Why is there a while loop in wait function of a semaphore, when if can be used too?

this is my code:

wait(){
    while(S<=0)
       //puts the thread in the block list until it wakes up(by calling post)
    S = S-1
 }

there is a while loop in the wait function of a semaphore, can’t I use an if statement simply?

Answer

Because we can’t assume that after a thread is woken up and it requires the lock another thread has not already come along and taken the resource this is guarding:

wait(){ 
    Some  lock_guard(mutex);         // You lock here.

    while(S<=0) {
        condition.wait(lock_guard);  // While you wait here
                                     // the lock is released.
                                     // when the condition/semaphore is signalled
                                     // one or more threads may be released
                                     // but they must aquire the lock before
                                     // they return from wait.
                                     //
                                     // Another thread may enter this function
                                     // aquire the lock and decrement S below
                                     // before the waiting thread aquires the
                                     // lock and thus mustbe resuspended.
    }
    S = S-1
 }