[英]Pthreads, confusion with pthread_join(pthread_t, void**)
我不明白为什么pthread_join
将第二个参数void**
作为返回值,而给定返回值的pthread_exit
将返回值参数作为void*
。
pthread_join waits for the thread to end, and the resulting value from pthread_exit is stored into *value_ptr. pthread_join等待线程结束,并将pthread_exit的结果值存储到* value_ptr中。 If you want to ignore the result, you can pass NULL for the value_ptr.
如果要忽略结果,则可以将NULL传递给value_ptr。 This is the common C practice of simulating pass by reference by passing a pointer to a variable.
这是通过将指针传递给变量来模拟按引用传递的通用C惯例。 See Passing by reference in C
请参见在C中通过引用传递
The pthread_join returns 0 as the function return value on success; pthread_join成功时返回0作为函数返回值;否则返回0。 then you know that the thread has been joined, and you can access the value from *value_ptr.
那么您就知道该线程已加入,您可以从* value_ptr中访问该值。
void *value = NULL;
if (pthread_join(thread, &value) == 0) {
// thread has ended, and the exit value is available in
// the value variable
}
Essentially, pthread_join()
wants to return two peices of information: 本质上,
pthread_join()
要返回两个信息:
void*
) void*
) In C, the typical way a function 'returns' two separate values is to have the function return one of those values normally and to 'return' the ofther value in a location provided by the caller where the caller passes in a pointer to that location. 在C语言中,函数“返回”两个单独值的典型方式是使函数正常返回这些值之一,并在调用者提供的位置(调用者将指针传递到该位置)中“返回”另一个值。 。 So
pthread_join()
: 所以
pthread_join()
:
void*
result in a caller-provided location that the caller passes a void**
to. void*
导致调用者将void**
传递给调用者提供的位置。 Note that in pthread_join()
's case, the caller-provided location is optional. 请注意,在
pthread_join()
的情况下,调用方提供的位置是可选的。 NULL can be passed in id the caller isn't interested in that result. 可以在呼叫者对该结果不感兴趣的id中传递NULL。 Making the caller-provided location optional is a common, by by no means universal, idiom.
将呼叫者提供的位置设置为可选是很普遍的,绝不是通用的。
pthread_exit()
doesn't need to use a void**
as it's argument because that argument isn't a result of the function. pthread_exit()
不需要使用void**
作为参数,因为该参数不是函数的结果 。 So it can simply take the value directly. 因此,它可以直接直接取值。
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