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

NAME         top

       fork - create a child process

SYNOPSIS         top

       #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)).

       *  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.1-2001.  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().

       *  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.

       *  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 descriptors
          share open file status flags, current 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 descriptor in the
          child refers to the same open message queue description as the
          corresponding descriptor in the parent.  This means that the two
          descriptors share the same flags (mq_flags).

       *  The child inherits copies of the parent's set of open directory
          streams (see opendir(3)).  POSIX.1-2001 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


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

       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.

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

CONFORMING TO         top

       SVr4, 4.3BSD, POSIX.1-2001.

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 ABI 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).

EXAMPLE         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), capabilities(7), credentials(7)

COLOPHON         top

       This page is part of release 3.81 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                            2015-02-21                          FORK(2)