How do the likely/unlikely macros in the Linux kernel work and what is their benefit?

Question

I\'ve been digging through some parts of the Linux kernel, and found calls like this:

if (unlikely(fd < 0))
{
    /* Do something */
}

o

Answer 1

(general comment - other answers cover the details)

There's no reason that you should lose portability by using them.

You always have the option of creating a simple nil-effect "inline" or macro that will allow you to compile on other platforms with other compilers.

You just won't get the benefit of the optimization if you're on other platforms.