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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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.
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.1‐2017 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.1‐2017 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.1‐2017. 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.1‐2017.
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.1‐2017, Section 4.12, Memory
Synchronization, sys_types.h(0p), unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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 2017 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), close(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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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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