NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON
FORK(2) Linux Programmer's Manual FORK(2)
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 record locks from its parent (fcntl(2)).
* 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 (seeio_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.
* 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)).
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 It was not possible to create a new process because the caller's
RLIMIT_NPROC resource limit was encountered. To exceed this limit, the
process must have either the CAP_SYS_ADMIN or the CAP_SYS_RESOURCE
capability.
ENOMEM fork() failed to allocate the necessary kernel structures because
memory is tight.
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. 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), setrlimit(2), unshare(2), vfork(2), wait(2), daemon(3),
capabilities(7), credentials(7)
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2009-04-27 FORK(2)