warning incompatible implicit declaration of built-in function xyz

Question

I m getting a number of these warnings when compiling a few binaries    warning  incompatible implicit declaration of built-in function    strcpy    warning  incompatible implicit declaration of built-in function    strlen    warning  incompatible implicit declaration of built-in function    exit      To try to resolve this  I have added   include  lt stdlib h gt    at the top of the C files associated with this warning  in addition to compiling with the following flags   CFLAGS   -fno-builtin-exit -fno-builtin-strcat -fno-builtin-strncat -fno-builtin-strcpy -fno-builtin-strlen -fno-builtin-calloc   I am using GCC 4 1 2     gcc --version gcc  GCC  4 1 2 20080704   What should I do to resolve these warnings

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In the case of some programs  these errors are normal and should not be fixed   I get these error messages when compiling the program phrap  for example    This program happens to contain code that modifies or replaces some built in functions  and when I include the appropriate header files to fix the warnings  GCC instead generates a bunch of errors   So fixing the warnings effectively breaks the build   If you got the source as part of a distribution that should compile normally  the errors might be normal   Consult the documentation to be sure

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I met these warnings on mempcpy function  Man page says this function is a GNU extension and synopsis shows    define  GNU SOURCE  include  lt string h gt    When  define is added to my source before the  include  declarations for the GNU extensions are made visible and warnings disappear

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Here is some C code that produces the above mentioned error   int main int argc  char   argv      exit 1       Compiled like this on Fedora 17 Linux 64 bit with gcc   el defiant   foo2   gcc -o n n2 c                                                                n2 c  In function    main     n2 c 2 3  warning  incompatible implicit declaration of built-in  function    exit     enabled by default  el defiant   foo2     n  el defiant   foo2      To make the warning go away  add this declaration to the top of the file    include  lt stdlib h gt

User · Answer

In C  using a previously undeclared function constitutes an implicit declaration of the function  In an implicit declaration  the return type is int if I recall correctly  Now  GCC has built-in definitions for some standard functions   If an implicit declaration does not match the built-in definition  you get this warning   To fix the problem  you have to declare the functions before using them  normally you do this by including the appropriate header  I recommend not to use the -fno-builtin-  flags if possible   Instead of stdlib h  you should try    include  lt string h gt    That s where strcpy and strncpy are defined  at least according to the strcpy 2  man page   The exit function is defined in stdlib h  though  so I don t know what s going on there

[c] warning: incompatible implicit declaration of built-in function ‘xyz’

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