Section (2) fork
fork — create a child process
#include <sys/types.h> #include <unistd.h>
fork() 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
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 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)) or session.
The child_zsingle_quotesz_s parent process ID is the same as the parent_zsingle_quotesz_s process ID.
The child_zsingle_quotesz_s set of pending signals is initially empty (sigpending(2)).
The child does not inherit semaphore adjustments from its parent (semop(2)).
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 process attributes:
The child does not inherit directory change notifications (dnotify) from its parent (see the description of
PR_SET_PDEATHSIGsetting 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_zsingle_quotesz_s current timer slack value. See the description of
Memory mappings that have been marked with the madvise(2)
MADV_DONTFORKflag are not inherited across a
Memory in address ranges that have been marked with the madvise(2)
MADV_WIPEONFORKflag is zeroed in the child after a
MADV_WIPEONFORKsetting remains in place for those address ranges in the child.)
The termination signal of the child is always
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_zsingle_quotesz_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
The child inherits copies of the parent_zsingle_quotesz_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 (
The child inherits copies of the parent_zsingle_quotesz_s set of open directory streams (see opendir(3)). POSIX.1 says that the corresponding directory streams in the parent and child
mayshare 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
errno is set
A system-imposed limit on the number of threads was encountered. There are a number of limits that may trigger this error:
RLIMIT_NPROCsoft resource limit (set via setrlimit(2)), which limits the number of processes and threads for a real user ID, was reached;
the kernel_zsingle_quotesz_s system-wide limit on the number of processes and threads,
/proc/sys/kernel/threads-max, was reached (see proc(5));
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_DEADLINEscheduling 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 pid_namespaces(7).
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.)
implemented using copy-on-write pages, so the only penalty
that it incurs is the time and memory required to duplicate
the parent_zsingle_quotesz_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_zsingle_quotesz_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
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
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Modified by Michael Haardt (michaelmoria.de)
Modified Sat Jul 24 13:22:07 1993 by Rik Faith (faithcs.unc.edu)
Modified 21 Aug 1994 by Michael Chastain (mecshell.portal.com):
Modified 1995-06-10, 1996-04-18, 1999-11-01, 2000-12-24
by Andries Brouwer (aebcwi.nl)
Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com>
Added notes on capability requirements
2006-09-04, Michael Kerrisk
Greatly expanded, to describe all attributes that differ
parent and child.