THREAD(3) THREAD(3)
NAME
alt, chancreate, chanfree, chanprint, chansetname,
mainstacksize, proccreate, procdata, recv, recvp, recvul,
send, sendp, sendul, nbrecv, nbrecvp, nbrecvul, nbsend,
nbsendp, nbsendul, threadcreate, threaddata, threadexec,
threadexecl, threadexits, threadexitsall, threadgetgrp,
threadgetname, threadint, threadintgrp, threadkill,
threadkillgrp, threadmain, threadnotify, threadid,
threadpid, threadpin, threadunpin, threadsetgrp,
threadsetname, threadsetstate, threadspawn, threadspawnd,
threadspawnl, threadwaitchan, yield - thread and proc
management
SYNOPSIS
#include <u.h>
#include <libc.h>
#include <thread.h>
#define CHANEND 0
#define CHANSND 1
#define CHANRCV 2
#define CHANNOP 3
#define CHANNOBLK 4
typedef struct Alt Alt;
struct Alt {
Channel *c;
void *v;
int op;
Channel **tag;
int entryno;
char *name;
};
void threadmain(int argc, char *argv[])
int mainstacksize
int proccreate(void (*fn)(void*), void *arg, uint stacksize)
int threadcreate(void (*fn)(void*), void *arg, uint stacksize)
void threadexits(char *status)
void threadexitsall(char *status)
void yield(void)
int threadpin(void)
int threadunpin(void)
int threadid(void)
int threadgrp(void)
int threadsetgrp(int group)
int threadpid(int id)
int threadint(int id)
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THREAD(3) THREAD(3)
int threadintgrp(int group)
int threadkill(int id)
int threadkillgrp(int group)
void threadsetname(char *name)
char* threadgetname(void)
void** threaddata(void)
void** procdata(void)
Channel* chancreate(int elsize, int nel)
void chanfree(Channel *c)
int alt(Alt *alts)
int recv(Channel *c, void *v)
void* recvp(Channel *c)
ulong recvul(Channel *c)
int nbrecv(Channel *c, void *v)
void* nbrecvp(Channel *c)
ulong nbrecvul(Channel *c)
int send(Channel *c, void *v)
int sendp(Channel *c, void *v)
int sendul(Channel *c, ulong v)
int nbsend(Channel *c, void *v)
int nbsendp(Channel *c, void *v)
int nbsendul(Channel *c, ulong v)
int chanprint(Channel *c, char *fmt, ...)
int threadspawnl(int fd[3], char *file, ...)
int threadspawn(int fd[3], char *file, char *args[])
int threadspawnd(int fd[3], char *file, char *args[], char *dir)
int threadexecl(Channel *cpid, int fd[3], char *file, ...)
int threadexec(Channel *cpid, int fd[3], char *file, char *args[])
Channel* threadwaitchan(void)
int threadnotify(int (*f)(void*, char*), int in)
DESCRIPTION
The thread library provides parallel programming support
similar to that of the languages Alef and Newsqueak.
Threads and procs occupy a shared address space,
communicating and synchronizing through channels and shared
variables.
A proc is a Plan 9 process that contains one or more cooper-
atively scheduled threads. Programs using threads must
replace main by threadmain. The thread library provides a
main function that sets up a proc with a single thread exe-
cuting threadmain on a stack of size mainstacksize (default
eight kilobytes). To set mainstacksize, declare a global
variable initialized to the desired value (e.g., int
mainstacksize = 1024).
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THREAD(3) THREAD(3)
Threadcreate creates a new thread in the calling proc,
returning a unique integer identifying the thread; the
thread executes fn(arg) on a stack of size stacksize. Thread
stacks are allocated in shared memory, making it valid to
pass pointers to stack variables between threads and procs.
Proccreate creates a new proc, and inside that proc creates
a single thread as threadcreate would, returning the id of
the created thread. Be aware that the calling thread may
continue execution before the newly created proc and thread
are scheduled. Because of this, arg should not point to
data on the stack of a function that could return before the
new process is scheduled.
Threadexits terminates the calling thread. If the thread is
the last in its proc, threadexits also terminates the proc,
using status as the exit status. Threadexitsall terminates
all procs in the program, using status as the exit status.
When the last thread in threadmain's proc exits, the program
will appear to its parent to have exited. The remaining
procs will still run together, but as a background program.
The threads in a proc are coroutines, scheduled nonpreemp-
tively in a round-robin fashion. A thread must explicitly
relinquish control of the processor before another thread in
the same proc is run. Calls that do this are yield,
proccreate, threadexec, threadexecl, threadexits,
threadspawn, threadspawnd, threadspawnl, alt, send, and recv
(and the calls related to send and recv-see their descrip-
tions further on). Procs are scheduled by the operating
system. Therefore, threads in different procs can preempt
one another in arbitrary ways and should synchronize their
actions using qlocks (see lock(3)) or channel communication.
System calls such as read(3) block the entire proc; all
threads in a proc block until the system call finishes.
Threadpin disables scheduling inside a proc, `pinning' the
current thread as the only runnable one in the current proc.
Threadunpin reenables scheduling, allowing other procs to
run once the current thread relinquishes the processor.
Threadpin and threadunpin can lead to deadlock. Used care-
fully, they can make library routines that use qlocks appear
atomic relative to the current proc, like a system call.
As mentioned above, each thread has a unique integer thread
id. Thread ids are not reused; they are unique across the
life of the program. Threadid returns the id for the cur-
rent thread. Each thread also has a thread group id. The
initial thread has a group id of zero. Each new thread
inherits the group id of the thread that created it.
Threadgrp returns the group id for the current thread;
threadsetgrp sets it. Threadpid returns the pid of the Plan
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THREAD(3) THREAD(3)
9 process containing the thread identified by id, or -1 if
no such thread is found.
Threadint interrupts a thread that is blocked in a channel
operation or system call. Threadintgrp interrupts all
threads with the given group id. Threadkill marks a thread
to die when it next relinquishes the processor (via one of
the calls listed above). If the thread is blocked in a
channel operation or system call, it is also interrupted.
Threadkillgrp kills all threads with the given group id.
Note that threadkill and threadkillgrp will not terminate a
thread that never relinquishes the processor.
Primarily for debugging, threads can have string names asso-
ciated with them. Threadgetname returns the current
thread's name; threadsetname sets it. The pointer returned
by threadgetname is only valid until the next call to
threadsetname.
Also for debugging, threads have a string state associated
with them. Threadsetstate sets the state string. There is
no threadgetstate; since the thread scheduler resets the
state to Running every time it runs the thread, it is only
useful for debuggers to inspect the state.
Threaddata returns a pointer to a per-thread pointer that
may be modified by threaded programs for per-thread storage.
Similarly, procdata returns a pointer to a per-proc pointer.
Threadexecl and threadexec are threaded analogues of exec
and execl (see exec(3)); on success, they replace the call-
ing thread and invoke the external program, never returning.
(Unlike on Plan 9, the calling thread need not be the only
thread in its proc-the other threads will continue execut-
ing.) On error, they return -1. If cpid is not null, the
pid of the invoked program will be sent along cpid (using
sendul) once the program has been started, or -1 will be
sent if an error occurs. Threadexec and threadexecl will
not access their arguments after sending a result along
cpid. Thus, programs that malloc the argv passed to
threadexec can safely free it once they have received the
cpid response.
Threadexecl and threadexec will duplicate (see dup(3)) the
three file descriptors in fd onto standard input, output,
and error for the external program and then close them in
the calling thread. Beware of code that sets
fd[0] = 0;
fd[1] = 1;
fd[2] = 2;
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THREAD(3) THREAD(3)
to use the current standard files. The correct code is
fd[0] = dup(0, -1);
fd[1] = dup(1, -1);
fd[2] = dup(2, -1);
Threadspawnl and threadspawn are like threadexecl and
threadexec but do not replace the current thread. They
return the pid of the invoked program on success, or -1 on
error. Threadspawnd is like threadspawn but takes as its
final argument the directory in which to run the invoked
program. The child will attempt to change into that direc-
tory before running the program, but it is only best effort:
failure to change into the directory does not stop the run-
ning of the program.
Threadwaitchan returns a channel of pointers to Waitmsg
structures (see wait(3)). When an exec'ed process exits, a
pointer to a Waitmsg is sent to this channel. These Waitmsg
structures have been allocated with malloc(3) and should be
freed after use.
A Channel is a buffered or unbuffered queue for fixed-size
messages. Procs and threads send messages into the channel
and recv messages from the channel. If the channel is
unbuffered, a send operation blocks until the corresponding
recv operation occurs and vice versa. Chancreate allocates a
new channel for messages of size elsize and with a buffer
holding nel messages. If nel is zero, the channel is
unbuffered. Chanfree frees a channel that is no longer
used. Chanfree can be called by either sender or receiver
after the last item has been sent or received. Freeing the
channel will be delayed if there is a thread blocked on it
until that thread unblocks (but chanfree returns immedi-
ately).
The name element in the Channel structure is a description
intended for use in debugging. Chansetname sets the name.
Send sends the element pointed at by v to the channel c. If
v is null, zeros are sent. Recv receives an element from c
and stores it in v. If v is null, the received value is dis-
carded. Send and recv return 1 on success, -1 if inter-
rupted. Nbsend and nbrecv behave similarly, but return 0
rather than blocking.
Sendp, nbsendp, sendul, and nbsendul send a pointer or an
unsigned long; the channel must have been initialized with
the appropriate elsize. Recvp, nbrecvp, recvul, and nbrecvul
receive a pointer or an unsigned long; they return zero when
a zero is received, when interrupted, or (for nbrecvp and
nbrecvul) when the operation would have blocked. To
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THREAD(3) THREAD(3)
distinguish between these three cases, use recv or nbrecv.
Alt can be used to recv from or send to one of a number of
channels, as directed by an array of Alt structures, each of
which describes a potential send or receive operation. In
an Alt structure, c is the channel; v the value pointer
(which may be null); and op the operation: CHANSND for a
send operation, CHANRECV for a recv operation; CHANNOP for
no operation (useful when alt is called with a varying set
of operations). The array of Alt structures is terminated
by an entry with op CHANEND or CHANNOBLK. If at least one
Alt structure can proceed, one of them is chosen at random
to be executed. Alt returns the index of the chosen struc-
ture. If no operations can proceed and the list is termi-
nated with CHANNOBLK, alt returns the index of the terminat-
ing CHANNOBLK structure. Otherwise, alt blocks until one of
the operations can proceed, eventually returning the index
of the structure executes. Alt returns -1 when interrupted.
The tag and entryno fields in the Alt structure are used
internally by alt and need not be initialized. They are not
used between alt calls.
Chanprint formats its arguments in the manner of print(3)
and sends the result to the channel c. The string delivered
by chanprint is allocated with malloc(3) and should be freed
upon receipt.
Thread library functions do not return on failure; if errors
occur, the entire program is aborted.
Threaded programs should use threadnotify in place of
atnotify (see notify(3)).
It is safe to use sysfatal(3) in threaded programs.
Sysfatal will print the error string and call
threadexitsall.
It is not safe to call rfork in a threaded program, except
to call rfork(RFNOTEG) from the main proc before any other
procs have been created. To create new processes, use
proccreate.
FILES
/usr/local/plan9/acid/thread contains useful acid(1) func-
tions for debugging threaded programs.
/usr/local/plan9/src/libthread/test contains some example
programs.
SOURCE
/usr/local/plan9/src/libthread
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SEE ALSO
intro(3), ioproc(3)
BUGS
To avoid name conflicts, alt, nbrecv, nbrecvp, nbrecvul,
nbsend, nbsendp, nbsendul, recv, recvp, recvul, send, sendp,
and sendul are defined as macros that expand to chanalt,
channbrecv, and so on. Yield is defined as a macro that
expands to threadyield. See intro(3).
Threadint, threadintgrp, threadkill, threadkillgrp and
threadpid are unimplemented.
The implementation of threadnotify may not be correct.
There appears to be a race in the Linux NPTL implementation
of pthread_exit . Call threadexitsall rather than coordinat-
ing a simultaneous threadexits among many threads.
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