fork

FORK(3POSIX)               POSIX Programmer's Manual              FORK(3POSIX)



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
           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_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_PRIVATE 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. Any use of asynchronous control
           blocks created by the parent produces undefined behavior.

        *  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 operations 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 async-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 POSIX.1‐2008 shall be the
       same in the parent and child processes. The inheritance of process
       characteristics not defined by POSIX.1‐2008 is unspecified by
       POSIX.1‐2008.

       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()
       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 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 POSIX.1‐2008 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 POSIX.1‐2008 and pass available
       verification suites. This behavior is only a consequence of the
       implementation failing to make the interval between signal generation
       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 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 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 during 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
       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
       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 POSIX.1‐2008.  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 POSIX.1‐2008.

       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 developer.
       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
       deadlock condition.  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_eventid_equal(), pthread_atfork(), semop(), signal(),
       times()

       The Base Definitions volume of POSIX.1‐2008, Section 4.11, Memory
       Synchronization, <sys_types.h>, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2013                         FORK(3POSIX)