What is the difference between include filename and include filename

Question

In the C and C   programming languages  what is the difference between using angle brackets and using quotes in an include statement  as follows     include  lt filename gt    include  filename

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include  lt abc h gt    is used to include standard library files  So the compiler will check in the locations where standard library headers are residing    include  xyz h    will tell the compiler to include user-defined header files  So the compiler will check for these header files in the current folder or -I defined folders

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It does    mypath myfile  is short for   mypath myfile   with   being either the directory of the file where the  include is contained in  and or the current working directory of the compiler  and or the default include paths  and   lt mypath myfile gt  is short for  lt defaultincludepaths gt  mypath myfile   If    is in  lt default include paths gt   then it doesn t make a difference   If mypath myfile is in another include directory  the behavior is undefined

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include  filename     User defined header  include  lt filename gt     Standard library header    Example   The filename here is Seller h    ifndef SELLER H        Header guard  define SELLER H        Header guard   include  lt string gt   include  lt iostream gt   include  lt iomanip gt   class Seller       private          char name 31           double sales total       public          Seller            Seller char    double           char getName      endif   In the class implementation  for example  Seller cpp  and in other files that will use the file Seller h   the header defined by the user should now be included  as follows    include  Seller h

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Form 1 -  include  lt  xxx  gt  First  looks for the presence of header file in the current directory from where directive is invoked  If not found  then it searches in the preconfigured list of standard system directories  Form 2 -  include  quot xxx quot  This looks for the presence of header file in the current directory from where directive is invoked   The exact search directory list depends on the target system  how GCC is configured  and where it is installed  You can find the search directory list of your GCC compiler by running it with -v option  You can add additional directories to the search path by using - Idir  which causes dir to be searched after the current directory  for the quote form of the directive  and ahead of the standard system directories   Basically  the form  quot xxx quot  is nothing but search in current directory  if not found falling back the form

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In C    include a file in two ways   The first one is  include  which tells the preprocessor to look for the file in the predefined default location  This location is often an INCLUDE environment variable that denotes the path to include files   And the second type is  include  filename  which tells the preprocessor to look for the file in the current directory first  then look for it in the predefined locations user have set up

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include  lt file h gt  tells the compiler to search for the header in its  includes  directory  e g  for MinGW the compiler would search for file h in C  MinGW include  or wherever your compiler is installed    include  file  tells the compiler to search the current directory  i e  the directory in which the source file resides  for file    You can use the -I flag for GCC to tell it that  when it encounters an include with angled brackets  it should also search for headers in the directory after -I  GCC will treat the directory after the flag as if it were the includes directory   For instance  if you have a file called myheader h in your own directory  you could say  include  lt myheader h gt  if you called GCC with the flag -I    indicating that it should search for includes in the current directory     Without the -I flag  you will have to use  include  myheader h  to include the file  or move myheader h to the include directory of your compiler

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Many of the answers here focus on the paths the compiler will search in order to find the file  While this is what most compilers do  a conforming compiler is allowed to be preprogrammed with the effects of the standard headers  and to treat  say   include  lt list gt  as a switch  and it need not exist as a file at all   This is not purely hypothetical  There is at least one compiler that work that way  Using  include  lt xxx gt  only with standard headers is recommended

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include  lt  gt  is for predefined header files   If the header file is predefined then you would simply write the header file name in angular brackets  and it would look like this  assuming we have a predefined header file name iostream     include  lt iostream gt      include     is for header files the programmer defines   If you  the programmer  wrote your own header file then you would write the header file name in quotes  So  suppose you wrote a header file called myfile h  then this is an example of how you would use the include directive to include that file    include  myfile h

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The  include  lt filename gt  is used when a system file is being referred to  That is a header file that can be found at system default locations like  usr include or  usr local include  For your own files that needs to be included in another program you have to use the  include  filename  syntax

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For  include    a compiler normally searches the folder of the file which contains that include and then the other folders  For  include  lt  gt  the compiler does not search the current file s folder

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The sequence of characters between  lt  and   uniquely refer to a header  which isn t necessarily a file  Implementations are pretty much free to use the character sequence as they wish   Mostly  however  just treat it as a file name and do a search in the include path  as the other posts state    If the  include  file  form is used  the implementation first looks for a file of the given name  if supported  If not  supported   or if the search fails  the implementation behaves as though the other   include  lt file gt   form was used   Also  a third form exists and is used when the  include directive doesn t match either of the forms above  In this form  some basic preprocessing  such as macro expansion  is done on the  operands  of the  include directive  and the result is expected to match one of the two other forms

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include  lt filename gt  is used when you want to use the header file of the C C   system or compiler libraries  These libraries can be stdio h  string h  math h  etc   include  quot path-to-file filename quot  is used when you want to use your own custom header file which is in your project folder or somewhere else  For more information about preprocessors and header  Read C - Preprocessors

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Thanks for the great answers  esp  Adam Stelmaszczyk and piCookie  and aib  Like many programmers  I have used the informal convention of using the  quot myApp hpp quot  form for application specific files  and the  lt libHeader hpp gt  form for library and compiler system files  i e  files specified in  I and the INCLUDE environment variable  for years thinking that was the standard  However  the C standard states that the search order is implementation specific  which can make portability complicated   To make matters worse  we use jam  which automagically figures out where the include files are   You can use relative or absolute paths for your include files  i e   include  quot       MyProgDir SourceDir1 someFile hpp quot   Older versions of MSVS required double backslashes       but now that s not required  I don t know when it changed  Just use forward slashes for compatibility with  nix  Windows will accept that   If you are really worried about it  use  quot   myHeader h quot  for an include file in the same directory as the source code  my current  very large project has some duplicate include file names scattered about--really a configuration management problem   Here s the MSDN explanation copied here for your convenience    Quoted form The preprocessor searches for include files in this order   In the same directory as the file that contains the  include statement  In the directories of the currently opened include files  in the reverse order in which they were opened  The search begins in the directory of the parent include file and continues upward through the directories of any grandparent include files  Along the path that s specified by each  I compiler option  Along the paths that are specified by the INCLUDE environment variable   Angle-bracket form The preprocessor searches for include files in this order   Along the path that s specified by each  I compiler option  When compiling occurs on the command line  along the paths that are specified by the INCLUDE environment variable

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In general the difference is where the preprocessor searches for the header file   include is a preprocessor directive to include header file  Both  include are used to add or include header file in the program  but first is to include system header files and later one for user defined header files    include  lt filename gt  is used to include the system library header file in the program  means the C C   preprocessor will search for the filename where the C library files are stored or predefined system header files are stored   include  quot filename quot  is used to include user defined header file in the program  means the C C   preprocessor will search for the filename in the current directory the program is in and then follows the search path used for the  include  lt filename gt   Check the gcc docs gcc include files

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quot  quot  will search    first  then search the default include path  you can use command like this to print the default include path   gcc -v -o a a c  here are some examples to make thing more clear  the code a c works    a c  include  quot stdio h quot  int main             int a   3          printf  quot a    d n quot   a           return 0      the code of b c works too    b c  include  lt stdio h gt  int main             int a   3          printf  quot a    d n quot   a           return 0      but when I create a new file named stdio h in current directory    stdio h inline int foo             return 10     a c will generate compile error  but b c still works and  quot  quot    lt  gt  can be used together with the same file name  since the search path priority is different  so d c also works    d c  include  lt stdio h gt   include  quot stdio h quot  int main             int a   0           a   foo             printf  quot a  d n quot   a            return 0

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Some good answers here make references to the C standard but forgot the POSIX standard  especially the specific behavior of the c99  e g  C compiler  command   According to The Open Group Base Specifications Issue 7      -I  directory      Change the algorithm for searching for headers whose names are not absolute pathnames to look in the directory named by the directory pathname before looking in the usual places  Thus  headers whose names are enclosed in double-quotes        shall be searched for first in the directory of the file with the  include line  then in directories named in -I options  and last in the usual places  For headers whose names are enclosed in angle brackets     lt       the header shall be searched for only in directories named in -I options and then in the usual places  Directories named in -I options shall be searched in the order specified  Implementations shall support at least ten instances of this option in a single c99 command invocation    So  in a POSIX compliant environment  with a POSIX compliant C compiler   include  file h  is likely going to search for   file h first  where   is the directory where is the file with the  include statement  while  include  lt file h gt   is likely going to search for  usr include file h first  where  usr include is your system defined usual places for headers  it s seems not defined by POSIX

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In practice  the difference is in the location where the preprocessor searches for the included file    For  include  lt filename gt  the preprocessor searches in an implementation dependent manner  normally in search directories pre-designated by the compiler IDE  This method is normally used to include standard library header files   For  include  filename  the preprocessor searches first in the same directory as the file containing the directive  and then follows the search path used for the  include  lt filename gt  form  This method is normally used to include programmer-defined header files   A more complete description is available in the GCC documentation on search paths

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By the standard - yes  they are different         A preprocessing directive of the form   include  lt h-char-sequence gt  new-line       searches a sequence of implementation-defined places for a header identified uniquely by the specified sequence between the  lt  and  gt  delimiters  and causes the replacement of that directive by the entire contents of the header  How the places are specified or the header identified is implementation-defined    A preprocessing directive of the form   include  q-char-sequence  new-line       causes the replacement of that directive by the entire contents of the source file identified by the specified sequence between the   delimiters  The named source file is searched for in an implementation-defined manner  If this search is not supported  or if the search fails  the directive is reprocessed as if it read   include  lt h-char-sequence gt  new-line       with the identical contained sequence  including  gt  characters  if any  from the original   directive    A preprocessing directive of the form   include pp-tokens new-line        that does not match one of the two previous forms  is permitted  The preprocessing tokens after include in the directive are processed just as in normal text   Each identifier currently defined as a macro name is replaced by its replacement list of preprocessing tokens   The directive resulting after all replacements shall match one of the two previous forms  The method by which a sequence of preprocessing tokens between a  lt  and a  gt  preprocessing token pair or a pair of   characters is combined into a single header name preprocessing token is implementation-defined          Definitions          h-char  any member of the source character set except the new-line character and  gt    q-char  any member of the source character set except the new-line character and        Note that the standard does not tell any relation between the implementation-defined manners  The first form searches in one implementation-defined way  and the other in a  possibly other  implementation-defined way  The standard also specifies that certain include files shall be present  for example   lt stdio h gt     Formally you d have to read the manual for your compiler  however normally  by tradition  the  include       form searches the directory of the file in which the  include was found first  and then the directories that the  include  lt     gt  form searches  the include path  eg system headers

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the    lt  filename     searches in standard C library locations      whereas  filename  searches in the current directory as well    Ideally  you would use  lt      for standard C libraries and       for libraries that you write and are present in the current directory

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An  include with angle brackets will search an  implementation-dependent list of places   which is a very complicated way of saying  system headers   for the file to be included   An  include with quotes will just search for a file  and   in an implementation-dependent manner   bleh   Which means  in normal English  it will try to apply the path filename that you toss at it and will not prepend a system path or tamper with it otherwise   Also  if  include    fails  it is re-read as  include  lt   by the standard   The gcc documentation has a  compiler specific  description which although being specific to gcc and not the standard  is a lot easier to understand than the attorney-style talk of the ISO standards

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The only way to know is to read your implementation s documentation   In the C standard  section 6 10 2  paragraphs 2 to 4 state         A preprocessing directive of the form   include  lt h-char-sequence gt  new-line       searches a sequence of implementation-defined places for a header identified uniquely by the specified sequence between the  lt  and  gt  delimiters  and causes the replacement of that directive by the entire contents of the header  How the places are specified or the header identified is implementation-defined    A preprocessing directive of the form   include  q-char-sequence  new-line       causes the replacement of that directive by the entire contents of the source file identified by the specified sequence between the   delimiters  The named source file is searched for in an implementation-defined manner  If this search is not supported  or if the search fails  the directive is reprocessed as if it read   include  lt h-char-sequence gt  new-line       with the identical contained sequence  including  gt  characters  if any  from the original   directive    A preprocessing directive of the form   include pp-tokens new-line        that does not match one of the two previous forms  is permitted  The preprocessing tokens after include in the directive are processed just as in normal text   Each identifier currently defined as a macro name is replaced by its replacement list of preprocessing tokens   The directive resulting after all replacements shall match one of the two previous forms  The method by which a sequence of preprocessing tokens between a  lt  and a  gt  preprocessing token pair or a pair of   characters is combined into a single header name preprocessing token is implementation-defined          Definitions          h-char  any member of the source character set except the new-line character and  gt    q-char  any member of the source character set except the new-line character and

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When you use  include  lt filename gt   the pre-processor looking for the file in directory of C C   header files  stdio h cstdio  string  vector  etc    But  when you use  include  filename   first  the pre-processor looking for the file in the current directory  and if it doesn t here - he looking for it in the directory of C C   header files

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include  lt filename gt      The preprocessor searches in an implementation-dependent manner  It tells the compiler to search directory where system header files are held   This method usually use to find standard header files     include  filename    This tell compiler to search header files where program is running  If it was failed it behave like  include  lt filename gt  and search that header file at where system header files stored  This method usually used for identify user defined header files header files which are created by user   There for don t use this if you want to call standard library because it takes more compiling time than  include  lt filename gt

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include  lt file gt     Includes a file where the default include directory is    include  file     Includes a file in the current directory in which it was compiled

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The  lt file gt  include tells the preprocessor to search in -I directories and in predefined directories first  then in the  c file s directory  The  file  include tells the preprocessor to search the source file s directory first  and then revert to -I and predefined  All destinations are searched anyway  only the order of search is different   The 2011 standard mostly discusses the include files in  16 2 Source file inclusion       2 A preprocessing directive of the form        include  lt h-char-sequence gt  new-line      searches a sequence of implementation-defined places for a header identified uniquely by the   specified sequence between the  lt  and   delimiters  and causes the   replacement of that directive by the entire contents of the header    How the places are specified or the header identified is   implementation-defined       3 A preprocessing directive of the form        include  q-char-sequence  new-line      causes the replacement of that directive by the entire contents of the source file identified by the   specified sequence between the   delimiters  The named source file is   searched for in an implementation-defined manner  If this search is   not supported  or if the search fails  the directive is reprocessed as   if it read        include  lt h-char-sequence gt  new-line      with the identical contained sequence  including   characters  if any  from the original directive    Note that  xxx  form degrades to  lt xxx gt  form if the file is not found  The rest is implementation-defined

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GCC documentation says the following about the difference between the two      Both user and system header files are included using the preprocessing     directive     include     It has two variants             include  lt file gt           This variant is used for system header files  It searches for a file named file in a standard list of system directories  You can prepend directories to this list with the -I option  see Invocation             include  file           This variant is used for header files of your own program  It searches for a file named file first in the directory containing the current file  then in the quote directories and then the same directories used for  lt file gt   You can prepend directories to the list of quote directories with the -iquote option      The argument of     include     whether delimited with quote marks or angle brackets  behaves like a string constant in that comments are not recognized  and macro names are not expanded  Thus   include  lt x  y gt  specifies inclusion of a system header file named x  y           However  if backslashes occur within file  they are considered ordinary text characters  not escape characters  None of the character escape sequences appropriate to string constants in C are processed  Thus  include  x n  y specifies a filename containing three backslashes   Some systems interpret         as a pathname separator  All of these also interpret         the same way  It is most portable to use only                    It is an error if there is anything  other than comments  on the line after the file name

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To see the search order on your system using gcc  based on current configuration   you can execute the following command   You can find more detail on this command here  cpp -v  dev null -o  dev null     Apple LLVM version 10 0 0  clang-1000 10 44 2     Target  x86 64-apple-darwin18 0 0    Thread model  posix InstalledDir  Library Developer CommandLineTools usr bin     Library Developer CommandLineTools usr bin clang  -cc1 -triple   x86 64-apple-macosx10 14 0 -Wdeprecated-objc-isa-usage   -Werror deprecated-objc-isa-usage -E -disable-free -disable-llvm-verifier -discard-value-names -main-file-name null -mrelocation-model pic -pic-level 2 -mthread-model posix -mdisable-fp-elim -fno-strict-return -masm-verbose -munwind-tables -target-cpu penryn -dwarf-column-info -debugger-tuning lldb -target-linker-version 409 12 -v -resource-dir  Library Developer CommandLineTools usr lib clang 10 0 0 -isysroot    Library Developer CommandLineTools SDKs MacOSX10 14 sdk   -I usr local include -fdebug-compilation-dir  Users hogstrom -ferror-limit 19 -fmessage-length 80 -stack-protector 1 -fblocks -fencode-extended-block-signature -fobjc-runtime macosx-10 14 0 -fmax-type-align 16 -fdiagnostics-show-option -fcolor-diagnostics -traditional-cpp -o - -x c  dev null    clang -cc1 version 10 0 0  clang-1000 10 44 2  default target x86 64-apple-darwin18 0 0 ignoring   nonexistent directory   Library Developer CommandLineTools SDKs MacOSX10 14 sdk usr local include    ignoring nonexistent directory   Library Developer CommandLineTools SDKs MacOSX10 14 sdk Library Frameworks     include       search starts here     include  lt      search starts here     usr local include      Library Developer CommandLineTools usr lib clang 10 0 0 include     Library Developer CommandLineTools usr include      Library Developer CommandLineTools SDKs MacOSX10 14 sdk usr include     Library Developer CommandLineTools SDKs MacOSX10 14 sdk System Library Frameworks  framework directory     End of search list

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There exists two ways to write  include statement These are    include filename   include lt filename gt    The meaning of each form is    include mylib h    This command would look for the file mylib h in the current directory as well as the specified list of directories as mentioned n the include search path that might have been set up    include lt mylib h gt    This command would look for the file mylib h in the specified list of directories only   The include search path is nothing but a list of directories that would be searched for the file being included Different C compilers let you set the search path in different manners

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The simple general rule is to use angled brackets to include header files that come with the compiler  Use double quotes to include any other header files  Most compilers do it this way   1 9     Header files explains in more detail about pre-processor directives  If you are a novice programmer  that page should help you understand all that  I learned it from here  and I have been following it at work

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At least for GCC version  lt   3 0  the angle-bracket form does not generate a dependency between the included file and the including one   So if you want to generate dependency rules  using the GCC -M option for exemple   you must use the quoted form for the files that should be included in the dependency tree    See http   gcc gnu org onlinedocs cpp Invocation html

[c++] What is the difference between #include <filename> and #include "filename"?

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