added OS theory code

OS/C/theory/sync/withsema.c

@@ -0,0 +1,32 @@
+#include <stdio.h>
+#include <pthread.h>
+#include <semaphore.h>
+#include <sys/semaphore.h>
+
+int counter = 0; // shared resource
+sem_t semaphore; // semaphore declaration
+
+void* increment(void* arg) {
+    for (int i = 0; i < 100000; i++) {
+        sem_wait(&semaphore); // lock
+        counter++; // critical section
+        sem_post(&semaphore); // unlock
+    }
+    return NULL;
+}
+
+int main() {
+    pthread_t t1, t2;
+    sem_init(&semaphore, 0, 1); // initialize semaphore with value 1
+
+    pthread_create(&t1, NULL, increment, NULL);
+    pthread_create(&t2, NULL, increment, NULL);
+    pthread_join(t1, NULL);
+    pthread_join(t2, NULL);
+
+    sem_destroy(&semaphore);
+
+    printf("Final Counter Value (with semaphores): %d\n", counter);
+
+    return 0;
+}