Semaphores are a synchronization primitive that provide two operations: "wait" and "signal". The "wait" operation checks the semaphore counter and blocks the thread if the counter is zero. The "signal" operation increases the semaphore counter but does not block. If another thread has blocked waiting for the semaphore that is signalled, the blocked thread will become runnable again.
Semaphores can be used to implement other, more structured, synchronization primitives such as monitors and message queues/bounded buffers (see below).
The following example shows how the producer-consumer problem, also known as the bounded buffer problem, can be solved using protothreads and semaphores. Notes on the program follow after the example.
#include "pt-sem.h" #define NUM_ITEMS 32 #define BUFSIZE 8 static struct pt_sem mutex, full, empty; PT_THREAD(producer(struct pt *pt)) { static int produced; PT_BEGIN(pt); for(produced = 0; produced < NUM_ITEMS; ++produced) { PT_SEM_WAIT(pt, &full); PT_SEM_WAIT(pt, &mutex); add_to_buffer(produce_item()); PT_SEM_SIGNAL(pt, &mutex); PT_SEM_SIGNAL(pt, &empty); } PT_END(pt); } PT_THREAD(consumer(struct pt *pt)) { static int consumed; PT_BEGIN(pt); for(consumed = 0; consumed < NUM_ITEMS; ++consumed) { PT_SEM_WAIT(pt, &empty); PT_SEM_WAIT(pt, &mutex); consume_item(get_from_buffer()); PT_SEM_SIGNAL(pt, &mutex); PT_SEM_SIGNAL(pt, &full); } PT_END(pt); } PT_THREAD(driver_thread(struct pt *pt)) { static struct pt pt_producer, pt_consumer; PT_BEGIN(pt); PT_SEM_INIT(&empty, 0); PT_SEM_INIT(&full, BUFSIZE); PT_SEM_INIT(&mutex, 1); PT_INIT(&pt_producer); PT_INIT(&pt_consumer); PT_WAIT_THREAD(pt, producer(&pt_producer) & consumer(&pt_consumer)); PT_END(pt); }
The program uses three protothreads: one protothread that implements the consumer, one thread that implements the producer, and one protothread that drives the two other protothreads. The program uses three semaphores: "full", "empty" and "mutex". The "mutex" semaphore is used to provide mutual exclusion for the buffer, the "empty" semaphore is used to block the consumer is the buffer is empty, and the "full" semaphore is used to block the producer is the buffer is full.
The "driver_thread" holds two protothread state variables, "pt_producer" and "pt_consumer". It is important to note that both these variables are declared as static. If the static keyword is not used, both variables are stored on the stack. Since protothreads do not store the stack, these variables may be overwritten during a protothread wait operation. Similarly, both the "consumer" and "producer" protothreads declare their local variables as static, to avoid them being stored on the stack.
Files | |
| file | pt-sem.h |
| Couting semaphores implemented on protothreads. | |
Data Structures | |
| struct | pt_sem |
Defines | |
| #define | PT_SEM_INIT(s, c) |
| Initialize a semaphore. | |
| #define | PT_SEM_WAIT(pt, s) |
| Wait for a semaphore. | |
| #define | PT_SEM_SIGNAL(pt, s) |
| Signal a semaphore. | |
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Initialize a semaphore. This macro initializes a semaphore with a value for the counter. Internally, the semaphores use an "unsigned int" to represent the counter, and therefore the "count" argument should be within range of an unsigned int.
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Signal a semaphore. This macro carries out the "signal" operation on the semaphore. The signal operation increments the counter inside the semaphore, which eventually will cause waiting protothreads to continue executing.
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Wait for a semaphore. This macro carries out the "wait" operation on the semaphore. The wait operation causes the protothread to block while the counter is zero. When the counter reaches a value larger than zero, the protothread will continue.
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1.4.6