String Prefix Validation

[Resilius]

Hello cboard! I'm both new to the forum, and new to C Programming and am struggling to achieve validation of the start of a string passed through terminal.


Below is both the code and my general thinking slung together:

#include <stdio.h>
#include <string.h>


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


    char *address;
    char *addresspre;


    //request input
    printf("Enter a URL (http://example.com):\n");


    //address is passed as argv[1] when run from terminal
    printf("Saving address \"%s\".\n", argv[1]);


    //copies argv[1] into 'address'
    strcpy(address, argv[1]);
    
    addresspre = address;
    address = address+7;


    strncmp(address, addresspre, strlen(addresspre));


}

Here is what I'd type to run from terminal:

./httptest http://blah.com

(or something to that effect)

I appreciate it doesn't achieve much but I would like this part of my program to check that 'argv[1]' (as passed through terminal on execution) starts with 'http://'. It's all part of a bigger program I am working on; here I have just taken out what I am struggling with.

Ideally I'd like an if statement to be satisfied with 'http://' being present and continue - with a block of working code I have all ready got - and an else to printf a failure message and return 0 (the latter I can do myself).

Apologies for for slinging in random working parts but I am new to C and have only past experience with Java, a whole different kettle of fish as I'm sure you'll know.

I'd much appreciate any help or direction with getting further forward with this. Thanking you in advance.

[oogabooga]

You need to allocate space for address. Something like

char address[200];

I have no idea what you're trying to do with addresspre.
Try something like

strncmp(address, "http://", 7)

[Subsonics]

Something like this could work.

    if(strncmp(argv[1], "http://", 7) != 0) {
        printf("start of input must be: http://\nyou wrote: %s\n", argv[1]);
        return -1;
    }

Preferably do this at the beginning of main, right after you checked argc, no point in going further if the input is not correct.

[Resilius]

Thank you oogabooga, and Subsonics.

Though the first response suits the code I put in the first post, Subsonics' solution fits nicely into the program I'm working on has a whole, (which isn't the same as here).

Anyhow, I got the validation to work nicely so thank you very much guys!

[Subsonics]

Thank you oogabooga, and Subsonics.

Though the first response suits the code I put in the first post, Subsonics' solution fits nicely into the program I'm working on has a whole, (which isn't the same as here).

Anyhow, I got the validation to work nicely so thank you very much guys!

Just, don't forget to check that argc == 2, before you do anything else. If you use argv without any arguments you will segfault. Or more precisely: if argc != 2 bail.

[Resilius]

Just, don't forget to check that argc == 2, before you do anything else. If you use argv without any arguments you will segfault. Or more precisely: if argc != 2 bail.

I've got that check at the start of the html stripper I've written (the main prog).