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