fork


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  (sigpend-
          ing(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_DONT-
          FORK flag are not inherited across a fork().
          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  descrip-
          tors.   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 correspond-
          ing 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).

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 dupli-
       cate 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

COLOPHON
       This page is part of release 3.54 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)
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