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FORK(P)			   POSIX Programmer's Manual		       FORK(P)

PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the	 corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.

NAME
       fork - create a new process

SYNOPSIS
       #include <unistd.h>

       pid_t fork(void);

DESCRIPTION
       The fork() function shall create a new process. The new process	(child
       process) shall be an exact copy of the calling process (parent process)
       except as detailed below:

	* The child process shall have a unique process ID.

	* The child process ID also shall not match any active	process	 group
	  ID.

	* The  child  process  shall have a different parent process ID, which
	  shall be the process ID of the calling process.

	* The child process shall have its  own	 copy  of  the	parent's  file
	  descriptors.	 Each  of  the child's file descriptors shall refer to
	  the same open file description with the corresponding file  descrip‐
	  tor of the parent.

	* The  child  process  shall  have  its	 own copy of the parent's open
	  directory streams. Each open directory stream in the	child  process
	  may share directory stream positioning with the corresponding direc‐
	  tory stream of the parent.

	* The child process shall have its own copy of	the  parent's  message
	  catalog descriptors.

	* The  child  process' values of tms_utime, tms_stime, tms_cutime, and
	  tms_cstime shall be set to 0.

	* The time left until an alarm clock signal shall be  reset  to	 zero,
	  and the alarm, if any, shall be canceled; see alarm() .

	* All semadj values shall be cleared.

	* File	locks  set by the parent process shall not be inherited by the
	  child process.

	* The set of signals pending for the child process shall  be  initial‐
	  ized to the empty set.

	* Interval timers shall be reset in the child process.

	* Any  semaphores  that	 are  open in the parent process shall also be
	  open in the child process.

	* The child process shall not inherit any address space	 memory	 locks
	  established  by  the	parent	process	 via  calls  to	 mlockall() or
	  mlock().

	* Memory mappings created in the parent shall be retained in the child
	  process.  MAP_PRIVATE	 mappings inherited from the parent shall also
	  be MAP_PRIVATE mappings in the child, and any modifications  to  the
	  data	in  these  mappings made by the parent prior to calling fork()
	  shall be visible to the child. Any  modifications  to	 the  data  in
	  MAP_PRIVATE  mappings	 made by the parent after fork() returns shall
	  be visible only to the parent. Modifications to the data in MAP_PRI‐
	  VATE mappings made by the child shall be visible only to the child.

	* For  the  SCHED_FIFO	and  SCHED_RR  scheduling  policies, the child
	  process shall inherit the policy and priority settings of the parent
	  process during a fork() function. For other scheduling policies, the
	  policy and priority settings on fork() are implementation-defined.

	* Per-process timers created by the parent shall not be	 inherited  by
	  the child process.

	* The  child  process  shall  have  its	 own copy of the message queue
	  descriptors of the parent. Each of the message  descriptors  of  the
	  child	 shall refer to the same open message queue description as the
	  corresponding message descriptor of the parent.

	* No asynchronous input or asynchronous	 output	 operations  shall  be
	  inherited by the child process.

	* A process shall be created with a single thread. If a multi-threaded
	  process calls fork(), the new process shall contain a replica of the
	  calling  thread and its entire address space, possibly including the
	  states of mutexes  and  other	 resources.   Consequently,  to	 avoid
	  errors,  the child process may only execute async-signal-safe opera‐
	  tions until such time as  one	 of  the  exec	functions  is  called.
	   Fork	 handlers  may be established by means of the pthread_atfork()
	  function in order to maintain application invariants	across	fork()
	  calls.

       When  the application calls fork() from a signal handler and any of the
       fork handlers registered by pthread_atfork() calls a function  that  is
       not asynch-signal-safe, the behavior is undefined.

	* If the Trace option and the Trace Inherit option are both supported:

       If  the calling process was being traced in a trace stream that had its
       inheritance policy set  to  POSIX_TRACE_INHERITED,  the	child  process
       shall  be  traced  into	that trace stream, and the child process shall
       inherit the parent's mapping of trace event names to trace  event  type
       identifiers. If the trace stream in which the calling process was being
       traced had its inheritance policy set  to  POSIX_TRACE_CLOSE_FOR_CHILD,
       the  child  process  shall  not	be  traced into that trace stream. The
       inheritance policy is set by a call to  the  posix_trace_attr_setinher‐
       ited() function.

	* If  the  Trace  option is supported, but the Trace Inherit option is
	  not supported:

       The child process shall not be traced into any of the trace streams  of
       its parent process.

	* If  the Trace option is supported, the child process of a trace con‐
	  troller process shall not control the trace  streams	controlled  by
	  its parent process.

	* The  initial	value of the CPU-time clock of the child process shall
	  be set to zero.

	* The initial value of the CPU-time clock of the single thread of  the
	  child process shall be set to zero.

       All other process characteristics defined by IEEE Std 1003.1-2001 shall
       be the same in the parent and  child  processes.	  The  inheritance  of
       process characteristics not defined by IEEE Std 1003.1-2001 is unspeci‐
       fied by IEEE Std 1003.1-2001.

       After fork(), both the parent and the child processes shall be  capable
       of executing independently before either one terminates.

RETURN VALUE
       Upon  successful completion, fork() shall return 0 to the child process
       and shall return the process ID of the  child  process  to  the	parent
       process. Both processes shall continue to execute from the fork() func‐
       tion. Otherwise, -1 shall be returned to the parent process,  no	 child
       process shall be created, and errno shall be set to indicate the error.

ERRORS
       The fork() function shall fail if:

       EAGAIN The  system  lacked  the	necessary  resources to create another
	      process, or the system-imposed limit on the total number of pro‐
	      cesses   under   execution  system-wide  or  by  a  single  user
	      {CHILD_MAX} would be exceeded.

       The fork() function may fail if:

       ENOMEM Insufficient storage space is available.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       None.

RATIONALE
       Many historical implementations have timing windows where a signal sent
       to  a  process group (for example, an interactive SIGINT) just prior to
       or during execution of fork() is delivered to the parent following  the
       fork()  but not to the child because the fork() code clears the child's
       set of pending signals.	This volume of IEEE Std 1003.1-2001  does  not
       require,	 or  even  permit,  this behavior. However, it is pragmatic to
       expect that problems of this nature may continue to exist in  implemen‐
       tations	that  appear to conform to this volume of IEEE Std 1003.1-2001
       and pass available verification suites.	This behavior is only a conse‐
       quence  of the implementation failing to make the interval between sig‐
       nal generation and delivery totally invisible. From  the	 application's
       perspective,  a fork() call should appear atomic. A signal that is gen‐
       erated prior to the fork() should be delivered prior to the fork().   A
       signal  sent  to the process group after the fork() should be delivered
       to both parent and child. The implementation  may  actually  initialize
       internal	 data  structures  corresponding to the child's set of pending
       signals to include signals sent to the process group during the fork().
       Since  the  fork()  call	 can be considered as atomic from the applica‐
       tion's perspective, the set would be initialized as empty and such sig‐
       nals would have arrived after the fork(); see also <signal.h>.

       One  approach  that has been suggested to address the problem of signal
       inheritance across fork() is to add an [EINTR] error,  which  would  be
       returned	 when  a  signal  is  detected	during the call. While this is
       preferable to losing signals, it was not considered  an	optimal	 solu‐
       tion.  Although	it  is not recommended for this purpose, such an error
       would be an allowable extension for an implementation.

       The [ENOMEM] error value is reserved  for  those	 implementations  that
       detect  and distinguish such a condition. This condition occurs when an
       implementation detects that there is not enough memory  to  create  the
       process. This is intended to be returned when [EAGAIN] is inappropriate
       because there can never be enough memory (either primary	 or  secondary
       storage) to perform the operation.  Since fork() duplicates an existing
       process, this must be a condition where there is sufficient memory  for
       one  such  process,  but	 not  for two. Many historical implementations
       actually return [ENOMEM] due to temporary lack of memory, a  case  that
       is  not generally distinct from [EAGAIN] from the perspective of a con‐
       forming application.

       Part of the reason for including the optional error [ENOMEM] is because
       the SVID specifies it and it should be reserved for the error condition
       specified there. The condition is not applicable	 on  many  implementa‐
       tions.

       IEEE Std 1003.1-1988  neglected	to require concurrent execution of the
       parent and child of fork(). A system that single-threads processes  was
       clearly not intended and is considered an unacceptable "toy implementa‐
       tion" of this volume of IEEE Std 1003.1-2001. The only objection antic‐
       ipated to the phrase "executing independently" is testability, but this
       assertion should be testable. Such tests require that both  the	parent
       and  child  can	block  on  a detectable action of the other, such as a
       write to a pipe or a signal. An interactive exchange  of	 such  actions
       should be possible for the system to conform to the intent of this vol‐
       ume of IEEE Std 1003.1-2001.

       The [EAGAIN] error exists to warn applications that  such  a  condition
       might  occur.  Whether  it  occurs or not is not in any practical sense
       under the control of the application because the condition is usually a
       consequence  of	the user's use of the system, not of the application's
       code. Thus, no application can or should rely upon its occurrence under
       any  circumstances,  nor	 should the exact semantics of what concept of
       "user" is used be of concern  to	 the  application  writer.  Validation
       writers should be cognizant of this limitation.

       There  are two reasons why POSIX programmers call fork(). One reason is
       to create a new thread of control within the same  program  (which  was
       originally only possible in POSIX by creating a new process); the other
       is to create a new process running a different program. In  the	latter
       case,  the call to fork() is soon followed by a call to one of the exec
       functions.

       The general problem with making fork() work in a	 multi-threaded	 world
       is  what to do with all of the threads. There are two alternatives. One
       is to copy all of the threads into the new process.   This  causes  the
       programmer or implementation to deal with threads that are suspended on
       system calls or that might be about to execute system calls that should
       not  be	executed  in the new process. The other alternative is to copy
       only the thread that calls fork(). This creates the difficulty that the
       state  of process-local resources is usually held in process memory. If
       a thread that is not calling fork() holds a resource, that resource  is
       never  released in the child process because the thread whose job it is
       to release the resource does not exist in the child process.

       When a programmer  is  writing  a  multi-threaded  program,  the	 first
       described  use  of fork(), creating new threads in the same program, is
       provided by the pthread_create() function.  The fork() function is thus
       used  only  to  run  new programs, and the effects of calling functions
       that require certain resources between the call to fork() and the  call
       to an exec function are undefined.

       The  addition  of the forkall() function to the standard was considered
       and rejected. The forkall() function lets all the threads in the parent
       be  duplicated  in  the child. This essentially duplicates the state of
       the parent in the child. This allows threads in the child  to  continue
       processing  and	allows	locks  and  the	 state to be preserved without
       explicit pthread_atfork() code. The calling process has to ensure  that
       the  threads  processing	 state	that  is shared between the parent and
       child (that is, file descriptors or MAP_SHARED memory) behaves properly
       after  forkall(). For example, if a thread is reading a file descriptor
       in the parent when forkall() is called, then two threads	 (one  in  the
       parent  and one in the child) are reading the file descriptor after the
       forkall(). If this is not desired behavior, the parent process  has  to
       synchronize with such threads before calling forkall().

       While  the fork() function is async-signal-safe, there is no way for an
       implementation to determine whether the fork  handlers  established  by
       pthread_atfork()	 are async-signal-safe.	 The fork handlers may attempt
       to execute portions of the implementation that  are  not	 async-signal-
       safe,  such  as those that are protected by mutexes, leading to a dead‐
       lock condition. It is therefore undefined for the fork handlers to exe‐
       cute  functions	that  are  not async-signal-safe when fork() is called
       from a signal handler.

       When forkall() is called, threads, other than the calling thread,  that
       are  in	functions that can return with an [EINTR] error may have those
       functions return [EINTR] if the implementation cannot ensure  that  the
       function	 behaves  correctly  in	 the  parent and child. In particular,
       pthread_cond_wait() and	pthread_cond_timedwait()  need	to  return  in
       order to ensure that the condition has not changed. These functions can
       be awakened by  a  spurious  condition  wakeup  rather  than  returning
       [EINTR].

FUTURE DIRECTIONS
       None.

SEE ALSO
       alarm()	 ,   exec()  ,	fcntl()	 ,  posix_trace_attr_getinherited()  ,
       posix_trace_trid_eventid_open() , pthread_atfork() , semop() , signal()
       ,  times()  ,  the  Base	 Definitions  volume  of IEEE Std 1003.1-2001,
       <sys/types.h>, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),	The  Open  Group  Base
       Specifications  Issue  6,  Copyright  (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open  Group.  In  the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard
       is  the	referee document. The original Standard can be obtained online
       at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group		     2003			       FORK(P)
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