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.

Debian                                          December 1, 2017                                         FORK(2)