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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  parent  (aio_read(3),
          aio_write(3)), nor does it inherit any asynchronous I/O contexts 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 dif-
       fer 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 fail-
       ure, -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 encoun-
              tered.  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
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-04-27                           FORK(2)