NAME
fork —
create a new process
LIBRARY
library “libc”
SYNOPSIS
#include
<sys/types.h>
#include <unistd.h>
pid_t
fork(void);
DESCRIPTION
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 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 setrlimit(2).
- All interval timers are cleared; see setitimer(2).
RETURN VALUES
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.
MULTI-THREADING CONSIDERATIONS
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.
ERRORS
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
KERN_MAXPROCPERUID. - [
EAGAIN] - The user is not the super user, and the soft resource limit corresponding
to the resource parameter
RLIMIT_NPROCwould be exceeded (see getrlimit(2)). - [
ENOMEM] - There is insufficient swap space for the new process.
SEE ALSO
execve(2), rfork(2), setitimer(2), setrlimit(2), vfork(2), wait(2)
HISTORY
A fork() function call appeared in
Version 6 AT&T UNIX.