Compare string literal command line parameters in C

Question

I need my program to run this way ./src c 2345 or ./src s 345, whereby the first character hs to be either c or s and second an integer. The program should throw an usage error if there's any less parameters and also any charcter other than c or s. Here's my code

int main(int argc, char **argv) {

    int num_of_connections = 0, client_sockfd;
    int max_sockfd, master_socket;

    fd_set socket_collection, read_collection;

    // Check validity of the user input
    if(argc < 3) {
        if((strcmp(argv[2], "s") != 0) || (strcmp(argv[2], "c") != 0)){
            fprintf(stderr, "Usage: ./src <s/c> <port>\n");
            exit(EXIT_FAILURE);
        }

    }

When I enter one argument I get a segmentation fault. Also it doesnt identify the C or S parameter. Any help will be appreciated.

You get a segmentation fault because there's no such thing as argv[2] if you only enter one argument... you should first check against argc to see whether the number of arguments is exactly what your program needs. If so (and only then) go ahead and check what they are. Of course then any of the above two checks fail, print out the usage message. — notadam
– notadam, Commented Sep 27, 2014 at 16:37

Basile Starynkevitch · Accepted Answer · 2014-09-27 16:40:05Z

1

Notice that main has a very specific convention: the argv array has argc+1 members, with the last being NULL and the others being non-null distinct pointers to zero-terminated strings.

So if argc is 1 (e.g. if your run ./src alone) or 2, argv[2] is NULL and you cannot pass it to strcmp

You can call strcmp(argv[2],"s") only when argc>=3

BTW, I would suggest to use getopt(3) or preferably (on Linux only) getopt_long and to accept the --help and --version arguments, per GNU conventions.

Also, compile with all warnings and debug info (gcc -Wall -g) and use the gdb debugger. It would be faster for you to use gdb than to ask here and wait for replies.

edited Sep 27, 2014 at 16:40

answered Sep 27, 2014 at 16:33

Basile Starynkevitch

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2 Comments

Kijewski Over a year ago

The if(argc < 3) is clearly a mistake, please read the invocation example.

David C. Rankin Over a year ago

if(argc < 3) <exit> is correct, but needed to be a preliminary and separate test. The strcmp comparisons could then follow if argc >= 3.

Kijewski · Accepted Answer · 2014-09-27 16:52:01Z

1

if(argc < 3) { does not make sense if you want exactly two parameters. In the inner if block you are confusion || (logical or) with && (logical and).

In your invocation example ./src s 345 the character is the first argument, so probably argv[2] should read argv[1].

if ((argc != 3) || ((strcmp(argv[1], "s") != 0) &&
                    (strcmp(argv[1], "c") != 0))) {
    fprintf(…);
    return EXIT_FAILURE;
}

Note: all parentheses in this if (…) condition are optional, because of C's operator precedence. I put them for readability.

edited Sep 27, 2014 at 16:52

answered Sep 27, 2014 at 16:33

Kijewski

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1 Comment

Umar Gul Over a year ago

This has the same issues if I enter anything other than s or c it still works.

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