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

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 <string.h>

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.

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.