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NAME
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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, threadmaybackground, threadnotify, threadid, threadpid,
threadpin, threadunpin, threadsetgrp, threadsetname, threadsetstate,
threadspawn, threadspawnd, threadspawnl, threadwaitchan, yield
– thread and proc management
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SYNOPSIS
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#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 {
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Channel *c;
void *v;
int op;
Channel **tag;
int entryno;
char *name;
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};
void threadmain(int argc, char *argv[])
int threadmaybackground(void)
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)
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)
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DESCRIPTION
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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 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 executing threadmain.
Every thread is backed by an operating system-provided pthread
and runs on its system-provided stack; mainstacksize and the the
stack size arguments to proccreate and threadcreate are ignored.
Although each thread is backed by a separate pthread, the threads
in a proc are still scheduled non-preemptively as on Plan 9 and
as described below.
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. This functionality
can only be relied upon if the program defines a function threadmaybackground
returning a non-zero result. Programs that do not define such
a
threadmaybackground will crash instead should the last thread
in threadmain’s proc exit leaving behind other running procs.
The threads in a proc are coroutines, scheduled nonpreemptively
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 descriptions 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 carefully, 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 current 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 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 associated
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 calling 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 executing.) 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
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fd[0] = 0;
fd[1] = 1;
fd[2] = 2;
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to use the current standard files. The correct code is
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fd[0] = dup(0, −1);
fd[1] = dup(1, −1);
fd[2] = dup(2, −1);
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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 directory before
running the program, but it is only best effort: failure to change
into the directory does not stop the running 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 immediately).
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 discarded. Send and
recv return 1 on success, –1 if interrupted. 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 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 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 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.
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FILES
SOURCE
SEE ALSO
BUGS
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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 coordinating a
simultaneous threadexits among many threads.
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