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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,
       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 par-
          ent  (aio_read(3),  aio_write(3)), nor does it inherit any asynchronous I/O con-
          texts from its parent (seeio_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 pro-
       cess 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  sig-
          nal-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 mes-
          sage 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 pro-
              cess  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), capa-
       bilities(7), credentials(7)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2009-04-27                           FORK(2)

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