Add a "run_this_when_volume_is_set" facility to call a program when the volume is set. The AirPlay volume is passed.

common.c

@@ -32,6 +32,7 @@
 #include <stdarg.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <time.h>
@@ -454,6 +455,53 @@ uint8_t *rsa_apply(uint8_t *input, int inlen, int *outlen, int mode) {
 }
 #endif
 
+void command_set_volume(double volume) {
+  if (config.cmd_set_volume) {
+    /*Spawn a child to run the program.*/
+    pid_t pid = fork();
+    if (pid == 0) { /* child process */
+    	size_t command_buffer_size = strlen(config.cmd_set_volume)+32;
+      char* command_buffer = (char*)malloc(command_buffer_size);
+            if (command_buffer==NULL) {
+      	inform("Couldn't allocate memory for set_volume argument string");
+      } else {
+      	memset(command_buffer,0,command_buffer_size);
+				sprintf(command_buffer, "%s%f", config.cmd_set_volume,volume);
+				// debug(1,"command_buffer is \"%s\".",command_buffer);
+				int argC;
+				char **argV;
+				// debug(1,"set_volume command found.");
+				if (poptParseArgvString(command_buffer, &argC, (const char ***)&argV) !=
+					0) {
+					// note that argV should be free()'d after use, but we expect this fork to exit
+					// eventually.
+					warn("Can't decipher on-set-volume command arguments \"%s\".",command_buffer);
+					free(argV);
+					free(command_buffer);
+				} else {
+					free(command_buffer);
+					// debug(1,"Executing on-set-volume command %s with %d arguments.",argV[0],argC);
+					execv(argV[0], argV);
+					warn("Execution of on-set-volume command \"%s\" failed to start", config.cmd_set_volume);
+					// debug(1, "Error executing on-set-volume command %s", config.cmd_set_volume);
+					exit(127); /* only if execv fails */
+				}
+      }
+    
+    } else {
+      if (config.cmd_blocking) { /* pid!=0 means parent process and if blocking is true, wait for
+                                    process to finish */
+        pid_t rc = waitpid(pid, 0, 0); /* wait for child to exit */
+        if (rc != pid) {
+          warn("Execution of on-set-volume command returned an error.");
+          debug(1, "on-set-volume command %s finished with error %d", config.cmd_set_volume,errno);
+        }
+      }
+      // debug(1,"Continue after on-set-volume command");
+    }
+  }
+}
+
 void command_start(void) {
   if (config.cmd_start) {
     /*Spawn a child to run the program.*/

common.h

@@ -113,7 +113,7 @@ typedef struct {
   int logOutputLevel;  // log output level
   int statistics_requested, use_negotiated_latencies;
   enum playback_mode_type playback_mode;
-  char *cmd_start, *cmd_stop;
+  char *cmd_start, *cmd_stop, *cmd_set_volume;
   int cmd_blocking;
   double tolerance; // allow this much drift before attempting to correct it
   enum stuffing_type packet_stuffing;
@@ -203,6 +203,7 @@ config_t config_file_stuff;
 
 void command_start(void);
 void command_stop(void);
+void command_set_volume(double volume);
 
 void shairport_shutdown();
 // void shairport_startup_complete(void);

player.c

@@ -2109,6 +2109,8 @@ void player_volume(double airplay_volume, rtsp_conn_info* conn) {
   // Thus, we ask our vol2attn function for an appropriate dB between -96.3 and 0 dB and translate
   // it back to a number.
 
+  command_set_volume(airplay_volume);
+  
   int32_t hw_min_db, hw_max_db, hw_range_db, range_to_use, min_db,
       max_db; // hw_range_db is a flag; if 0 means no mixer
 

shairport.c

@@ -499,6 +499,10 @@ int parse_options(int argc, char **argv) {
         config.volume_max_db_set = 1;
       }
 
+      if (config_lookup_string(config.cfg, "general.run_this_when_volume_is_set", &str)) {
+        config.cmd_set_volume = (char *)str;
+      }
+
       /* Get the playback_mode setting */
       if (config_lookup_string(config.cfg, "general.playback_mode", &str)) {
         if (strcasecmp(str, "stereo") == 0)