add sem_create(), wrappign sem_init() for linux and sem_open() for macOS

src/platform.h

@@ -2,13 +2,20 @@
 #define _PLATFORM_H_
 
 #include <pthread.h>
-
-#ifndef PSEMNAMLEN	/* defined in macOS, but not in Linux */
-#define PSEMNAMLEN	31
-#endif
+#include <semaphore.h>
 
 int nr_cpus(void);
 int set_thread_affinity(pthread_t tid, int core);
-int get_random(int max);
+
+/*
+ * macOS does not support sem_init(). macOS (seems to) releases the
+ * named semaphore when associated mscp process finished. In linux,
+ * program (seems to) need to release named semaphore in /dev/shm by
+ * sem_unlink() explicitly. So, using sem_init() (unnamed semaphore)
+ * in linux and using sem_open() (named semaphore) in macOS without
+ * sem_unlink() are reasonable (?).
+ */
+sem_t *sem_create(int value);
+int sem_release(sem_t *sem);
 
 #endif /* _PLATFORM_H_ */