FORK(2) manual page
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fork - create a child process
#include <unistd.h>
pid_t fork(void);
fork() creates a
new process by duplicating the calling process. The new process, referred
to as the child, is an exact duplicate of the calling process, referred
to as the parent, except for the following points:
- *
- The child has its own
unique process ID, and this PID does not match the ID of any existing process
group (setpgid(2)
).
- *
- The child’s parent process ID is the same as the parent’s
process ID.
- *
- The child does not inherit its parent’s memory locks (mlock(2)
,
mlockall(2)
).
- *
- Process resource utilizations (getrusage(2)
) and CPU time
counters (times(2)
) are reset to zero in the child.
- *
- The child’s set of pending
signals is initially empty (sigpending(2)
).
- *
- The child does not inherit
semaphore adjustments from its parent (semop(2)
).
- *
- The child does not inherit
process-associated record locks from its parent (fcntl(2)
). (On the other
hand, it does inherit fcntl(2)
open file description locks and flock(2)
locks from its parent.)
- *
- The child does not inherit timers from its parent
(setitimer(2)
, alarm(2)
, timer_create(2)
).
- *
- The child does not inherit outstanding
asynchronous I/O operations from its parent (aio_read(3)
, aio_write(3)
),
nor does it inherit any asynchronous I/O contexts from its parent (see
io_setup(2)
).
The process attributes in the preceding list are all specified
in POSIX.1-2001. The parent and child also differ with respect to the following
Linux-specific process attributes:
- *
- The child does not inherit directory
change notifications (dnotify) from its parent (see the description of
F_NOTIFY in fcntl(2)
).
- *
- The prctl(2)
PR_SET_PDEATHSIG setting is reset so
that the child does not receive a signal when its parent terminates.
- *
- The
default timer slack value is set to the parent’s current timer slack value.
See the description of PR_SET_TIMERSLACK in prctl(2)
.
- *
- Memory mappings that
have been marked with the madvise(2)
MADV_DONTFORK flag are not inherited
across a fork().
- *
- The termination signal of the child is always SIGCHLD
(see clone(2)
).
- *
- The port access permission bits set by ioperm(2)
are not
inherited by the child; the child must turn on any bits that it requires
using ioperm(2)
.
Note the following further points:
- *
- The child process is
created with a single thread--the one that called fork(). The entire virtual
address space of the parent is replicated in the child, including the states
of mutexes, condition variables, and other pthreads objects; the use of
pthread_atfork(3)
may be helpful for dealing with problems that this can
cause.
- *
- The child inherits copies of the parent’s set of open file descriptors.
Each file descriptor in the child refers to the same open file description
(see open(2)
) as the corresponding file descriptor in the parent. This means
that the two descriptors share open file status flags, current file offset,
and signal-driven I/O attributes (see the description of F_SETOWN and F_SETSIG
in fcntl(2)
).
- *
- The child inherits copies of the parent’s set of open message
queue descriptors (see mq_overview(7)
). Each descriptor in the child refers
to the same open message queue description as the corresponding descriptor
in the parent. This means that the two descriptors share the same flags
(mq_flags).
- *
- The child inherits copies of the parent’s set of open directory
streams (see opendir(3)
). POSIX.1-2001 says that the corresponding directory
streams in the parent and child may share the directory stream positioning;
on Linux/glibc they do not.
On success, the PID of the child
process is returned in the parent, and 0 is returned in the child. On failure,
-1 is returned in the parent, no child process is created, and errno is
set appropriately.
- EAGAIN
- fork() cannot allocate sufficient memory
to copy the parent’s page tables and allocate a task structure for the child.
- EAGAIN
- NOTE! The following should match the description in pthread_create(3)
A system-imposed limit on the number of threads was encountered. There are
a number of limits that may trigger this error: the RLIMIT_NPROC soft resource
limit (set via setrlimit(2)
), which limits the number of processes and
threads for a real user ID, was reached; the kernel’s system-wide limit on
the number of processes and threads, /proc/sys/kernel/threads-max, was reached
(see proc(5)
); or the maximum number of PIDs, /proc/sys/kernel/pid_max,
was reached (see proc(5)
).
- EAGAIN
- The caller is operating under the SCHED_DEADLINE
scheduling policy and does not have the reset-on-fork flag set. See sched(7)
.
- ENOMEM
- fork() failed to allocate the necessary kernel structures because
memory is tight.
- ENOSYS
- fork() is not supported on this platform (for example,
hardware without a Memory-Management Unit).
SVr4, 4.3BSD, POSIX.1-2001.
Under Linux, fork() is implemented using copy-on-write pages, so the
only penalty that it incurs is the time and memory required to duplicate
the parent’s page tables, and to create a unique task structure for the
child.
Since version 2.3.3, rather than invoking the kernel’s fork() system
call, the glibc fork() wrapper that is provided as part of the NPTL threading
implementation invokes clone(2)
with flags that provide the same effect
as the traditional system call. (A call to fork() is equivalent to a call
to clone(2)
specifying flags as just SIGCHLD.) The glibc wrapper invokes
any fork handlers that have been established using pthread_atfork(3)
.
See
pipe(2)
and wait(2)
.
clone(2)
, execve(2)
, exit(2)
, setrlimit(2)
,
unshare(2)
, vfork(2)
, wait(2)
, daemon(3)
, capabilities(7)
, credentials(7)
This page is part of release 3.78 of the Linux man-pages project.
A description of the project, information about reporting bugs, and the
latest version of this page, can be found at http://www.kernel.org/doc/man-pages/.
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