pthread_once - Calls an initialization routine to be executed by a single thread, once.
DECthreads POSIX 1003.1c Library (libpthread.so)
Interfaces documented on this reference page conform to industry standards as follows:
IEEE Std 1003.1c-1995, POSIX System Application Program Interface
Address of a record that defines the one-time initialization code. Each one-time initialization routine must have its own unique pthread_once_t record. Address of a procedure that performs the initialization. This routine is called only once, regardless of the number of times it and its associated once_control are passed to pthread_once(3).
The first call to this routine by any thread in a process with a given once_control will call the specified init_routine with no arguments. Subsequent calls to pthread_once(3) with the same once_control will not call the init_routine. On return from pthread_once(3), it is guaranteed that the initialization routine has completed.
For example, a mutex or a per-thread context key must be created exactly once. Calling pthread_once(3) ensures that the initialization is serialized across multiple threads. Other threads that reach the same point in the code would be delayed until the first thread is finished.
If you specify an init_routine that directly or indirectly results in a recursive call to pthread_once(3) and that specifies the same init_routine argument, the recursive call can result in a deadlock.
To initialize the once_control record, your program can zero out the entire structure, or you can use the PTHREAD_ONCE_INIT macro, which is defined in the pthread.h header file, to statically initialize that structure. If using PTHREAD_ONCE_INIT, declare the once_control record as follows:
pthread_once_t once_control = PTHREAD_ONCE_INIT;
Note that it is often easier to simply lock a statically initialized mutex,
check a control flag, and perform necessary initialization (in-line) rather
than using pthread_once(3). For example, code an initialization routine
that begins with the following basic logic:
static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
static int flag = FALSE;
flag = TRUE;
/* initialize code */
If an error condition occurs, this routine returns an integer indicating the type of error. Possible return values are as follows: Successful completion. Invalid argument.
Manuals: Guide to DECthreads and Programmer's Guide