From Linux Man Pages
fork - create a child process
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
pid_t fork(void);
DESCRIPTION
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 utilisations (getrusage(2)) and CPU time counters (times(2)) are reset to zero in the child.
* The child 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 parent does not inherit timers from its parent (setitimer(2) alarm(3), timer_create(3)).
* The child does not inherit outstanding asynchronous I/O operations from its parent (aio_read(3),
aio_write(3)).
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(2).
* 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(2). 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).
RETURN VALUE
On success, the PID of the child process is returned in the parent's thread of execution, and a 0 is returned in
the child's thread of execution. On failure, a -1 will be returned in the parent's context, no child process
will be created, and errno will be set appropriately.
ERRORS
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 encoun-
tered. To exceed this limit, the process must have either the CAP_SYS_ADMIN or the CAP_SYS_RESOURCE capa-
bility.
ENOMEM fork() failed to allocate the necessary kernel structures because memory is tight.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
EXAMPLE
See pipe(2) and wait(2).
NOTES
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.
RELATED
clone(2), execve(2), setrlimit(2), unshare(2), vfork(2), wait(2), capabilities(7)
CATEGORY