How does sig_atomic_t actually work?

Question

How does the compiler or OS distinguish between sig_atomic_t type and a normal int type variable, and ensures that the operation will be atomic? Programs using both have sam

Answer 1

Programs using both have same assembler code. How extra care is taken to make the operation atomic?

Although this is an old question, I think it's still worth addressing this part of the question specifically. On Linux, sig_atomic_t is provided by glibc. sig_atomic_t in glibc is a typedef for int and has no special treatment (as of this post). The glibc docs address this:

In practice, you can assume that int is atomic. You can also assume that pointer types are atomic; that is very convenient. Both of these assumptions are true on all of the machines that the GNU C Library supports and on all POSIX systems we know of.

In other words, it just so happens that regular int already satisfies the requirements of sig_atomic_t on all the platforms that glibc supports and no special support is needed. Nonetheless, the C and POSIX standards mandate sig_atomic_t because there could be some exotic machine on which we want to implement C and POSIX for which int does not fulfill the requirements of sig_atomic_t.