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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).

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 3.65 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/.



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