manipulate process resources
Standard C Library (libc, -lc)
Forking, vforking or rforking are the only ways new processes are created. The
flags argument to
selects which resources of the invoking process (parent) are shared by the new
process (child) or initialized to their default values. The resources include
the open file descriptor table (which, when shared, permits processes to open
and close files for other processes), and open files. The
flags argument is either
or the logical OR of some subset of:
- If set a new process is created; otherwise changes affect the current
- If set, the child process will be dissociated from the parent. Upon exit
the child will not leave a status for the parent to collect. See
- If set, the invoker's file descriptor table (see
is copied; otherwise the two processes share a single table.
- If set, the new process starts with a clean file descriptor table. Is
mutually exclusive with
- If set, the new process shares file descriptor to process leaders table
with its parent. Only applies when neither
RFCFDG are set.
- If set, the kernel will force sharing of the entire address space,
typically by sharing the hardware page table directly. The child will thus
inherit and share all the segments the parent process owns, whether they
are normally shareable or not. The stack segment is not split (both the
parent and child return on the same stack) and thus
rfork() with the RFMEM flag may not generally be
called directly from high level languages including C. May be set only
RFPROC. A helper function is provided to
assist with this problem and will cause the new process to run on the
provided stack. See
for information. Note that a lot of code will not run correctly in such an
- If set, the kernel will force sharing the sigacts structure between the
child and the parent.
- If set, the kernel will deliver a specified signal to the parent upon the
child exit, instead of default SIGCHLD. The signal number
signum is specified by oring the
RFTSIGFLAGS(signum) expression into
flags. Specifying signal number 0 disables signal
delivery upon the child exit.
- If set, the kernel will deliver SIGUSR1 instead of SIGCHLD upon thread
exit for the child. This is intended to mimic certain Linux clone
File descriptors in a shared file descriptor table are kept open
until either they are explicitly closed or all processes sharing the table
RFSPAWN is passed,
rfork will use
semantics but reset all signal actions in the child to default. This flag is
used by the
implementation in libc.
RFPROC is set, the value returned in
the parent process is the process id of the child process; the value
returned in the child is zero. Without
return value is zero. Process id's range from 1 to the maximum integer
(int) value. The
system call will sleep, if necessary, until required process resources are
Upon successful completion,
fork() system call can be implemented
as a call to
RFPROC) but is not for backwards compatibility.
rfork() 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.
rfork() 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
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
- The user is not the super user, and the soft resource limit corresponding
to the resource argument
RLIMIT_NOFILE would be exceeded (see
- Both the RFFDG and the RFCFDG flags were specified.
- Any flags not listed above were specified.
- An invalid signal number was specified.
- There is insufficient swap space for the new process.
rfork() function first appeared in Plan9.