Why does my compiler not accept fork(), despite my inclusion of <unistd.h>?

unistd.h and fork are part of the POSIX standard. They aren't available on windows (text.exe in your gcc command hints that's you're not on *nix).

It looks like you're using gcc as part of MinGW, which does provide the unistd.h header but does not implement functions like fork. Cygwin does provide implementations of functions like fork.

However, since this is homework you should already have instructions on how to obtain a working environment.

You have got #include <unistd.h> which is where fork() is declared.

So, you probably need to tell the system to show the POSIX definitions before you include the system headers:

#define _XOPEN_SOURCE 600

You can use 700 if you think your system is mostly POSIX 2008 compliant, or even 500 for an older system. Because fork() has been around forever, it will show up with any of those.

If you are compiling with -std=c99 --pedantic, then all the declarations for POSIX will be hidden unless you explicitly request them as shown.

You can also play with _POSIX_C_SOURCE, but using _XOPEN_SOURCE implies the correct corresponding _POSIX_C_SOURCE (and _POSIX_SOURCE, and so on).

As you've already noted, fork() should be defined in unistd.h - at least according to the man pages that come with Ubuntu 11.10. The minimal:

#include <unistd.h>int main( int argc, char* argv[]){    pid_t procID;    procID = fork();    return procID;}

...builds with no warnings on 11.10.

Speaking of which, what UNIX/Linux distribution are you using? For instance, I've found several non-remarkable functions that should be defined in Ubuntu 11.10's headers aren't. Such as:

// string.hchar* strtok_r( char* str, const char* delim, char** saveptr);char* strdup( const char* const qString);// stdio.hint fileno( FILE* stream);// time.hint nanosleep( const struct timespec* req, struct timespec* rem);// unistd.hint getopt( int argc, char* const argv[], const char* optstring);extern int opterr;int usleep( unsigned int usec);

As long as they're defined in your C library it won't be a huge problem. Just define your own prototypes in a compatibility header and report the standard header problems to whoever maintains your OS distribution.