Implement a debug mode ("--debug").

src/info.c

@@ -120,7 +120,8 @@ help (void)
           "  -nd, --no-duplicates\t\tIgnore duplicate messages\n"
 	  "  -o,  --output=MAILBOX\t\tWrite messages to MAILBOX\n"
 	  "  -p,  --pipe=COMMAND\t\tPipe each found message to COMMAND\n"
-	  "  -s,  --no-messages\t\tSuppress most error messages\n\n"
+	  "  -s,  --no-messages\t\tSuppress most error messages\n"
+    "       --debug\t\t\tPrint messages useful for debugging\n\n"
 	  "Matching criteria:\n\n"
 	  "  -E,  --extended-regexp\tPATTERN is an extended regular expression\n"
 	  "  -G,  --basic-regexp\t\tPATTERN is a basic regular expression\n");

src/main.c

@@ -76,6 +76,7 @@ main (int argc, char **argv)
       {"output", 1, 0, 'o'},
       {"no-duplicates", 0, 0, 200},
       {"no-file-lock", 0, 0, 201},
+      {"debug", 0, 0, 202},
       {"file-lock", 1, 0, 'l'},
       {"recursive", 0, 0, 'r'},
       {0, 0, 0, 0}

src/mboxgrep.h

@@ -107,6 +107,7 @@ typedef struct
   int merr;
   int pid;
   int haveregex;
+  int debug;
 
   char hostname[HOST_NAME_SIZE];
   char *boxname, *outboxname, *pipecmd, *tmpfilename, *regex_s;

src/misc.c

@@ -61,7 +61,7 @@ folder_format (const char *name)
   else
     {
       if (config.merr)
-        fprintf (stderr, "mboxgrep: %s: unknown folder type\n", name);
+        fprintf (stderr, "%s: %s: unknown folder type\n", APPNAME, name);
       exit (2);
     }
 
@@ -181,6 +181,7 @@ set_default_options (void)
   config.format = MBOX; /* default mailbox format */
   config.lock = FCNTL; /* default file locking method */
   config.merr = 1; /* report errors by default */
+  config.debug = 0;
 }
 
 void
@@ -227,7 +228,7 @@ get_runtime_options (int *argc, char **argv, struct option *long_options)
 #else
             fprintf(stderr,
               "%s: Support for Perl regular expressions not "
-              "compiled in\n");
+              "compiled in\n", APPNAME);
             exit(2);
 #endif /* HAVE_LIBPCRE */
             break;
@@ -265,6 +266,11 @@ get_runtime_options (int *argc, char **argv, struct option *long_options)
           case 201:
             config.lock = 0;
             break;
+          case 202:
+            config.debug = 1;
+            fprintf (stderr, "%s: %s, line %d: enable debugging\n",
+              APPNAME, __FILE__, __LINE__);
+            break;
           case 'r':
             config.recursive = 1;
             break;