RFORK(2) OpenBSD Programmer's Manual RFORK(2)NAMErfork - control new processes
SYNOPSIS
#include <sys/param.h>
#include <unistd.h>
int
rfork(int flags);
DESCRIPTION
The fork functions (fork(2), vfork(2), and rfork()) create new processes.
The new process (child process) is an exact copy of the calling process
(parent process), except as outlined in the fork(2) manual page. rfork()
is used to manipulate the resources of the parent process and the child
process.
Operations currently supported include whether to copy or share the file
descriptor table between the two processes, whether to share the address
space, and whether the parent should wait(2) for the child process to
_exit(2). rfork() takes a single argument, flags, which controls which
of these resources should be manipulated. They are defined in the header
file <sys/param.h> and are the logical OR of one or more of the
following:
RFFDG Copy the parent's file descriptor table. If this flag is
unset, the parent and child will share the parent's file
descriptor table. Descriptors will remain in existence until
they are closed by all child processes using the table copies
as well as by the parent process. May not be used in
conjunction with RFCFDG.
RFPROC Create a new process. The current implementation requires this
flag to always be set.
RFMEM Force sharing of the entire address space between the parent
and child processes. The child will then inherit all the
shared segments the parent process owns. Subsequent forks by
the parent will then propagate the shared data and BSS segments
among children.
RFNOWAIT Child processes will have their resources reaped immediately
and implicitly when they terminate instead of turning into
zombies, so the parent process may not call wait(2) to collect
their exit statuses and have their resources released
explicitly.
RFCFDG Zero the child's file descriptor table (i.e. start with a blank
file descriptor table). May not be used in conjunction with
RFFDG.
RFTHREAD Create a kernel thread in the current process instead of a
separate process. Must be combined with RFMEM. Automatically
enables RFNOWAIT. The kern.rthreads sysctl must be enabled for
this to succeed.
fork(2) can be implemented as a call to rfork() using "RFFDG|RFPROC", but
isn't for backwards compatibility. If a process has file descriptor
table sharing active, setuid or setgid programs will not execve(2) with
extra privileges.
RETURN VALUES
The parent process returns the process ID (PID) of the child process.
The child process returns 0. The range of the process ID is defined in
<sys/proc.h> and is currently between 1 and 32766, inclusive.
ERRORSrfork() will fail and no child process will be created if:
[ENOMEM] Cannot allocate memory. The new process image required
more memory than was allowed by the hardware or by system-
imposed memory management constraints. A lack of swap
space is normally temporary; however, a lack of core is
not. Soft limits may be increased to their corresponding
hard limits.
[EINVAL] Invalid argument. Some invalid argument was supplied.
[EAGAIN] Resource temporarily unavailable. The system-imposed limit
on the total number of processes under execution would be
exceeded. This limit is configuration-dependent.
[EAGAIN] Resource temporarily unavailable. The system-imposed limit
MAXUPRC on the total number of processes under execution by
a single user would be exceeded. MAXUPRC is currently
defined in <sys/param.h> as CHILD_MAX, which is currently
defined as 80 in <sys/syslimits.h>.
[ENOTSUP] The RFTHREAD flag was set but the kern.rthreads sysctl was
not enabled.
SEE ALSO_exit(2), execve(2), fork(2), intro(2), vfork(2)HISTORY
The rfork() function first appeared in Plan 9.
BUGS
RFTHREAD cannot be used from C, as the two threads would return on the
same stack.
OpenBSD 4.9 June 29, 2010 OpenBSD 4.9
