fork - create a child process
() creates a new process by duplicating the calling process. The new
process is referred to as the child
process. The calling process is
referred to as the parent
The child process and the parent process run in separate memory spaces. At the
time of fork
() both memory spaces have the same content. Memory writes,
file mappings (mmap(2)
), and unmappings (munmap(2)
) performed by
one of the processes do not affect the other.
The child process is an exact duplicate of the parent process except for the
- The child has its own unique process ID, and this PID does not match the
ID of any existing process group (setpgid(2)) or session.
- 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),
- 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
- The child does not inherit semaphore adjustments from its parent
- 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
- The child does not inherit timers from its parent (setitimer(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
The process attributes in the preceding list are all specified in POSIX.1. The
parent and child also differ with respect to the following Linux-specific
- The child does not inherit directory change notifications (dnotify) from
its parent (see the description of F_NOTIFY in
- 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
- Memory mappings that have been marked with the madvise(2)
MADV_DONTFORK flag are not inherited across a fork().
- Memory in address ranges that have been marked with the madvise(2)
MADV_WIPEONFORK flag is zeroed in the child after a fork().
(The MADV_WIPEONFORK setting remains in place for those address
ranges in the child.)
- The termination signal of the child is always SIGCHLD (see
- 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
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.
- After a fork() in a multithreaded program, the child can safely
call only async-signal-safe functions (see signal-safety(7)) until
such time as it calls execve(2).
- 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 file descriptors share open file status flags,
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 file descriptor in the child
refers to the same open message queue description as the corresponding
file descriptor in the parent. This means that the two file 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 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.
- 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
- the kernel's system-wide limit on the number of processes and threads,
/proc/sys/kernel/threads-max, was reached (see
- the maximum number of PIDs, /proc/sys/kernel/pid_max, was reached
(see proc(5)); or
- the PID limit (pids.max) imposed by the cgroup "process
number" (PIDs) controller was reached.
- The caller is operating under the SCHED_DEADLINE scheduling policy
and does not have the reset-on-fork flag set. See sched(7).
- fork() failed to allocate the necessary kernel structures because
memory is tight.
- An attempt was made to create a child process in a PID namespace whose
"init" process has terminated. See
- fork() is not supported on this platform (for example, hardware
without a Memory-Management Unit).
- ERESTARTNOINTR (since Linux 2.6.17)
- System call was interrupted by a signal and will be restarted. (This can
be seen only during a trace.)
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
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.
C library/kernel differences¶
Since version 2.3.3, rather than invoking the kernel's fork
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
equivalent to a call to clone(2)
.) The glibc wrapper invokes any fork handlers that have been
established using pthread_atfork(3)
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