我们可以使用信号量重新看一下上面的读/写程序。涉及信号量的操作是semaphore_up、semaphore_down、semaphore_init、semaphore_destroy和semaphore_decrement。所有这些操作都只有一个参数，一个指向信号量目标的指针。 void reader_function(void); void writer_function(void); char buffer ; Semaphore writers_turn; Semaphore readers_turn; main( ) { pthread_t reader; semaphore_init( &readers_turn ); semaphore_init( &writers_turn ); /* writer must go first */ semaphore_down( &readers_turn ); pthread_create( &reader, pthread_attr_default, (void *)&reader_function, NULL); w r i t e r _ f u n c t i o n ( ) ; } void writer_function(void) { w h i l e ( 1 ) { semaphore_down( &writers_turn ); b u ffer = make_new_item(); semaphore_up( &readers_turn ); } } void reader_function(void) { w h i l e ( 1 ) { semaphore_down( &readers_turn ); consume_item( buffer ); semaphore_up( &writers_turn ); } } 这个例子也没有完全地利用一般信号量的所有函数。我们可以使用信号量重新编写“Hello world” 的程序： void print_message_function( void *ptr ); Semaphore child_counter; Semaphore worlds_turn; main( ) { pthread_t thread1, thread2; char *message1 = "Hello"; char *message2 = "Wo r l d " ; semaphore_init( &child_counter ); semaphore_init( &worlds_turn ); semaphore_down( &worlds_turn ); /* world goes second */ semaphore_decrement( &child_counter ); /* value now 0 */ semaphore_decrement( &child_counter ); /* value now -1 */ /* * child_counter now must be up-ed 2 times for a thread blocked on it * to be released * * / pthread_create( &thread1, pthread_attr_default, (void *) &print_message_function, (void *) message1); semaphore_down( &worlds_turn ); pthread_create(&thread2, pthread_attr_default, (void *) &print_message_function, (void *) message2); semaphore_down( &child_counter ); /* not really necessary to destroy since we are exiting anyway */ semaphore_destroy ( &child_counter ); semaphore_destroy ( &worlds_turn ); e x i t ( 0 ) ; } void print_message_function( void *ptr ) { char *message; message = (char *) ptr; printf("%s ", message); fflush(stdout); semaphore_up( &worlds_turn ); semaphore_up( &child_counter ); p t h r e a d _ e x i t ( 0 ) ; } 信号量c h i l d _ c o u n t e r用来强迫父线程阻塞，直到两个子线程执行完p r i n t f语句和其后的semaphore_up( &child_counter )语句才继续执行。
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