that in C    the conversion to and from void   is not implicit  C   has a whole set of different flavors of casting

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Casting is only for C   not C In case you are using a C   compiler you better change it to C compiler

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You don t cast the result of malloc  because doing so adds pointless clutter to your code  The most common reason why people cast the result of malloc is because they are unsure about how the C language works  That s a warning sign  if you don t know how a particular language mechanism works  then don t take a guess  Look it up or ask on Stack Overflow  Some comments   A void pointer can be converted to from any other pointer type without an explicit cast  C11 6 3 2 3 and 6 5 16 1    C   will however not allow an implicit cast between void  and another pointer type  So in C    the cast would have been correct  But if you program in C    you should use new and not malloc    And you should never compile C code using a C   compiler  If you need to support both C and C   with the same source code  use compiler switches to mark the differences  Do not attempt to sate both language standards with the same code  because they are not compatible   If a C compiler cannot find a function because you forgot to include the header  you will get a compiler linker error about that  So if you forgot to include  lt stdlib h gt  that s no biggie  you won t be able to build your program   On ancient compilers that follow a version of the standard which is more than 25 years old  forgetting to include  lt stdlib h gt  would result in dangerous behavior  Because in that ancient standard  functions without a visible prototype implicitly converted the return type to int  Casting the result from malloc explicitly would then hide away this bug  But that is really a non-issue  You aren t using a 25 years old computer  so why would you use a 25 years old compiler

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Just adding my experience  studying computer engineering I see that the two or three professors that I have seen writing in C always cast malloc  however the one I asked  with an immense CV and understanding of C  told me that it is absolutely unnecessary but only used to be absolutely specific  and to get the students into the mentality of being absolutely specific  Essentially casting will not change anything in how it works  it does exactly what it says  allocates memory  and casting does not effect it  you get the same memory  and even if you cast it to something else by mistake  and somehow evade compiler errors  C will access it the same way   Edit  Casting has a certain point  When you use array notation  the code generated has to know how many memory places it has to advance to reach the beginning of the next element  this is achieved through casting  This way you know that for a double you go 8 bytes ahead while for an int you go 4  and so on  Thus it has no effect if you use pointer notation  in array notation it becomes necessary

[c] Do I cast the result of malloc?

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