SDL Wiki
(This is the documentation for SDL3, which is under heavy development and the API is changing! SDL2 is the current stable version!)


Wait until a semaphore has a positive value and then decrements it.


int SDL_SemWait(SDL_sem *sem);

Function Parameters

sem the semaphore wait on

Return Value

Returns 0 on success or a negative error code on failure; call SDL_GetError() for more information.


This function suspends the calling thread until either the semaphore pointed to by sem has a positive value or the call is interrupted by a signal or error. If the call is successful it will atomically decrement the semaphore value.

This function is the equivalent of calling SDL_SemWaitTimeout() with a time length of SDL_MUTEX_MAXWAIT.


This function is available since SDL 3.0.0.

Code Examples

#define NB_WAITER 10

SDL_sem *sem;

// Increments the semaphore every 2s
int poster_thread() {
  for (int i = 0; i < NB_WAITER; i++) {
    SDL_Delay(2 * 1000);
  return 0;

int waiter_thread() {
  int status;

  status = SDL_SemWait(sem);
  if (status == 0) {
    SDL_Log("Semaphore was decremented.\n");
  } else {
    SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "An error has occured while waiting: %s\n", SDL_GetError());
  return 0;

int main() {
  sem = SDL_CreateSemaphore(0);
  create_and_wait_threads(); // 1 poster, 10 waiters

CategoryAPI, CategoryMutex

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