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

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 <stdlib.h>

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?

I met these warnings on mempcpy function. Man page says this function is a GNU extension and synopsis shows:

#define _GNU_SOURCE
#include <string.h>

When #define is added to my source before the #include, declarations for the GNU extensions are made visible and warnings disappear.

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.

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 <stdlib.h>