GLIB: g_atomic_int_get becomes NO-OP?

问题:

In a larger piece of code, I noticed that the g_atomic_* functions in glib were not doing what I expected, so I wrote this simple example:

#include <stdlib.h> #include "glib.h" #include "pthread.h" #include "stdio.h"  void *set_foo(void *ptr) {   g_atomic_int_set(((int*)ptr), 42);   return NULL; }  int main(void) {   int foo = 0;   pthread_t other;    if (pthread_create(&other, NULL, set_foo, &foo)== 0) {     pthread_join(other, NULL);     printf("Got %d\n", g_atomic_int_get(&foo));   } else {     printf("Thread did not run\n");     exit(1);   } } 

When I compile this with GCC's '-E' option (stop after pre-processing), I notice that the call to g_atomic_int_get(&foo) has become:

(*(&foo)) 

and g_atomic_int_set(((int*)ptr), 42) has become:

((void) (*(((int*)ptr)) = (42))) 

Clearly I was expecting some atomic compare and swap operations, not just simple (thread-unsafe) assignments. What am I doing wrong?

For reference my compile command looks like this:

gcc -m64 -E -o foo.E `pkg-config --cflags glib-2.0` -O0 -g foo.c 

回答1:

The architecture you are on does not require a memory barrier for atomic integer set/get operations, so the transformation is valid.

Here's where it's defined: http://git.gnome.org/browse/glib/tree/glib/gatomic.h#n60

This is a good thing, because otherwise you'd need to lock a global mutex for every atomic operation.

文章来源: GLIB: g_atomic_int_get becomes NO-OP?