init CAS example

threads-locks/README.md

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+
+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
+```
+
+

threads-locks/compare-and-swap.c

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+#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;
+}
+