RENDEZVOUS(2) System Calls Manual RENDEZVOUS(2)NAME
rendezvous - user level process synchronization
SYNOPSIS
#include <u.h>
#include <libc.h>
ulong rendezvous(ulong tag, ulong value)
DESCRIPTION
The rendezvous system call allows two processes to synchronize and exchange a value. In conjunction with the shared memory system calls
(see segattach(2) and fork(2)), it enables parallel programs to control their scheduling.
Two processes wishing to synchronize call rendezvous with a common tag, typically an address in memory they share. One process will arrive
at the rendezvous first; it suspends execution until a second arrives. When a second process meets the rendezvous the value arguments are
exchanged between the processes and returned as the result of the respective rendezvous system calls. Both processes are awakened when the
rendezvous succeeds.
The tag space is common to processes in the same file name space, so rendezvous only works between processes in the same file name space.
If a rendezvous is interrupted the return value is ~0, so that value should not be used in normal communication.
SOURCE
/sys/src/libc/9syscall
SEE ALSO segattach(2), fork(2)DIAGNOSTICS
Sets errstr.
BUGS
The correlation of rendezvous tags and file name space is a historical accident. If two unrelated processes happen to be in the same name
space and do a rendezvous, trouble will result. The solution is to call rfork(RFNAMEG) (see fork(2)) in programs that use rendezvous
unless they need to share the name space with their parent. This is especially important in Alef programs.
RENDEZVOUS(2)
Check Out this Related Man Page
FORK(2) System Calls Manual FORK(2)NAME
fork, rfork - manipulate process resources
SYNOPSIS
#include <u.h>
#include <libc.h>
int fork(void)
int rfork(int flags)
DESCRIPTION
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)), 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.
RFNOWAIT
If set, the child process will be dissociated from the parent. Upon exit the child will leave no Waitmsg (see wait(2)) for the par-
ent to collect.
RFNAMEG
If set, the new process inherits a copy of the parent's name space; otherwise the new process shares the parent's name space. The
tag space for rendezvous(2) is considered part of the name space. Is mutually exclusive with RFCNAMEG.
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.
RFENVG If set, the environment variables are copied; otherwise the two processes share environment variables. Is mutually exclusive with
RFCENVG.
RFCENVG
If set, the new process starts with an empty environment. Is mutually exclusive with RFENVG.
RFNOTEG
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.
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 mark segments of type data and bss as shared. 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 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|RFPROC).
SOURCE
/sys/src/libc/9syscall
/sys/src/libc/9sys/fork.c
SEE ALSO intro(2), proc(3),
DIAGNOSTICS
These functions set errstr.
FORK(2)