__sync_val_compare_and_swap()

__sync_val_compare_and_swap() is a built-in function in GCC that performs an atomic compare-and-swap operation on a given memory location. This function compares the value of a memory location with a given old value and, if they match, replaces the old value with a new value. The function returns the original value of the memory location, allowing the caller to determine if the compare-and-swap operation was successful.

The function takes the following parameters:

- A pointer to the memory location to perform the operation on
- The old value to compare against
- The new value to replace the old value with

The function is useful in multi-threaded programming, where multiple threads may attempt to update the same memory location simultaneously. By using this function, only one thread will successfully update the memory location, while the others will receive an indication that the update failed. This can help prevent race conditions and other synchronization issues.