added PCM_NONBLOCK macro

include/tinyalsa/pcm.h

@@ -88,6 +88,15 @@ extern "C" {
  */
 #define PCM_MONOTONIC 0x00000008
 
+/** If used with @pcm_open and @pcm_params_get,
+ * it will not cause the function to block if
+ * the PCM is not available. It will also cause
+ * the functions @ref pcm_readi and @ref pcm_writei
+ * to exit if they would cause the caller to wait.
+ * @ingroup libtinyalsa-pcm
+ * */
+#define PCM_NONBLOCK 0x00000010
+
 /** For inputs, this means the PCM is recording audio samples.
  * For outputs, this means the PCM is playing audio samples.
  * @ingroup libtinyalsa-pcm

src/pcm.c

@@ -818,9 +818,13 @@ struct pcm_params *pcm_params_get(unsigned int card, unsigned int device,
     snprintf(fn, sizeof(fn), "/dev/snd/pcmC%uD%u%c", card, device,
              flags & PCM_IN ? 'c' : 'p');
 
-    fd = open(fn, O_RDWR);
+    if (flags & PCM_NONBLOCK)
+        fd = open(fn, O_RDWR | O_NONBLOCK);
+    else
+        fd = open(fn, O_RDWR);
+
     if (fd < 0) {
-        fprintf(stderr, "cannot open device '%s'\n", fn);
+        fprintf(stderr, "cannot open device '%s': %s\n", fn, strerror(errno));
         goto err_open;
     }
 
@@ -1073,7 +1077,12 @@ struct pcm *pcm_open(unsigned int card, unsigned int device,
              flags & PCM_IN ? 'c' : 'p');
 
     pcm->flags = flags;
-    pcm->fd = open(fn, O_RDWR);
+
+    if (flags & PCM_NONBLOCK)
+        pcm->fd = open(fn, O_RDWR | O_NONBLOCK);
+    else
+        pcm->fd = open(fn, O_RDWR);
+
     if (pcm->fd < 0) {
         oops(pcm, errno, "cannot open device '%s'", fn);
         return pcm;