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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.

       *  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)).

       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.

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.  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),  setrlimit(2),  unshare(2),  vfork(2),   wait(2),
       daemon(3), capabilities(7), credentials(7)

COLOPHON

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       be found at http://man7.org/linux/man-pages/.