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     NAME
          fork - create a child process

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

          pid_t fork(void);

     DESCRIPTION
          fork() creates a new process by duplicating the calling pro-
          cess.  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 other.

          The child process is an exact duplicate of the parent pro-
          cess 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 session.

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

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

          *  The child does not inherit semaphore adjustments from its
             parent (semop(2)).

          *  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 parent.)

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

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          *  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 speci-
          fied 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 fcntl(2)).

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

          *  The default timer slack value is set to the parent's cur-
             rent timer slack value.  See the description of
             PR_SET_TIMERSLACK in prctl(2).

          *  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 clone(2)).

          *  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

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             the corresponding file descriptor in the parent.  This
             means that the two file descriptors share open file sta-
             tus 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 mes-
             sage queue description as the corresponding file descrip-
             tor in the parent.  This means that the two file descrip-
             tors 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
          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
          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 pro-
                  cesses and threads, /proc/sys/kernel/threads-max,
                  was reached (see proc(5));

               *  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 "pro-
                  cess 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 sched(7).

          ENOMEM

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               fork() failed to allocate the necessary kernel struc-
               tures because memory is tight.

          ENOMEM
               An attempt was made to create a child process in a PID
               namespace whose "init" process has terminated.  See
               pid_namespaces(7).

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

     NOTES
          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 pro-
          vided as part of the NPTL threading implementation invokes
          clone(2) with flags that provide the same effect as the tra-
          ditional 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
          See pipe(2) and wait(2).

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

     COLOPHON
          This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.

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