DragonFly On-Line Manual Pages
FORK(2) DragonFly System Calls Manual FORK(2)
fork -- create a new process
Standard C Library (libc, -lc)
Fork() causes creation of a new process. The new process (child process)
is an exact copy of the calling process (parent process) except for the
o The child process has a unique process ID.
o The child process has a different parent process ID (i.e., the
process ID of the parent process).
o 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.
o The child process' resource utilizations are set to 0; see
o 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
indicate the error.
fork() can create severe issues for multi-threaded programs due to the
fact that the memory state of the child process will record the
asynchronous state of the threads that are running in the parent. Fork()
will only be synchronous for the specific thread making the call. In
particular, locks used internally by pthread(3) and rtld(1) can be caught
in a bad state. To deal with these issues, the pthreads library goes to
great lengths to synchronize internal locks when a fork() call is issued.
The threaded program itself as well as third party libraries used by the
program might or might not properly handle these issues when it comes to
their own internal state.
If at all possible, programs should use vfork(2) instead of fork() when
forking for the purposes of issuing an exec of some sort. Attempting to
fork a threaded program without issuing an exec is not recommended.
Attempting to bypass pthreads and implement threading manually is also
not recommended as it is doubtful that homegrown implementations could
properly deal with rtld races.
Fork() 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
[EAGAIN] The user is not the super user, and the soft resource
limit corresponding to the resource parameter
RLIMIT_NPROC would be exceeded (see getrlimit(2)).
[ENOMEM] There is insufficient swap space for the new process.
execve(2), rfork(2), setitimer(2), setrlimit(2), vfork(2), wait(2)
A fork() function call appeared in Version 6 AT&T UNIX.
DragonFly 4.9 June 4, 1993 DragonFly 4.9