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rfork - manipulate process resources
Standard C Library (libc, -lc)
Forking, vforking or rforking are the only ways new processes are
created. The flags argument to rfork() 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 the
logical OR of some subset of:
RFPROC If set a new process is created; otherwise changes affect
the current process.
RFNOWAIT 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 wait(2).
RFFDG If set, the invoker’s file descriptor table (see intro(2))
is copied; otherwise the two processes share a single table.
RFCFDG If set, the new process starts with a clean file descriptor
table. Is mutually exclusive with RFFDG.
RFMEM 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 with
RFPROC. A helper function is provided to assist with this
problem and will cause the new process to run on the
provided stack. See rfork_thread(3) for information.
RFSIGSHARE If set, the kernel will force sharing the sigacts structure
between the child and the parent.
RFLINUXTHPN If set, the kernel will return SIGUSR1 instead of SIGCHILD
upon thread exit for the child. This is intended to mimic
certain Linux clone behaviour.
File descriptors in a shared file descriptor table are kept open until
either they are explicitly closed or all processes sharing the table
If 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 RFPROC, the return value is zero. Process id’s range from 1 to
the maximum integer (int) value. The rfork() system call will sleep, if
necessary, until required process resources are available.
The fork() system call can be implemented as a call to rfork(RFFDG |
RFPROC) but is not for backwards compatibility.
Upon successful completion, 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.
The rfork() system call will fail and no child process will be created
[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 argument
RLIMIT_NOFILE would be exceeded (see getrlimit(2)).
[EINVAL] Both the RFFDG and the RFCFDG flags were specified.
[EINVAL] Any flags not listed above were specified.
[ENOMEM] There is insufficient swap space for the new process.
fork(2), intro(2), minherit(2), vfork(2), rfork_thread(3)
The rfork() function first appeared in Plan9.
FreeBSD does not yet implement a native clone() library call, and the
current pthreads implementation does not use rfork() with RFMEM. A
native port of the linux threads library, /usr/ports/devel/linuxthreads,
contains a working clone() call that utilizes RFMEM. The rfork_thread(3)
function can often be used instead of clone().