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
- The child's parent process ID is the same as the parent's
- The child does not inherit its parent's memory locks
- 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
- 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
- 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
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
- The default timer slack value is set to the parent's
current timer slack value. See the description of PR_SET_TIMERSLACK
- Memory mappings that have been marked with the
madvise(2) MADV_DONTFORK flag are not inherited across a
- 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.
- 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
- 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
- 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
- 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
- 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
- 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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