FORK(2) BSD System Calls Manual FORK(2)NAMEfork — 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.
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
· All interval timers are cleared; see setitimer(2).
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 indi‐
cate 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 pro‐
cesses 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
[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.
SEE ALSOexecve(2), rfork(2), setitimer(2), setrlimit(2), vfork(2), wait(2)HISTORY
The fork() function appeared in Version 6 AT&T UNIX.
BSD June 4, 1993 BSD