create a new process
Standard C Library (libc, -lc)
() 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
- 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
Upon successful completion,
() 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.
() system call will fail and no child
process will be created if:
- 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).
- 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
- The user is not the super user, and the soft resource limit corresponding
to the resource argument
RLIMIT_NPROC would be exceeded (see
- There is insufficient swap space for the new process.
() function appeared in
Version 6 AT&T UNIX