fork(2) - phpMan
FORK(2) Linux Programmer's Manual FORK(2)
fork - create a child process
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_cre‐
* 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 par‐
ent 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 allo‐
cate a task structure for the child.
A system-imposed limit on the number of threads was encountered. There are a num‐
ber 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-
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 cre‐
ate 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
See pipe(2) and wait(2).
clone(2), execve(2), exit(2), setrlimit(2), unshare(2), vfork(2), wait(2), daemon(3),
This page is part of release 3.74 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/.
Linux 2014-05-28 FORK(2)