How do I get the directory that a program is running from?

Is there a platform-agnostic and filesystem-agnostic method to obtain the full path of the directory from where a program is running using C/C++? Not to be confused with the current working directory. (Please don't suggest libraries unless they're standard ones like clib or STL.)

(If there's no platform/filesystem-agnostic method, suggestions that work in Windows and Linux for specific filesystems are welcome too.)

Here's code to get the full path to the executing app:

Variable declarations:

char pBuf[256];
size_t len = sizeof(pBuf);

Windows:

int bytes = GetModuleFileName(NULL, pBuf, len);
return bytes ? bytes : -1;

Linux:

int bytes = MIN(readlink("/proc/self/exe", pBuf, len), len - 1);
if(bytes >= 0)
    pBuf[bytes] = '\0';
return bytes;

Stack Overflow Q#143174, score: 202