fork, rfork – manipulate process resources


#include <u.h>
#include <libc.h>

int fork(void)

int rfork(int flags)


Forking is the only way 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 file name space, the open file descriptor table (which, when shared, permits processes to open and close files for other processes), the set of environment variables (see env(3)), the note group (the set of processes that receive notes written to a member’s notepg file; see proc(3)), the set of rendezvous tags (see rendezvous(2)); and open files. Flags is the logical OR of some subset of


If set a new process is created; otherwise changes affect the current process.  


If set, the child process will be dissociated from the parent. Upon exit the child will leave no Waitmsg (see wait(2)) for the parent to collect.  


If set, the new process inherits a copy of the parent’s name space; otherwise the new process shares the parent’s name space. Is mutually exclusive with RFCNAMEG.  


If set, the new process starts with a clean name space. A new name space must be built from a mount of an open file descriptor. Is mutually exclusive with RFNAMEG.  


If set, subsequent mounts into the new name space and dereferencing of pathnames starting with # are disallowed.  


If set, the environment variables are copied; otherwise the two processes share environment variables. Is mutually exclusive with RFCENVG.  


If set, the new process starts with an empty environment. Is mutually exclusive with RFENVG.  


Each process is a member of a group of processes that all receive notes when a note is written to any of their notepg files (see proc(3)). The group of a new process is by default the same as its parent, but if RFNOTEG is set (regardless of RFPROC), the process becomes the first in a new group, isolated from previous processes.  


If set, the invoker’s file descriptor table (see intro(2)) is copied; otherwise the two processes share a single table.  


If set, the new process starts with a clean file descriptor table. Is mutually exclusive with RFFDG.  


If set, the process will be unable to rendezvous(2) with any of its ancestors; its children will, however, be able to rendezvous with it. In effect, RFREND makes the process the first in a group of processes that share a space for rendezvous tags.  


If set, the child and the parent will share data and bss segments. Otherwise, the child inherits a copy of those segments. Other segment types, in particular stack segments, will be unaffected. 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 ids range from 1 to the maximum integer (int) value. Rfork will sleep, if necessary, until required process resources are available.

Fork is just a call of rfork(RFFDG|RFREND|RFPROC).




intro(2), proc(3),


These functions set errstr.