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

       fork - create a child process

       #include <unistd.h>

       pid_t fork(void);

       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 other.

       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 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),

       *  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

       *  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)).

       *  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 fcntl(2)).

       *  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 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

       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.

       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 to indicate the error.

       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 sched(7).

       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 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.)

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

       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

   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

       See pipe(2) and wait(2).

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

       This page is part of release 5.13 of the Linux man-pages project.  A
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       latest version of this page, can be found at

Linux                              2021-03-22                            FORK(2)