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FORK(2)                    Linux Programmer's Manual                   FORK(2)



NAME
       fork - create a child process

SYNOPSIS
       #include <&lt;unistd.h>&gt;

       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  (sigpend-
          ing(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(3)).

       *  The  child  does not inherit outstanding asynchronous I/O operations
          from its parent (aio_read(3), aio_write(3)).

       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_DONT-
          FORK 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  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 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 correspond-
          ing 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  dupli-
       cate  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), capa-
       bilities(7), credentials(7)

COLOPHON
       This  page  is  part of release 3.05 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/.



Linux                             2008-04-22                           FORK(2)