C代码如何调用汇编代码（例如，优化的strlen）？(How does C code call assembly code (e.g. optimized strlen)?)

 我总是阅读有关C编程语言中某些函数如何通过汇编写入来优化的内容。 如果这句话听起来有点误导，让我道歉。 
 
 所以，我会说清楚：当你在UNIX / C系统上调用像strlen这样的函数时，你调用的实际函数是用汇编语言编写的？ 你能否以某种方式将程序集编写成C程序，还是外部呼叫情况？ 它能够做到这一点是C标准的一部分，还是它是一个特定于操作系统的东西？ 

I always read things about how certain functions within the C programming language are optimized by being written in assembly. Let me apologize if that sentence sounds a little misguided.
 
So, I'll put it clearly: How is it that when you call some functions like strlen on UNIX/C systems, the actual function you're calling is written in assembly? Can you write assembly right into C programs somehow or is it an external call situation? Is it part of the C standard to be able to do this, or is it an operating system specific thing?

原文：https://stackoverflow.com/questions/7300160