sem_init(...):pshared 参数有什么用? [英] sem_init(...): What is the pshared parameter for?

问题描述

在研究生课程中，我们不得不使用信号量来完成线程工作.

In a graduate class, we've had to use semaphores to accomplish work with threads.

我们被指示将 sem_init 与其他一些 sem_* 过程一起使用，但我们没有得到关于这些 sem_* 方法中每一个的详细信息的太多信息.

We were directed to use sem_init along with a bunch of other sem_* procedure but we were not given much information about the details of each of these sem_* methods.

sem_init 的原型(和头文件)是 以下:

The prototype (and header file) of sem_init is the following:

#include <semaphore.h>

int sem_init(sem_t *sem, int pshared, unsigned int value);

但我不明白 pshared 值的用途.根据 opengroup.org:

but I don't understand what the pshared value is used for. According to opengroup.org:

如果 pshared 参数有一个非零值，然后共享信号量进程之间；在这种情况下，任何可以访问信号量的进程sem 可以使用 sem 来执行sem_wait()、sem_trywait()、sem_post()、和 sem_destroy() 操作.

If the pshared argument has a non-zero value, then the semaphore is shared between processes; in this case, any process that can access the semaphore sem can use sem for performing sem_wait(), sem_trywait(), sem_post(), and sem_destroy() operations.

但我想我不明白 1,2, 10, 25, 50000 等之间的区别.我认为这是说如果值为 0 则不共享信号量.(但是，重点是什么?)

but I guess I don't understand the difference between say 1,2, 10, 25, 50000, etc. I think it is saying that if the value is 0 then the semaphore is not shared. (But then, what is the point?)

如何恰当地使用这个 pshared 参数?

How do I appropriately use this pshared parameter?

推荐答案

sem_init 的 GLIBC 版本(如果你在 Linux 上 man sem_init 会得到什么)有这个说:

The GLIBC version of sem_init (what you get if you man sem_init on Linux) has this to say:

"pshared 参数表明这个信号量是否被在进程的线程之间或进程之间共享."

"The pshared argument indicates whether this semaphore is to be shared between the threads of a process, or between processes."

所以 pshared 是一个布尔值:实际上传递给它的有意义的值是 false (0) 和 true (1)，但任何非 0 值都将被视为真.如果传递它 0，您将获得一个信号量，该信号量可以被同一进程中的其他线程访问——本质上是一个进程内锁.您可以将其用作互斥锁，也可以更广泛地将其用于信号量的资源计数属性.可以说，如果 pthreads 支持信号量 API，你就不需要 sem_init 的这个特性，但是 Unix 中的信号量比 pthreads 早了很多时间.

So pshared is a boolean value: in practice meaningful values passed to it are false (0) and true (1), though any non-0 value will be treated as true. If you pass it 0 you will get a semaphore that can be accessed by other threads in the same process -- essentially an in-process lock. You can use this as a mutex, or you can use it more generally for the resource-counting properties of a semaphore. Arguably if pthreads supported a semaphore API you wouldn't need this feature of sem_init, but semaphores in Unix precede pthreads by quite a bit of time.

如果布尔值是某种枚举会更好(例如 SEM_PROCESS_PRIVATE 与 SEM_PROCESS_SHARED)，因为那样你就不会遇到这个问题，但是 POSIX 信号量随着这些事情的发展，它们是一个相当古老的 API.

It would be better if the boolean was some kind of enumeration (e.g. SEM_PROCESS_PRIVATE vs SEM_PROCESS_SHARED), because then you wouldn't have had this question, but POSIX semaphores are a fairly old API as these things go.

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