— create a
() 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, which also
does not match any existing process group ID.
- The child process has a different parent process ID
(i.e., the process ID of the parent process).
- The child process has a single thread.
- 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
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
- The child process has no
- The child process' resource utilizations are set to 0;
- All interval timers are cleared; see
- The child process' semaphore undo values are set to 0;
- The child process' pending signals set is empty.
- The child process has no memory locks; see
In general, the child process should call
any stdio buffers that exist both in the parent and child will be flushed
used to prevent
from being called twice (once in the parent and once in the child).
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
is set to indicate the error.
() will fail and no child process will be
- The system-imposed limits on the total number of processes
or total number of threads under execution would be exceeded. These limits
are configuration dependent.
- The limit
on the total number of processes under execution by the user ID would be
- There is insufficient swap space for the new process.
() function conforms to
IEEE Std 1003.1-2008
() system call first appeared in
Version 1 AT&T UNIX