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NAME

dispatch_once — execute a block only once

SYNOPSIS

#include <dispatch/dispatch.h>

void
dispatch_once(dispatch_once_t *predicate, void (^block)(void));

void
dispatch_once_f(dispatch_once_t *predicate, void *context, void (*function)(void *));

DESCRIPTION

The dispatch_once() function provides a simple and efficient mechanism to run an initializer exactly once, similar to pthread_once(3). Well designed code hides the use of lazy initialization. For example:

FILE *getlogfile(void)
{
	static dispatch_once_t pred;
	static FILE *logfile;

	dispatch_once(&pred, ^{
		logfile = fopen(MY_LOG_FILE, "a");
	});

	return logfile;
}

SCOPE OF PREDICATE

The variable of type dispatch_once_t must have global or static scope. The result of using this type with automatic or dynamic allocation is undefined.

FUNDAMENTALS

The dispatch_once() function is a wrapper around dispatch_once_f().

SEE ALSO

dispatch(3)