thread(3) - Plan 9 from User Space

| 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) 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 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 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`).
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 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
| `fd[0] = 0; fd[1] = 1; fd[2] = 2;
`
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 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.
FILES
| `/usr/lib/plan9/acid/thread` contains useful acid(1) functions for debugging threaded programs.
`/usr/lib/plan9/src/libthread/test` contains some example programs.
SOURCE
| `/usr/lib/plan9/src/libthread`
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 coordinating a simultaneous threadexits among many threads.