fork

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



NAME
       fork - create a child process

SYNOPSIS
       #include <unistd.h>

       pid_t fork(void);

DESCRIPTION
       fork() creates a new process by duplicating the calling process.  The
       new process, referred to as the child, is an exact duplicate of the
       calling process, referred to as the parent, 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
          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 record locks from its parent (fcntl(2)).

       *  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 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().

       *  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.

       *  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
       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 fork() cannot allocate sufficient memory to copy the parent's
              page tables and allocate a task structure for the child.

       EAGAIN It was not possible to create a new process because the caller's
              RLIMIT_NPROC resource limit was encountered.  To exceed this
              limit, the process must have either the CAP_SYS_ADMIN or the
              CAP_SYS_RESOURCE capability.

       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).

       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
       SVr4, 4.3BSD, POSIX.1-2001.

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.

       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
       See pipe(2) and wait(2).

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

COLOPHON
       This page is part of release 3.53 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2013-03-12                           FORK(2)