GCC by itself attempts to be a conforming freestanding implementation. See Language Standards Supported by GCC , for details of what this means. Beyond the library facilities required of such an implementation, the rest of the C library is supplied by the vendor of the operating system. If that C library doesn't conform to the C standards, then your programs might get warnings (especially when using -Wall ) that you don't expect.
For example, the
function on SunOS 4.1.3 returns
while the C standard says that
program could make the prototype for this function match the Standard, but that would be wrong, since the function will still return
If you need a Standard compliant library, then you need to find one, as GCC does not provide one. The GNU C library (called
) provides ISO C, POSIX, BSD, SystemV and X/Open compatibility for GNU/Linux and HURD-based GNU systems; no recent version of it supports other systems, though some very old versions did. Version 2.2 of the GNU C library includes nearly complete C99 support. You could also ask your operating system vendor if newer libraries are available.