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Linux 2.6 - man page for fork (linux section 3posix)

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

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 descriptor 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 directory 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 initialized 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_PRI-
	  VATE 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_PRI-
	  VATE mappings made by the parent after fork() returns shall be visible only to the par-
	  ent. Modifications to the data in MAP_PRIVATE mappings made by the child shall be visi-
	  ble 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 implementa-
	  tion-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  par-
	  ent.	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.  Conse-
	  quently, 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 reg-
       istered 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_setinherited() 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 controller 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 unspecified by IEEE Std 1003.1-2001.

       After  fork(), both the parent and the child processes shall be capable of executing inde-
       pendently 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() function.  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 processes 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 deliv-
       ered  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 implementations that appear  to	conform  to  this
       volume  of  IEEE Std 1003.1-2001 and pass available verification suites.  This behavior is
       only a consequence of the implementation failing to make the interval between signal  gen-
       eration	and delivery totally invisible. From the application's perspective, a fork() call
       should appear atomic. A signal that is generated prior to the fork() should  be	delivered
       prior to the fork().  A signal sent to the process group after the fork() should be deliv-
       ered 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 application's perspective, the set would be initialized as empty and such signals
       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 dur-
       ing the call. While this is preferable to losing signals, it was not considered an optimal
       solution.  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 stor-
       age) 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 conforming
       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 implementations.

       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 implementation" of this  volume  of  IEEE Std 1003.1-2001.	The  only
       objection  anticipated  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 volume 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 con-
       cern 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 func-
       tions.

       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  pro-
       cessing	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 par-
       ent 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 call-
       ing 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 deadlock con-
       dition. It is therefore undefined for the fork handlers to execute 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_even-
       tid_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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					  FORK(P)


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