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/.