[techtalk] switch function in C (or how to read commandline args?)

[JamesA.Sutherland]

On Sat, 30 Jun 2001 21:32:38 +0100, you wrote:

>I want to read a bunch of commandline args or config options from file 
>in a C program.  Now, in bash, I can use a switch structure to do stuff
>based on the *string* presented.  AFAIR, in C, you can only switch on an
>*int*.
>
>So, how do I use a switch to process my config options? Or do I have to
>build an "if; elseif; else" ladder instead?  I have options in a config
>file that look like
>
><opt> <arg>
>location Belmullet
>name "Joe Bloggs"
>
>and commandline switches like
>
>--force
>-C /path/to/config.file
>
>I can read these and put the values into "opt" and "arg" but what I need
>to do next is the C equivalent to the following (bash)
>
>case $opt in
>	name)
>		NAME=$arg
>	;;
>	location)
>		LOCATION=$arg
>	;;
>	--force)
>		FORCE=TRUE
>	;;
>	-C)
>		CONFIG_FILE=$arg
>		readconfig($CONFIG_FILE)
>	;;
>	*)
>		echo "AARRRGGGHHHH!!!!!"
>	;;
>esac
>
>I even tried 
>
>case (! strcmp(opt,"config-file")):
>
>to try and return an integer for the case statement but it 
>seems to need a constant rather than the return value from 
>function to work properly.

Unfortunately, C can't do this in a very nice way. As you've seen,
each case statement will only work with a constant - a single
character, or a number. What you can do is switch() on the FIRST
character of the word, then use strcmp() to check for each possibility
- something like this:

#include <string.h>

int main(int argc, char* argv[])
{
	if (argc != 2)
		return 0;
	switch(*argv[1])
	{
	case 'f':
		if (!strcmp("foo",argv[1]))
			printf("foo\n");
		if (!strcmp("far",argv[1]))
			printf("far\n");
		break;
	default:
		break;
	}
	return 0;
}


This takes "foo" and "far" as being valid statements, and just prints
them; anything else is just ignored. Obviously, if you had lots of
statements starting with the same character (like all the --enable...
options for GNU autoconf's configure scripts) you can use a second or
third level of switch() statements.

It would be nice if C could be used with case "foo", but I suppose
implementing that efficiently would have been too stressfull for the
original C compilers?

You can do something similar using the GNU getopt library, I think,
but I haven't looked very closely at that - and besides, hand-rolling
code is much more rewarding ;-)


James.