48 lines
1.1 KiB
Markdown
48 lines
1.1 KiB
Markdown
|
||
A simple example of compare-and-swap shown in actual C code (which calls into
|
||
assembly).
|
||
|
||
The code, in entirety, is shown here:
|
||
|
||
```c
|
||
#include <stdio.h>
|
||
|
||
int global = 0;
|
||
|
||
char compare_and_swap(int *ptr, int old, int new) {
|
||
unsigned char ret;
|
||
// Note that sete sets a ’byte’ not the word
|
||
__asm__ __volatile__ (
|
||
" lock\n"
|
||
" cmpxchgl %2,%1\n"
|
||
" sete %0\n"
|
||
: "=q" (ret), "=m" (*ptr)
|
||
: "r" (new), "m" (*ptr), "a" (old)
|
||
: "memory");
|
||
return ret;
|
||
}
|
||
|
||
int main(int argc, char *argv[]) {
|
||
printf("before successful cas: %d\n", global);
|
||
int success = compare_and_swap(&global, 0, 100);
|
||
printf("after successful cas: %d (success: %d)\n", global, success);
|
||
|
||
printf("before failing cas: %d\n", global);
|
||
success = compare_and_swap(&global, 0, 200);
|
||
printf("after failing cas: %d (old: %d)\n", global, success);
|
||
|
||
return 0;
|
||
}
|
||
```
|
||
|
||
The first call to `compare_and_swap()` succeeds because the old value is
|
||
correct; the second call does not because the old value is wrong.
|
||
|
||
To compile and run:
|
||
```sh
|
||
prompt> gcc -o compare_and_swap compare_and_swap.c -Wall
|
||
prompt> ./compare_and_swap
|
||
```
|
||
|
||
|