DragonFly On-Line Manual Pages
RFORK(2) DragonFly System Calls Manual RFORK(2)
rfork -- manipulate process resources
Standard C Library (libc, -lc)
Forking, vforking or rforking are the only ways new processes are cre-
ated. The flags argument to rfork() selects which resources of the
invoking process (parent) are shared by the new process (child) or ini-
tialized 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. Flags is the logical OR of
some subset of:
RFPROC If set a new process is created; otherwise changes
affect the current process. The current implementa-
tion requires this flag to always be set.
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 lan-
guages including C. May be set only with RFPROC. A
helper function is provided to assist with this prob-
lem and will cause the new process to run on the pro-
vided 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. With-
out RFPROC, the return value is zero. Process id's range from 1 to the
maximum integer (int) value. Rfork() will sleep, if necessary, until
required process resources are available.
Fork() can be implemented as a call to rfork(RFFDG | RFPROC) but isn't
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 indi-
cate the error.
Rfork() will fail and no child process will be created if:
[EAGAIN] The system-imposed limit on the total number of pro-
cesses 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 parameter
RLIMIT_NOFILE would be exceeded (see getrlimit(2)).
[EINVAL] The RFPROC flag was not specified.
[EINVAL] Both the RFFDG and the RFCFDG flags were specified.
[ENOMEM] There is insufficient swap space for the new process.
fork(2), intro(2), lwp_create(2), minherit(2), vfork(2), rfork_thread(3)
The rfork() function call first appeared in Plan 9.
DragonFly 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()
library call can often be used instead of clone().
DragonFly 3.5 January 12, 1996 DragonFly 3.5