fork(2)



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 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 (sigpend-
          ing(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 follow-
       ing 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().

       *  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-safety(7))  until  such
          time as it calls execve(2).

       *  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 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 correspond-
          ing  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));

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

              *  the  PID limit (pids.max) imposed by the cgroup "process num-
                 ber" (PIDs) controller was reached.

       EAGAIN The caller is operating under the SCHED_DEADLINE scheduling pol-
              icy 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  dupli-
       cate  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), pthread_atfork(3), capabilities(7), credentials(7)

COLOPHON
       This  page  is  part of release 4.13 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/.

Linux                             2017-09-15                           FORK(2)

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