fork
— create a
new process
Standard C Library (libc, -lc)
The
fork
()
system call causes creation of a new process. The new process (child
process) is an exact copy of the calling process (parent process) except for
the following:
- The child process has a unique process ID.
- The child process has a different parent process ID (i.e., the process ID
of the parent process).
- The child process has its own copy of the parent's descriptors, except for
descriptors returned by kqueue(2), which are not
inherited from the parent process. These descriptors reference the same
underlying objects, so that, for instance, file pointers in file objects
are shared between the child and the parent, so that an
lseek(2) on a descriptor in the child process can affect
a subsequent read(2) or write(2) by
the parent. This descriptor copying is also used by the shell to establish
standard input and output for newly created processes as well as to set up
pipes.
- The child process' resource utilizations are set to 0; see
setrlimit(2).
- All interval timers are cleared; see setitimer(2).
- The child process has only one thread, corresponding to the calling thread
in the parent process. If the process has more than one thread, locks and
other resources held by the other threads are not released and therefore
only async-signal-safe functions (see sigaction(2)) are
guaranteed to work in the child process until a call to
execve(2) or a similar function.
Upon successful completion, fork
() returns
a value of 0 to the child process and returns the process ID of the child
process to the parent process. Otherwise, a value of -1 is returned to the
parent process, no child process is created, and the global variable
errno is set to indicate the error.
The fork
() system call will fail and no
child process will be created if:
- [
EAGAIN
]
- The system-imposed limit on the total number of processes under execution
would be exceeded. The limit is given by the sysctl(3)
MIB variable
KERN_MAXPROC
. (The limit is actually
ten less than this except for the super user).
- [
EAGAIN
]
- The user is not the super user, and the system-imposed limit on the total
number of processes under execution by a single user would be exceeded.
The limit is given by the sysctl(3) MIB variable
KERN_MAXPROCPERUID
.
- [
EAGAIN
]
- The user is not the super user, and the soft resource limit corresponding
to the resource argument
RLIMIT_NPROC
would be exceeded (see
getrlimit(2)).
- [
ENOMEM
]
- There is insufficient swap space for the new process.
The fork
() function appeared in
Version 1 AT&T UNIX.