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



NAME
       fork - create a child process

SYNOPSIS
       #include 

       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(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 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),  dae-
       mon(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 information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux				  2009-04-27			       FORK(2)