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Optional target level argument to --norm

Let the --norm global option accept a target dB level as an optional
argument that is passed on to the gain effect; e.g.,
  sox --norm=-3.5 infile outfile
will normalize to -3.5 dBFS instead of 0 dBFS.

This resolves feature request 3418124.

--- a/sox.1

+++ b/sox.1

@@ -903,7 +903,7 @@

 alias, script, or batch file may be an appropriate way of permanently

 enabling it.

 .TP

-\fB\-\-norm\fR

+\fB\-\-norm\fR[\fB=\fIdB-level\fR]

 Automatically invoke the

 .B gain

 effect to guard against clipping and to normalise the audio. E.g.

@@ -914,6 +914,12 @@

 .EX

    sox infile \-b 16 outfile gain \-h rate 44100 gain \-nh dither \-s

 .EE

+Optionally, the audio can be normalized to a given level (usually)

+below 0 dBFS:

+.EX

+   sox \-\-norm=\-3 infile outfile

+.EE

+.SP

 See also

 .BR \-V,

 .BR \-G,

@@ -1016,7 +1022,7 @@

 .TP

 \fB\-\-version\fR

 Show SoX's version number and exit.

-.IP \fB\-V\fB[\fIlevel\fB]\fP

+.IP \fB\-V\fR[\fIlevel\fR]

 Set verbosity. This is particularly useful for seeing how any automatic

 effects have been invoked by SoX.

 .SP

--- a/src/sox.c

+++ b/src/sox.c

@@ -183,6 +183,7 @@

 static int eff_chain_count = 0;

 static char *effects_filename = NULL;

 static char * play_rate_arg = NULL;

+static char *norm_level = NULL;

 /* Flowing */

@@ -256,6 +257,7 @@

   free(play_rate_arg);

   free(effects_filename);

+  free(norm_level);

   sox_quit();

 }

@@ -1069,9 +1071,11 @@

     auto_effect(chain, "rate", rate_arg != NULL, &rate_arg, &signal, &guard);

   if (is_guarded && (do_guarded_norm || !(signal.mult && *signal.mult == 1))) {

-    char * arg = do_guarded_norm? "-nh" : guard? "-rh" : "-h";

+    char *args[2];

     int no_guard = -1;

-    auto_effect(chain, "gain", 1, &arg, &signal, &no_guard);

+    args[0] = do_guarded_norm? "-nh" : guard? "-rh" : "-h";

+    args[1] = norm_level;

+    auto_effect(chain, "gain", norm_level ? 2 : 1, args, &signal, &no_guard);

     guard = 1;

   }

@@ -2155,7 +2159,7 @@

   {"temp"            , lsx_option_arg_required, NULL, 0},

   {"single-threaded" , lsx_option_arg_none    , NULL, 0},

   {"ignore-length"   , lsx_option_arg_none    , NULL, 0},

-  {"norm"            , lsx_option_arg_none    , NULL, 0},

+  {"norm"            , lsx_option_arg_optional, NULL, 0},

   {"magic"           , lsx_option_arg_none    , NULL, 0},

   {"play-rate-arg"   , lsx_option_arg_required, NULL, 0},

   {"clobber"         , lsx_option_arg_none    , NULL, 0},

@@ -2333,7 +2337,9 @@

       case 16: sox_globals.tmp_path = lsx_strdup(optstate.arg); break;

       case 17: sox_globals.use_threads = sox_false; break;

       case 18: f->signal.length = SOX_IGNORE_LENGTH; break;

-      case 19: do_guarded_norm = is_guarded = sox_true; break;

+      case 19: do_guarded_norm = is_guarded = sox_true;

+        norm_level = lsx_strdup(optstate.arg);

+        break;

       case 20:

         if (info->flags & sox_version_have_magic)

           sox_globals.use_magic = sox_true;