NAME¶
rfork —
manipulate process
resources
LIBRARY¶
Standard C Library (libc, -lc)
SYNOPSIS¶
#include <unistd.h>
pid_t
rfork(
int
flags);
DESCRIPTION¶
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
.
RFTHREAD
- If set, the new process shares file descriptor to process
leaders table with its parent. Only applies when neither
RFFDG
nor RFCFDG
are
set.
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 exit.
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.
RETURN VALUES¶
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.
ERRORS¶
The
rfork() system call will fail and no child process will be
created if:
- [
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
KERN_MAXPROCPERUID
.
- [
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.
SEE ALSO¶
fork(2),
intro(2),
minherit(2),
vfork(2),
rfork_thread(3)
HISTORY¶
The
rfork() function first appeared in Plan9.
BUGS¶
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().