lib: shm_ap_rbuff: sleep when read on empty rbuff

When the ring buffer is empty, a read call will sleep. A write call on
an empty ring buffer will wake up sleeping readers.

1 files changed, 23 insertions, 9 deletions

diff --git a/src/lib/shm_ap_rbuff.c b/src/lib/shm_ap_rbuff.c
index 1cfafeda..63aa8813 100644
--- a/src/lib/shm_ap_rbuff.c
+++ b/src/lib/shm_ap_rbuff.c
@@ -36,7 +36,8 @@
 #include <errno.h>
 
 #define SHM_RBUFF_FILE_SIZE (SHM_RBUFF_SIZE * sizeof(struct rb_entry)          \
-                             + 2 * sizeof(size_t) + sizeof(pthread_mutex_t))
+                             + 2 * sizeof(size_t) + sizeof(pthread_mutex_t)    \
+                             + sizeof (pthread_cond_t))
 
 #define shm_rbuff_used(rb)((*rb->ptr_head + SHM_RBUFF_SIZE - *rb->ptr_tail)    \
                           & (SHM_RBUFF_SIZE - 1))
@@ -49,6 +50,7 @@ struct shm_ap_rbuff {
         size_t *          ptr_head;    /* start of ringbuffer head */
         size_t *          ptr_tail;    /* start of ringbuffer tail */
         pthread_mutex_t * shm_mutex;   /* lock all free space in shm */
+        pthread_cond_t *  work;        /* threads will wait for a signal */
         pid_t             pid;         /* pid to which this rb belongs */
         int               fd;
 };
@@ -58,7 +60,8 @@ struct shm_ap_rbuff * shm_ap_rbuff_create()
         struct shm_ap_rbuff * rb;
         int                   shm_fd;
         struct rb_entry *     shm_base;
-        pthread_mutexattr_t   attr;
+        pthread_mutexattr_t   mattr;
+        pthread_condattr_t    cattr;
         char                  fn[25];
 
         sprintf(fn, SHM_AP_RBUFF_PREFIX "%d", getpid());
@@ -111,10 +114,15 @@ struct shm_ap_rbuff * shm_ap_rbuff_create()
         rb->ptr_head  = (size_t *) (rb->shm_base + SHM_RBUFF_SIZE);
         rb->ptr_tail  = rb->ptr_head + 1;
         rb->shm_mutex = (pthread_mutex_t *) (rb->ptr_tail + 1);
+        rb->work      = (pthread_cond_t *) (rb->shm_mutex + 1);
 
-        pthread_mutexattr_init(&attr);
-        pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
-        pthread_mutex_init(rb->shm_mutex, &attr);
+        pthread_mutexattr_init(&mattr);
+        pthread_mutexattr_setpshared(&mattr, PTHREAD_PROCESS_SHARED);
+        pthread_mutex_init(rb->shm_mutex, &mattr);
+
+        pthread_condattr_init(&cattr);
+        pthread_condattr_setpshared(&cattr, PTHREAD_PROCESS_SHARED);
+        pthread_cond_init(rb->work, &cattr);
 
         *rb->ptr_head = 0;
         *rb->ptr_tail = 0;
@@ -169,6 +177,7 @@ struct shm_ap_rbuff * shm_ap_rbuff_open(pid_t pid)
         rb->ptr_head  = (size_t *) (rb->shm_base + SHM_RBUFF_SIZE);
         rb->ptr_tail  = rb->ptr_head + 1;
         rb->shm_mutex = (pthread_mutex_t *) (rb->ptr_tail + 1);
+        rb->work      = (pthread_cond_t *) (rb->shm_mutex + 1);
 
         rb->fd = shm_fd;
         rb->pid = pid;
@@ -231,6 +240,9 @@ int shm_ap_rbuff_write(struct shm_ap_rbuff * rb, struct rb_entry * e)
                 return -1;
         }
 
+        if (shm_rbuff_used(rb) == 0)
+                pthread_cond_broadcast(rb->work);
+
         *head_el_ptr = *e;
         *rb->ptr_head = (*rb->ptr_head + 1) & (SHM_RBUFF_SIZE -1);
 
@@ -246,12 +258,14 @@ struct rb_entry * shm_ap_rbuff_read(struct shm_ap_rbuff * rb)
         if (rb == NULL)
                 return NULL;
 
+        pthread_cleanup_push((void(*)(void *))pthread_mutex_unlock,
+                             (void*) rb->shm_mutex);
+
         pthread_mutex_lock(rb->shm_mutex);
+        while(shm_rbuff_used(rb) == 0)
+                pthread_cond_wait(rb->work, rb->shm_mutex);
 
-        if (shm_rbuff_used(rb) == 0) {
-                pthread_mutex_unlock(rb->shm_mutex);
-                return NULL;
-        }
+        pthread_cleanup_pop(0);
 
         e = malloc(sizeof(*e));
         if (e == NULL) {