fork - create a child process
fork creates a child process that differs from the parent
process only in its PID and PPID, and in the fact that
resource utilizations are set to 0. File locks and pend
ing signals are not inherited.
Under Linux, fork is implemented using copy-on-write
pages, so the only penalty incurred by fork is the time
and memory required to duplicate the parent's page tables,
and to create a unique task structure for the child.
On success, the PID of the child process is returned in
the parent's thread of execution, and a 0 is returned in
the child's thread of execution. On failure, a -1 will be
returned in the parent's context, no child process will be
created, and errno will be set appropriately.
EAGAIN fork cannot allocate sufficient memory to copy the
parent's page tables and allocate a task structure
for the child.
ENOMEM fork failed to allocate the necessary kernel struc
tures because memory is tight.
The fork call conforms to SVr4, SVID, POSIX, X/OPEN, BSD
clone(2), execve(2), vfork(2), wait(2)