pthreadonce

pthread_once is a synchronization function in POSIX threads (pthreads) that allows a variable to be initialized only once in a multi-threaded program. It provides a way to ensure that a given block of code is executed only once in a program, regardless of the number of threads.

The pthread_once function takes two arguments: the name of the variable to be initialized, and a control structure containing initialization code. When the function is called, it checks if the variable has already been initialized. If it has, the function returns immediately. If it has not, the function proceeds with the initialization code, sets the variable to an initialized state, and returns.

pthread_once is particularly useful in situations where avoiding unnecessary repetition of initialization code is critical. For example, it can be used to initialize a statically allocated object in C or C++, where multiple threads may attempt to initialize the object simultaneously.

pthread_once is a low-level synchronization function that provides a basic building block for more complex synchronization functionality. It can be used in conjunction with other synchronization primitives, such as mutexes and semaphores, to implement more sophisticated synchronization policies.

The pthread_once function is defined in the `pthread.h` header file and is implemented as a POSIX thread primitive. Its use is generally discouraged in favor of higher-level synchronization abstractions, such as `tod_initonce` (the multidomain version) or `m_ignored` in applications where security is a concern. However, in certain specialized contexts, pthread_once can still be a useful tool for avoiding unnecessary overhead introduced by higher-level synchronization interfaces.