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FORK(3P) POSIX Programmer's Manual FORK(3P)
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.
fork — create a new process
#include <unistd.h>
pid_t fork(void);
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(3p).
* 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.
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.
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.
None.
None.
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].
None.
alarm(3p), exec(1p), fcntl(3p), posix_trace_attr_getinherited(3p),
posix_trace_eventid_equal(3p), pthread_atfork(3p), semop(3p),
signal(3p), times(3p)
The Base Definitions volume of POSIX.1‐2008, Section 4.11, Memory
Synchronization, sys_types.h(0p), unistd.h(0p)
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(3P)
Pages that refer to this page: unistd.h(0p), sh(1p), aio_error(3p), aio_read(3p), aio_return(3p), aio_write(3p), alarm(3p), exec(3p), getpgid(3p), getpgrp(3p), getpid(3p), getppid(3p), getrlimit(3p), getsid(3p), lio_listio(3p), mlock(3p), mlockall(3p), mmap(3p), pclose(3p), popen(3p), posix_spawn(3p), posix_trace_attr_getinherited(3p), pthread_atfork(3p), pthread_create(3p), semop(3p), setpgrp(3p), shmat(3p), shmdt(3p), system(3p), times(3p), wait(3p)
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