rfork (2)

Leading comments

This manual page is taken directly from Plan9, and modified to
describe the actual BSD implementation. Permission for
use of this page comes from Rob Pike <rob@plan9.att.com>.

$FreeBSD: releng/11.0/lib/libc/sys/rfork.2 223967 2011-07-12 20:38:42Z kib $

(The comments found at the beginning of the groff file "man2/rfork.2freebsd".)

NAME

rfork - manipulate process resources

LIBRARY

Lb libc

SYNOPSIS

In unistd.h Ft pid_t Fn rfork int flags

DESCRIPTION

Forking, vforking or rforking are the only ways new processes are created. The Fa flags argument to Fn 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 Fa 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 Fn 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_thread3 for information. Note that a lot of code will not run correctly in such an environment.

RFSIGSHARE

If set, the kernel will force sharing the sigacts structure between the child and the parent.

RFTSIGZMB

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 Fa flags . Specifying signal number 0 disables signal delivery upon the child exit.

RFLINUXTHPN

If set, the kernel will deliver SIGUSR1 instead of SIGCHLD 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 Ft ( int ) value. The Fn rfork system call will sleep, if necessary, until required process resources are available.

The Fn fork system call can be implemented as a call to Fn rfork RFFDG | RFPROC but is not for backwards compatibility.

RETURN VALUES

Upon successful completion, Fn 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 Fn rfork system call will fail and no child process will be created if:

Bq Er 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).

Bq Er 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

Bq Er EAGAIN

The user is not the super user, and the soft resource limit corresponding to the Fa resource argument RLIMIT_NOFILE would be exceeded (see getrlimit(2)).

Bq Er EINVAL

Both the RFFDG and the RFCFDG flags were specified.

Bq Er EINVAL

Any flags not listed above were specified.

Bq Er EINVAL

An invalid signal number was specified.

Bq Er ENOMEM

There is insufficient swap space for the new process.

SEE ALSO

fork(2), intro(2), minherit(2), vfork(2), pthread_create3, rfork_thread3

HISTORY

The Fn rfork function first appeared in Plan9.