RFORK(2)

NAME
rfork - manipulate process resources

SYNOPSIS
#include <unistd.h>
int
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. 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 implementation 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. The child will then inherit all the shared segments the parent process owns. Other segment types will be unaffected. Subsequent forks by the parent will then propagate the shared data and bss between children. The stack segment is always split. May be set only with RFPROC.

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. 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.

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
Rfork() 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 one 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 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.

SEE ALSO
fork(2), intro(2), minherit(2), vfork(2)

HISTORY
The rfork() function call first appeared in Plan9.