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FORK(2)                     BSD System Calls Manual                    FORK(2)

NAME
     fork -- create a new process

SYNOPSIS
     #include <&lt;unistd.h>&gt;

     pid_t
     fork(void);

DESCRIPTION
     fork() causes creation of a new process.  The new process (child process)
     is an exact copy of the calling process (parent process) except for the
     following:

           o   The child process has a unique process ID, which also does not
               match any existing process group ID.

           o   The child process has a different parent process ID (i.e., the
               process ID of the parent process).

           o   The child process has a single thread.

           o   The child process has its own copy of the parent's descriptors.
               These descriptors reference the same underlying objects, so
               that, for instance, file pointers in file objects are shared
               between the child and the parent, so that an lseek(2) on a
               descriptor in the child process can affect a subsequent read(2)
               or write(2) by the parent.  This descriptor copying is also
               used by the shell to establish standard input and output for
               newly created processes as well as to set up pipes.

           o   The child process has no fcntl(2)-style file locks.

           o   The child process' resource utilizations are set to 0; see
               getrusage(2).

           o   All interval timers are cleared; see setitimer(2).

           o   The child process' semaphore undo values are set to 0; see
               semop(2).

           o   The child process' pending signals set is empty.

           o   The child process has no memory locks; see mlock(2) and
               mlockall(2).

     In general, the child process should call _exit(2) rather than exit(3).
     Otherwise, any stdio buffers that exist both in the parent and child will
     be flushed twice.  Similarly, _exit(2) should be used to prevent
     atexit(3) routines from being called twice (once in the parent and once
     in the child).

RETURN VALUES
     Upon successful completion, fork() returns a value of 0 to the child
     process and returns the process ID of the child process to the parent
     process.  Otherwise, a value of -1 is returned to the parent process, no
     child process is created, and the global variable errno is set to indi-
     cate the error.

ERRORS
     fork() will fail and no child process will be created if:

     [EAGAIN]  The system-imposed limits on the total number of processes or
               total number of threads under execution would be exceeded.
               These limits are configuration dependent.

     [EAGAIN]  The limit RLIMIT_NPROC on the total number of processes under
               execution by the user ID would be exceeded.

     [ENOMEM]  There is insufficient swap space for the new process.

SEE ALSO
     execve(2), getrusage(2), wait(2)

STANDARDS
     The fork() function conforms to IEEE Std 1003.1-2008 (``POSIX.1'').

HISTORY
     The fork() system call first appeared in Version 1 AT&T UNIX.

BSD                             March 31, 2013                             BSD