fork(2) — Linux manual page


FORK(2)                   Linux Programmer's Manual                  FORK(2)

NAME         top

       fork - create a child process

SYNOPSIS         top

       #include <sys/types.h>
       #include <unistd.h>

       pid_t fork(void);

DESCRIPTION         top

       fork() creates a new process by duplicating the calling process.  The
       new process is referred to as the child process.  The calling process
       is referred to as the parent process.

       The child process and the parent process run in separate memory
       spaces.  At the time of fork() both memory spaces have the same
       content.  Memory writes, file mappings (mmap(2)), and unmappings
       (munmap(2)) performed by one of the processes do not affect the

       The child process is an exact duplicate of the parent process except
       for the following points:

       *  The child has its own unique process ID, and this PID does not
          match the ID of any existing process group (setpgid(2)) or

       *  The child's parent process ID is the same as the parent's process

       *  The child does not inherit its parent's memory locks (mlock(2),

       *  Process resource utilizations (getrusage(2)) and CPU time counters
          (times(2)) are reset to zero in the child.

       *  The child's set of pending signals is initially empty

       *  The child does not inherit semaphore adjustments from its parent

       *  The child does not inherit process-associated record locks from
          its parent (fcntl(2)).  (On the other hand, it does inherit
          fcntl(2) open file description locks and flock(2) locks from its

       *  The child does not inherit timers from its parent (setitimer(2),
          alarm(2), timer_create(2)).

       *  The child does not inherit outstanding asynchronous I/O operations
          from its parent (aio_read(3), aio_write(3)), nor does it inherit
          any asynchronous I/O contexts from its parent (see io_setup(2)).

       The process attributes in the preceding list are all specified in
       POSIX.1.  The parent and child also differ with respect to the
       following Linux-specific process attributes:

       *  The child does not inherit directory change notifications
          (dnotify) from its parent (see the description of F_NOTIFY in

       *  The prctl(2) PR_SET_PDEATHSIG setting is reset so that the child
          does not receive a signal when its parent terminates.

       *  The default timer slack value is set to the parent's current timer
          slack value.  See the description of PR_SET_TIMERSLACK in

       *  Memory mappings that have been marked with the madvise(2)
          MADV_DONTFORK flag are not inherited across a fork().

       *  Memory in address ranges that have been marked with the madvise(2)
          MADV_WIPEONFORK flag is zeroed in the child after a fork().  (The
          MADV_WIPEONFORK setting remains in place for those address ranges
          in the child.)

       *  The termination signal of the child is always SIGCHLD (see

       *  The port access permission bits set by ioperm(2) are not inherited
          by the child; the child must turn on any bits that it requires
          using ioperm(2).

       Note the following further points:

       *  The child process is created with a single thread—the one that
          called fork().  The entire virtual address space of the parent is
          replicated in the child, including the states of mutexes,
          condition variables, and other pthreads objects; the use of
          pthread_atfork(3) may be helpful for dealing with problems that
          this can cause.

       *  After a fork() in a multithreaded program, the child can safely
          call only async-signal-safe functions (see signal-safety(7)) until
          such time as it calls execve(2).

       *  The child inherits copies of the parent's set of open file
          descriptors.  Each file descriptor in the child refers to the same
          open file description (see open(2)) as the corresponding file
          descriptor in the parent.  This means that the two file
          descriptors share open file status flags, file offset, and signal-
          driven I/O attributes (see the description of F_SETOWN and
          F_SETSIG in fcntl(2)).

       *  The child inherits copies of the parent's set of open message
          queue descriptors (see mq_overview(7)).  Each file descriptor in
          the child refers to the same open message queue description as the
          corresponding file descriptor in the parent.  This means that the
          two file descriptors share the same flags (mq_flags).

       *  The child inherits copies of the parent's set of open directory
          streams (see opendir(3)).  POSIX.1 says that the corresponding
          directory streams in the parent and child may share the directory
          stream positioning; on Linux/glibc they do not.

RETURN VALUE         top

       On success, the PID of the child process is returned in the parent,
       and 0 is returned in the child.  On failure, -1 is returned in the
       parent, no child process is created, and errno is set appropriately.

ERRORS         top

       EAGAIN A system-imposed limit on the number of threads was
              encountered.  There are a number of limits that may trigger
              this error:

              *  the RLIMIT_NPROC soft resource limit (set via
                 setrlimit(2)), which limits the number of processes and
                 threads for a real user ID, was reached;

              *  the kernel's system-wide limit on the number of processes
                 and threads, /proc/sys/kernel/threads-max, was reached (see

              *  the maximum number of PIDs, /proc/sys/kernel/pid_max, was
                 reached (see proc(5)); or

              *  the PID limit (pids.max) imposed by the cgroup "process
                 number" (PIDs) controller was reached.

       EAGAIN The caller is operating under the SCHED_DEADLINE scheduling
              policy and does not have the reset-on-fork flag set.  See

       ENOMEM fork() failed to allocate the necessary kernel structures
              because memory is tight.

       ENOMEM An attempt was made to create a child process in a PID
              namespace whose "init" process has terminated.  See

       ENOSYS fork() is not supported on this platform (for example,
              hardware without a Memory-Management Unit).

       ERESTARTNOINTR (since Linux 2.6.17)
              System call was interrupted by a signal and will be restarted.
              (This can be seen only during a trace.)

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

NOTES         top

       Under Linux, fork() is implemented using copy-on-write pages, so the
       only penalty that it incurs is the time and memory required to
       duplicate the parent's page tables, and to create a unique task
       structure for the child.

   C library/kernel differences
       Since version 2.3.3, rather than invoking the kernel's fork() system
       call, the glibc fork() wrapper that is provided as part of the NPTL
       threading implementation invokes clone(2) with flags that provide the
       same effect as the traditional system call.  (A call to fork() is
       equivalent to a call to clone(2) specifying flags as just SIGCHLD.)
       The glibc wrapper invokes any fork handlers that have been
       established using pthread_atfork(3).

EXAMPLES         top

       See pipe(2) and wait(2).

SEE ALSO         top

       clone(2), execve(2), exit(2), setrlimit(2), unshare(2), vfork(2),
       wait(2), daemon(3), pthread_atfork(3), capabilities(7),

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-06-09                          FORK(2)

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