(This is the documentation for SDL3, which is the current stable version. SDL2 was the previous version!)

SDL_CreateSemaphore

Create a semaphore.

Header File

Syntax

SDL_Semaphore * SDL_CreateSemaphore(Uint32 initial_value);

Function Parameters

Uint32

initial_value

the starting value of the semaphore.

Return Value

(SDL_Semaphore *) Returns a new semaphore or NULL on failure; call SDL_GetError() for more information.

Remarks

This function creates a new semaphore and initializes it with the value initial_value. Each wait operation on the semaphore will atomically decrement the semaphore value and potentially block if the semaphore value is 0. Each post operation will atomically increment the semaphore value and wake waiting threads and allow them to retry the wait operation.

Version

This function is available since SDL 3.1.3.

Code Examples

Typical use of semaphores:

void add_data_to_queue(void);
void get_data_to_queue(void);
void get_data_from_queue(void);
int data_available(void);
void wait_for_threads(void);

SDL_AtomicInt done;
SDL_Semaphore *sem;

SDL_SetAtomicInt(&done, 0);
sem = SDL_CreateSemaphore(0);


Thread_A:
    while (!SDL_GetAtomicInt(&done)) {
        add_data_to_queue();
        SDL_SignalSemaphore(sem);
    }

Thread_B:
    while (!SDL_GetAtomicInt(&done)) {
        SDL_WaitSemaphore(sem);
        if (data_available()) {
            get_data_from_queue();
        }
    }


SDL_SetAtomicInt(&done, 1);
SDL_SignalSemaphore(sem);
wait_for_threads();
SDL_DestroySemaphore(sem);