NAME

     fork — create a new process

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <unistd.h>

     pid_t
     fork(void);

DESCRIPTION

     The fork() system call 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:

           •   The child process has a unique process ID.

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

           •   The child process has its own copy of the parent's descriptors, except for
               descriptors returned by kqueue(2), which are not inherited from the parent
               process.  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.

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

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

           •   The child process has only one thread, corresponding to the calling thread in the
               parent process.  If the process has more than one thread, locks and other
               resources held by the other threads are not released and therefore only async-
               signal-safe functions (see sigaction(2)) are guaranteed to work in the child
               process until a call to execve(2) or a similar function.

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
     indicate the error.

ERRORS

     The fork() system call will fail and no child process will be created if:

     [EAGAIN]           The system-imposed limit on the total number of processes under execution
                        would be exceeded.  The limit is given by the sysctl(3) MIB variable
                        KERN_MAXPROC.  (The limit is actually ten less than this except for the
                        super user).

     [EAGAIN]           The user is not the super user, and the system-imposed limit on the total
                        number of processes under execution by a single user would be exceeded.
                        The limit is given by the sysctl(3) MIB variable KERN_MAXPROCPERUID.

     [EAGAIN]           The user is not the super user, and the soft resource limit corresponding
                        to the resource argument RLIMIT_NPROC would be exceeded (see
                        getrlimit(2)).

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

SEE ALSO

     execve(2), rfork(2), setitimer(2), setrlimit(2), sigaction(2), vfork(2), wait(2)

HISTORY

     The fork() function appeared in Version 1 AT&T UNIX.