vcgencmd: Add some very basic usage info. Also detect when the videocore returns an error

host_applications/linux/apps/gencmd/gencmd.c

@@ -2,6 +2,9 @@
 Copyright (c) 2012, Broadcom Europe Ltd
 All rights reserved.
 
+Copyright (c) 2019 Andrew Pattison
+All rights reserved.
+
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are met:
     * Redistributions of source code must retain the above copyright
@@ -36,6 +39,18 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include "interface/vmcs_host/vc_vchi_gencmd.h"
 
+void show_usage()
+{
+   printf( "Usage: vcgencmd [-t] [COMMAND]\n" );
+   printf( "Send a command to the Videocore and print the result.\n\n" );
+   printf( "  -t         Time how long the command takes to complete\n");
+   printf( "  commands   Display a list of commands\n\n" );
+   printf( "Exit status:\n" );
+   printf( "   0    command completed successfully\n" );
+   printf( "  -1    problem with VCHI\n" );
+   printf( "  -2    Videocore returned error\n" );
+}
+
 int main( int argc, char **argv )
 {
    int instNum = 0;
@@ -50,6 +65,9 @@ int main( int argc, char **argv )
            argv++;
            argc--;
        }
+   } else {
+      // no arguments passed, so show basic usage
+      show_usage();
    }
 
    vcos_init();
@@ -71,6 +89,14 @@ int main( int argc, char **argv )
 
     if (argc > 1)
     {
+       // first check if we were invoked with either -? or --help
+       // in which case show basic usage and exit
+      if( ( strcmp( argv[1], "-?" ) == 0) || strcmp( argv[1], "--help" ) == 0 )
+      {
+         show_usage();
+         return(0);
+      }
+
       int i = 1;
       char buffer[ 1024 ];
       size_t buffer_offset = 0;
@@ -130,6 +156,14 @@ int main( int argc, char **argv )
           {
               printf("%s\n", buffer );
           }
+          if (strncmp( buffer, "error=", 5) == 0 )
+          {
+             if ( strcmp( buffer, "error=1 error_msg=\"Command not registered\"" ) == 0 )
+             {
+                printf( "Use 'vcgencmd commands' to get a list of commands\n" );
+             }
+             return -2;
+          }
       }
     }