recursive_mutex的最大所有权的下限？ [英] Lower bound for the maximum level of ownership for recursive_mutex?

问题描述

引用 [thread.mutex.recursive] ：

拥有 recursive_mutex 对象的线程可以通过调用 lock（）或 try_lock（）。未指定单个线程可以获取多少级别的所有权。如果线程已经获得了 recursive_mutex 对象的最大所有权级别，对 try_lock（）的额外调用将​​失败，并且对 lock（）的额外调用将​​引发类型 system_error 的异常。

A thread that owns a recursive_mutex object may acquire additional levels of ownership by calling lock() or try_lock() on that object. It is unspecified how many levels of ownership may be acquired by a single thread. If a thread has already acquired the maximum level of ownership for a recursive_mutex object, additional calls to try_lock() shall fail, and additional calls to lock() shall throw an exception of type system_error.

最高拥有权级别的下限是否大于1？对于递归的pthread互斥体呢？

Is there a lower bound greater than 1 for the "maximum level of ownership"? What about recursive pthread mutexes?

推荐答案

标准中没有指定下限。这可能是故意的。

There is no lower limit specified in the standard. This is probably deliberate.

旧标准（我认为）用来为这样的事情提供较低的限制。结果是人们编写编码标准，说你不能使用超过这些下限。例如：这是（我认为仍然是）实现定义外部符号的多少个字符在比较相等时有意义。因此， a_very_ouble_long_name_indeed_with_extra_padding 和 a_very_very_long_name_indeed_with_extra_paddingX 可能被视为相同的符号。最小长度被指定为8，并且编码标准被写成指定外部符号的最大长度是八个字符。

Older standards (C I think) did use to provide lower limits for things like this. The result was that people wrote coding standards which said you couldn't use more than these lower limits. For example: It was (and I think still is) implementation defined how many characters of an external symbol were significant when comparing for equality. So a_very_very_long_name_indeed_with_extra_padding and a_very_very_long_name_indeed_with_extra_paddingX might be treated as the same symbol. The minimum length was specified as 8, and coding standards were written specifying "maximum length of a external symbol is eight characters".

在该值的合理的下限：我可以容易地想象，计数可能被打包成其他字段，以便整个事情可以用一个合适的指令进行原子性更新。因此，它可能是一个不到32位的交易。 （它只需要足够大的最大调用堆栈深度，因此在受限环境中，31可能是足够好的。）

On the plausible lower bound for this value: I can easily imagine that the count might be bit-packed into some other field so that the whole thing can be updated atomically with a suitable instruction. As such, it might be a good deal less than 32 bits. (It only really needs to be large enough for the maximum call stack depths, so in a constrained environment, 31 might be good enough.)

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