sssgun · March 29, 2018 07:05 · Mar 29, 2018
diff --git a/gistfile1.txt b/gistfile1.txt
@@ -0,0 +1,17 @@
+#ifdef __cplusplus
+extern "C" {
+#endif
+        struct simplefu_semaphore {
+                int avail;
+                int waiters;
+        };
+
+        typedef struct simplefu_semaphore *simplefu;
+
+        void simplefu_down(simplefu who);
+        void simplefu_up(simplefu who);
+
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/main_futex.c b/main_futex.c
@@ -0,0 +1,34 @@
+#include <stdio.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <sys/mman.h>
+#include <stdlib.h>
+#include <assert.h>
+
+#include "simple_futex.h"
+
+int main()
+{
+        int lockfile = open("ipc_lock", O_RDWR);
+        assert(lockfile != -1);
+        simplefu sema = mmap(NULL, sizeof(*sema), PROT_READ|PROT_WRITE,
+                        MAP_SHARED, lockfile, 0);
+        assert(sema != MAP_FAILED);
+
+        int pid = fork();
+        assert(pid != -1);
+        if( pid == 0 ) { // child
+                printf("Initializing...\n");
+                sleep(10);
+                printf("Done initializing\n");
+                simplefu_up(sema);
+                exit(0);
+        }
+        sleep(15);
+        printf("Waiting for child...\n");
+        simplefu_down(sema);
+        printf("Child done initializing\n");
+
+        return 0;
+}
diff --git a/simple_futex.c b/simple_futex.c
@@ -0,0 +1,25 @@
+#include "simple_futex.h"
+#include <linux/futex.h>
+#include <sys/syscall.h>
+
+void simplefu_down(simplefu who)
+{
+        int val;
+        do {
+                val = who->avail;
+                if( val > 0 && __sync_bool_compare_and_swap(&who->avail, val, val - 1) )
+                        return;
+                __sync_fetch_and_add(&who->waiters, 1);
+                syscall(__NR_futex, &who->avail, FUTEX_WAIT, val, NULL, 0, 0);
+                __sync_fetch_and_sub(&who->waiters, 1);
+        } while(1);
+}
+
+void simplefu_up(simplefu who)
+{
+        int nval = __sync_add_and_fetch(&who->avail, 1);
+        if( who->waiters > 0 )
+        {
+                syscall(__NR_futex, &who->avail, FUTEX_WAKE, nval, NULL, 0, 0);
+        }
+}