tis_once - Calls an initialization routine that can be executed by only one thread, once.
Standard C Library (libc.so, libc.a)
Address of a record (control block) that defines the one-time initialization code. Each one-time initialization routine in static storage 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 tis_once(3).
The first call to this routine by a process with a given once_control calls the init_routine with no arguments. Thereafter, subsequent calls to tis_once(3) with the same once_control do not call the init_routine. On return from tis_once(3), it is guaranteed that the initialization routine has completed.
For example, a mutex or a thread-specific data key must be created exactly once. In a threaded environment, calling tis_once(3) ensures that the initialization is serialized across multiple threads.
The once_control argument must be statically initialized using the PTHREAD_ONCE_INIT macro or by zeroing out the entire structure.
If you specify an init_routine that directly or indirectly results in a recursive call to tis_once(3) and that specifies the same init_block argument, the recursive call results in a deadlock.
The PTHREAD_ONCE_INIT macro, defined in the <pthread.h> header file, must be used to initialize a once_control record. Thus, your program must declare a 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 tis_once(3). For example, you can code an "init" routine
that begins with the following basic logic:
static pthread_mutex_t mutex = PTHREAD_MUTEX_INIT;
static int flag = FALSE;
flag = TRUE;
/* initialize code */
If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows: Successful completion. Invalid argument.
Manuals: Guide to DECthreads and Programmer's Guide