THREAD(2) THREAD(2)
NAME
alt, chanclose, chancreate, chanfree, chanclosing,
chanprint, mainstacksize, proccreate, procdata, procexec,
procexecl, procrfork, recv, recvp, recvul, send, sendp,
sendul, nbrecv, nbrecvp, nbrecvul, nbsend, nbsendp,
nbsendul, threadcreate, threaddata, threadexits,
threadexitsall, threadgetgrp, threadgetname, threadint,
threadintgrp, threadkill, threadkillgrp, threadmain,
threadnotify, threadid, threadpid, threadsetgrp,
threadsetname, threadwaitchan, yield - thread and proc
management
SYNOPSIS
#include <u.h>
#include <libc.h>
#include <thread.h>
typedef enum {
CHANEND,
CHANSND,
CHANRCV,
CHANNOP,
CHANNOBLK,
} ChanOp;
typedef struct Alt Alt;
struct Alt {
Channel *c; /* channel */
void *v; /* pointer to value */
ChanOp op; /* operation */
char *err; /* did the op fail? */
/*
* the next variables are used internally to alt
* they need not be initialized
*/
Channel **tag; /* pointer to rendez-vous tag */
int entryno; /* entry number */
};
void threadmain(int argc, char *argv[])
int mainstacksize
int proccreate(void (*fn)(void*), void *arg, uint stacksize)
int procrfork(void (*fn)(void*), void *arg, uint stacksize,
int rforkflag)
int threadcreate(void (*fn)(void*), void *arg, uint stacksize)
void threadexits(char *status)
void threadexitsall(char *status)
void yield(void)
int threadid(void)
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THREAD(2) THREAD(2)
int threadgetgrp(void)
int threadsetgrp(int group)
int threadpid(int id)
void threadint(int id)
void threadintgrp(int group)
void threadkill(int id)
void threadkillgrp(int group)
void threadsetname(char *fmt, ...)
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 chanclose(Channel *c);
int chanclosing(Channel *c);
void procexecl(Channel *cpid, char *file, ...)
void procexec(Channel *cpid, 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, communicat-
ing and synchronizing through channels and shared variables.
A proc is a Plan 9 process that contains one or more
cooperatively-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
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THREAD(2) THREAD(2)
eight kilobytes). To set mainstacksize, declare a global
variable initialized to the desired value (e.g., int
mainstacksize = 1024).
Creation
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.
Procrfork creates a new proc, and inside that proc creates a
single thread as threadcreate would, returning the id of the
created thread. Procrfork creates the new proc by calling
rfork (see fork(2)) with flags
RFPROC|RFMEM|RFNOWAIT|rforkflag. (The thread library
depends on all its procs running in the same rendezvous
group. Do not include RFREND in rforkflag.) Proccreate is
identical to procrfork with rforkflag set to zero. 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 func-
tion 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.
Scheduling
The threads in a proc are coroutines, scheduled non-
preemptively 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, procexec, procexecl, threadexits,
alt, send, and recv (and the calls related to send and
recv-see their descriptions further on), plus these from
lock(2): qlock, rlock, wlock, rsleep. Procs are scheduled by
the operating system. Therefore, threads in different procs
can preempt one another in arbitrary ways and should syn-
chronize their actions using qlocks (see lock(2)) or channel
communication. System calls such as read(2) block the
entire proc; all threads in a proc block until the system
call finishes.
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.
Threadgetgrp returns the group id for the current thread;
threadsetgrp sets it. Threadpid returns the pid of the Plan
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THREAD(2) THREAD(2)
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.
Names and per-thread data
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.
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.
Executing new programs
Procexecl and procexec are threaded analogues of exec and
execl (see exec(2)); on success, they replace the calling
thread (which must be the only thread in its proc) and
invoke the external program, never returning. On error,
they return and set errstr. If cpid is not null, the pid of
the invoked program will be sent along cpid once the program
has been started, or -1 will be sent if an error occurs.
Procexec and procexecl will not access their arguments after
sending a result along cpid. Thus, programs that malloc the
argv passed to procexec can safely free it once they have
received the cpid response.
Threadwaitchan returns a channel of pointers to Waitmsg
structures (see wait(2)). When an exec'ed process exits, a
pointer to a Waitmsg is sent to this channel. These Waitmsg
structures have been allocated with malloc(2) and should be
freed after use.
Channels
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
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THREAD(2) THREAD(2)
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).
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 distin-
guish 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, CHANRCV 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 structure.
If no operations can proceed and the list is terminated with
CHANNOBLK, alt returns the index of the terminating
CHANNOBLK structure. Otherwise, alt blocks until one of the
operations can proceed, eventually returning the index of
the structure it 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(2)
and sends the result to the channel c. The string delivered
by chanprint is allocated with malloc(2) and should be freed
upon receipt.
Chanclose prevents further elements being sent to the chan-
nel c. After closing a channel, send and recv never block.
Send always returns -1. Recv returns -1 if the channel is
empty. Alt may choose a CHANSND or CHANRCV that failed
because the channel was closed. In this case, the err field
of the Alt entry points to an error string stating that the
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THREAD(2) THREAD(2)
channel was closed and the operation was completed with
failure. If all entries have been selected and failed
because they were closed, alt returns -1.
Errors, notes and resources
Thread library functions do not return on failure; if errors
occur, the entire program is aborted.
Chanclosing returns -1 if no one called chanclose on the
channel, and otherwise the number of elements still in the
channel.
Threaded programs should use threadnotify in place of
atnotify (see notify(2)).
It is safe to use sysfatal (see perror(2)) in threaded pro-
grams. Sysfatal will print the error string and call
threadexitsall.
It is safe to use rfork (see fork(2)) to manage the names-
pace, file descriptors, note group, and environment of a
single process. That is, it is safe to call rfork with the
flags RFNAMEG, RFFDG, RFCFDG, RFNOTEG, RFENVG, and RFCENVG.
(To create new processes, use proccreate and procrfork.) As
mentioned above, the thread library depends on all procs
being in the same rendezvous group; do not change the ren-
dezvous group with rfork.
FILES
/sys/lib/acid/thread useful acid(1) functions for debugging
threaded programs.
/sys/src/libthread/example.c
a full example program.
SOURCE
/sys/src/libthread
SEE ALSO
intro(2), ioproc(2), lock(2)
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