Unable to get value of argv[i+1]

Question

I'm calling a simple executable as follows:

$ ./test -l 123

I have some relatively simple code that checks whether there are enough args provided, and if not prompts the user to enter the value instead.

The code looks like this:

#include <stdio.h>
#include <stdlib.h>
#include <strings.h>

int main(int argc, char *argv[])
{
    long max_number;
    char out[1024];

    // Check whether max_number was passed via args
    if (argc < 2) {
        printf("Enter the maximum number: ");
        fflush(stdout);
        scanf("%d", &max_number);

    } else {
        for (int i=1; i<argc; i++) {
            sprintf(out, "%sArg %d of %d is '%s'\n", out, i, argc, argv[i]);

            if (strcmp(argv[i], "-l") && argc > i) {
                sprintf(out, "%sFound '-l' argument at index %d\n", out, i);
                int argTarget = i + 1;
                sprintf(out, "%sLooking for the maximum number at argument %d\n", out, argTarget);
                max_number = atoi(argv[argTarget]);
            }
        }
    }

    printf("%s", out);
    printf("Maximum number received: %d\n", max_number);

    return 0;
}

What I would expect to come from the command above is:

Arg 1 of 3 is '-l'
Arg 2 of 3 is '123'
Found '-l' argument at index 1
Looking for the maximum number at argument 2
Maximum number received: 123

What actually happens is this:

Arg 1 of 3 is '-l'
Arg 2 of 3 is '123'
Found '-l' argument at index 2
Looking for the maximum number at argument 3
Maximum number received: 0

So for some reason, despite the fact that I can get the value from index i no problem, when I try to get the value of i it seems to be one higher than it's supposed to be. I don't understand why it's correct when I check it on the first line of the for loop, but when I check it after the strcomp() it seems to be incorrect.

Can anyone help me understand what's going on here?

I'm using GCC on Windows 10 and running in a MinGW-64 terminal. To compile I'm simply running:

$ gcc args.c -o args

Answer 1

strcmp returns 0 if the strings are equal. Therefore:

if (strcmp(argv[i], "-l") && argc > i) {

should be:

if (strcmp(argv[i], "-l") == 0 && argc > i) {

Also, note that there are a few other issues with the code:

• strings.h should be string.h
• No need for out -- remove it and use printf instead. You are currently breaking aliasing, since you pass out to both the output and the input of sprintf.
• Use the proper printf format string.

Try to compile your code with full warnings enabled! This is the code without warnings:

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

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

    // Check whether max_number was passed via args
    if (argc < 2) {
        printf("Enter the maximum number: ");
        fflush(stdout);
        scanf("%d", &max_number);
    } else {
        for (int i=1; i<argc; i++) {
            printf("Arg %d of %d is '%s'\n", i, argc, argv[i]);

            if (strcmp(argv[i], "-l") == 0 && argc > i) {
                printf("Found '-l' argument at index %d\n", i);
                int argTarget = i + 1;
                printf("Looking for the maximum number at argument %d\n", argTarget);
                max_number = atoi(argv[argTarget]);
            }
        }
    }

    printf("Maximum number received: %d\n", max_number);

    return 0;
}