How does sig_atomic_t actually work?
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 same assembler code. How extra care is taken to make the operation atomic?
Answer
sig_atomic_t is not an atomic data type. It is just the data type that you are allowed to use in the context of a signal handler, that is all. So better read the name as "atomic relative to signal handling".
To guarantee communication with and from a signal handler, only one of the properties of atomic data types is needed, namely the fact that read and update will always see a consistent value. Other data types (such as perhaps long long) could be written with several assembler instructions for the lower and higher part, e.g. sig_atomic_t is guaranteed to be read and written in one go.
So a platform may choose any integer base type as sig_atomic_t for which it can make the guarantee that volatile sig_atomic_t can be safely used in signal handlers. Many platforms chose int for this, because they know that for them int is written with a single instruction.
The latest C standard, C11, has atomic types, but which are a completely different thing. Some of them (those that are "lockfree") may also be used in signal handlers, but that again is a completely different story.