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

       fork - create a child process

       #include <unistd.h>

       pid_t fork(void);

       fork()  creates	a new process by duplicating the calling process.  The
       new process is referred to as the child process.	 The  calling  process
       is referred to as the parent process.

       The child process and the parent process run in separate memory spaces.
       At the time of fork() both memory spaces have the same content.	Memory
       writes,	file  mappings (mmap(2)), and unmappings (munmap(2)) performed
       by one of the processes do not affect the other.

       The child process is an exact duplicate of the  parent  process	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)) or session.

       *  The child's parent process ID is the same as	the  parent's  process

       *  The  child  does  not	 inherit  its parent's memory locks (mlock(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-

       *  The child does not inherit semaphore	adjustments  from  its	parent

       *  The  child does not inherit process-associated record locks from its
	  parent (fcntl(2)).  (On the other hand,  it  does  inherit  fcntl(2)
	  open file description locks and flock(2) locks from its parent.)

       *  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.	  The parent and child also differ with respect to the follow-
       ing 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_DONT-
	  FORK flag are not inherited across a fork().

       *  The  termination  signal  of	the  child  is	always	SIGCHLD	  (see

       *  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

       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.

       *  After a fork() in a multithreaded program, the child can safely call
	  only async-signal-safe functions (see signal-safety(7))  until  such
	  time as it calls execve(2).

       *  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 file descriptors share  open
	  file	status	flags,	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 file descriptor in the child
	  refers to the same open message queue description as the correspond-
	  ing  file  descriptor	 in  the parent.  This means that the two file
	  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	 says  that  the corresponding
	  directory streams in the parent and child may	 share	the  directory
	  stream positioning; on Linux/glibc they do not.

       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.

       EAGAIN A system-imposed limit on the number of threads was encountered.
	      There are a number of limits that may trigger this error:

	      *	 the RLIMIT_NPROC soft resource limit (set via	setrlimit(2)),
		 which	limits	the number of processes and threads for a real
		 user ID, was reached;

	      *	 the kernel's system-wide limit on the number of processes and
		 threads,   /proc/sys/kernel/threads-max,   was	 reached  (see

	      *	 the maximum number  of	 PIDs,	/proc/sys/kernel/pid_max,  was
		 reached (see proc(5)); or

	      *	 the  PID limit (pids.max) imposed by the cgroup "process num-
		 ber" (PIDs) controller was reached.

       EAGAIN The caller is operating under the SCHED_DEADLINE scheduling pol-
	      icy and does not have the reset-on-fork flag set.	 See sched(7).

       ENOMEM fork()  failed  to  allocate  the	 necessary  kernel  structures
	      because memory is tight.

       ENOMEM An attempt was made to create a child process in a PID namespace
	      whose "init" process has terminated.  See pid_namespaces(7).

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

       ERESTARTNOINTR (since Linux 2.6.17)
	      System call was interrupted by a signal and will	be  restarted.
	      (This can be seen only during a trace.)

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

       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.

   C library/kernel differences
       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).

       See pipe(2) and wait(2).

       clone(2),  execve(2),  exit(2),	setrlimit(2),  unshare(2),   vfork(2),
       wait(2), daemon(3), capabilities(7), credentials(7)

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Linux				  2017-03-13			       FORK(2)