getopt - portable way to treat a single dash as stdin filename

**incorrect** · 12-15-2013

I am writing a program that takes several parameters on the command line and a filename as the last parameter. If the filename is a single dash - `-' I want my program to read from stdin. Otherwise, I want to read from specified file. I want this program to be portable.

However, this doesn't work with getopt:

Code:

while ((c = getopt (argc, argv, "ab-:")) != -1)
         switch (c)
           {
           case 'a':
             aflag = 1;
             break;
           case 'b':
             bflag = 1;
             break;
           case '-':
             cvalue = optarg;
         printf("%s\n", cvalue);

$ ./main  -"stuff"
Unknown option `-s'.
$ ./main  - "stuff"

No output in second case. I came up with this:

Code:

while ((c = getopt (argc, argv, "ab-:")) != -1)
         switch (c)
           {
           case 'a':
             aflag = 1;
             break;
           case 'b':
             bflag = 1;
             break;
}
[...]
printf("last is: %s\n", argv[argc - 1]);
/* after getopt loop is finished, I will use strcmp to check if last argument is `-' and read from stdin, or if it's not and read from file */

Is this a good idea?

**tabstop** · 12-15-2013

The example from the manual (here: Example of Getopt - The GNU C Library) uses - as the last example to show that getopt won't process a single hyphen. Note that this is why you have the variable optind, to go through those arguments.

**incorrect** · 12-15-2013

Cool, I will use argv[optind] instead of argv[argc - 1]. Would you make any changes to my code, is this a good approach to this problem in general?

**tabstop** · 12-15-2013

I don't really know your setup, but unless you've got something really interesting I would just have a FILE* object that always gets read from, and something like

Code:

FILE *data = NULL;
//in the getopt
    case 'f': data = fopen(optarg, "r"); //assuming you can specify the file with an option; also don't skimp on error checking like I am
//afterwards in the post-processing
    if (strcmp(argv[optind], "-")==0)
        data = stdin;
//at the very end
    if (!data)
        data = fopen(default_filename, "r");

EDIT: Upon rereading the original, I see that the filename is supposed to be the last argument, so never mind case 'f' then. I also meant to ask originally: if we're using GNU extensions (getopt) how portable are you expecting this to be?

**oogabooga** · 12-15-2013

freopen is useful for reassigning the standard streams, if that's useful to you.

Code:

if (!freopen(filename, "r", stdin)) {
    ; // failed
}

**incorrect** · 12-15-2013

Originally Posted by tabstop

I also meant to ask originally: if we're using GNU extensions (getopt) how portable are you expecting this to be?

When I say portable, I want it to run on Linux and BSD and on ARM machines. getopt is a part of glibc and EGLIBC or uclibc are used on ARM computers very often. Is there getopt in them? I am afraid it may not be the case. So what's the most portable way to implement getopt functionality? Probably rolling your own function. I don't like reinventing the wheel and I am sure that I would make it worse that people who have faced this problem before. Do you have something to recommend?

EDIT:

I found that getopt itself actually *is* a part of C library but its implementation may differ. From http://en.wikipedia.org/wiki/Getopt

getopt is a C library function used to parse command-line options.
[...]
getopt is a system dependent function. The implementation of getopt in GNU C Library does permute the contents of the argument vector as it scans, so that eventually all the non-option arguments are at the end. On the contrary, the implementation of getopt in BSD C Library does not permute the argument vector and returns -1 if it encounters a non-option argument.

Only getopt_long is GNU extension.

**tabstop** · 12-15-2013

It's a part of the GNU C library, not the standard C library (won't find it in Microsoft's C library for instance). Having said that, it is part of the POSIX standard, and the source code tree at the uClibc website shows that there is a file called "getopt.c" so you are probably okay there.

**c99tutorial** · 12-15-2013

If you want portability I recommend APR, as it provides the same kind of functionality. Here is an article with an example posted:

libapr(apache portable runtime) programming tutorial: command line options

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