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 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),
      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 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().

   *  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.

   *  After a fork() in a multithreaded program, the child can safely call
      only async-signal-safe functions (see signal(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.

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 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   proc(5));   or    the    maximum    number    of    PIDs,
          /proc/sys/kernel/pid_max, was reached (see proc(5)).

   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.)

CONFORMING TO

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

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.

   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 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 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at
   https://www.kernel.org/doc/man-pages/.

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