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.  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 6 AT&T UNIX.

Debian                                            May 31, 2013                                           FORK(2)