FORK(3P) POSIX Programmer's Manual FORK(3P)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.
NAMEfork — 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
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 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_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 par‐
ent process during a fork() function. For other scheduling poli‐
cies, 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 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, possi‐
bly including the states of mutexes and other resources. Conse‐
quently, 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 invari‐
ants 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 sup‐
ported:
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 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 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() 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 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 imple‐
mentation 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 perspec‐
tive, 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 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 implemen‐
tation'' of this volume of POSIX.1‐2008. The only objection antici‐
pated 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 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. Valida‐
tion 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
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 ALSOalarm(), 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 Syn‐
chronization, <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 Electri‐
cal 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.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 FORK(3P)
