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 (see io_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 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.

       *  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
       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.

       ENOSYS fork() is not supported on this platform (for example, hardware without a Memory-Management Unit).

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.  (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 3.54 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                                              2013-03-12                                            FORK(2)