UNIX Portable Atomic Operations

Question

Is there a (POSIX-)portable way in C for atomic variable operations similar to a portable threading with pthread?

Atomic operations are operations like "increment and get" that are executed atomically that means that no context switch can interfere with the operation. In Linux kernel space, we have to atomic_t type, in Java we have the java.util.concurrent.atomic package.

On Linux, the atomic.h file provides atomic operations, but the include is platform dependent e.g. #include <asm-x86_64/atomic.h> and it is not available on Mac OS X in a similar way.

Answer 1

As of C11 there is an optional Atomic library which provides atomic operations. This is portable to whatever platform that has a C11 compiler (like gcc-4.9) with this optional feature.

The presence of the atomic can be checked with __STDC_NO_ATOMICS__and the presence of <stdatomic.h>

atomic.c

#include <stdio.h>
#include <stdlib.h>
#ifndef __STDC_NO_ATOMICS__
#include <stdatomic.h>
#endif

int main(int argc, char**argv) {
    _Atomic int a;
    atomic_init(&a, 42);
    atomic_store(&a, 5);
    int b = atomic_load(&a);
    printf("b = %i\n", b);

    return EXIT_SUCCESS;
}

Compiler invocations

clang -std=c11 atomic.c
gcc -std=c11 atomic.c