How to parse integer command line arguments in C?

Question

I would like a user to pass either two parameters or leave it blank. For instance:

./program 50 50

or

./program

When I tried to use int main(int argc, char *argv[]), first thing I have done was to change char *argv[] to int *argv[] but it did not work. What I want is from the user is just to enter two integers between 0 and 100. So if it is not two integers then it should give an error.

I was sort of thinking to give out an error with types (as I used to program on C#) but whatever I enter, argv[1] would be 'char' type all the time.

So what I have done is

for (int i = 0; i <= 100; i++) {
    //printf("%d", i);
    if (argv[1] == i) {
        argcheck++;
        printf("1st one %d\n", i);
    }
    else if (argv[2] == i) {
        argcheck++;
        printf("2nd one %d\n", i);
    }

This does not work as well. Also it gives warning when compiling, but if I change argv with atoi(argv[1]) for instance, then it gives a Segmentation fault (core dumped) error.

I need a simple way to solve this problem.

EDIT:

So I fixed with atoi(), the reason why it was giving segmentation fault was because I was trying it with null value when I have no parameter. So I fixed it up by adding an extra cond. But now the problem is if the value is let's say

./program asd asd

Then the output of atoi(argv[1]) would be 0. Is there a way to change this value?

Answer 1

Don't use atoi() and don't use strtol(). atoi() has no error checking (as you found out!) and strtol() has to be error-checked using the global errno variable, which means you have to set errno to 0, then call strtol(), then check errno again for errors. A better way is to use sscanf(), which also lets you parse any primitive type from a string, not just an integer, and it lets you read fancy formats (like hex).

For example, to parse integer "1435" from a string:

if (sscanf (argv[1], "%i", &intvar) != 1) {
    fprintf(stderr, "error - not an integer");
}

To parse a single character 'Z' from a string

if (sscanf (argv[1], "%c", &charvar)!=1) {
    fprintf(stderr, "error - not a char");
}

To parse a float "3.1459" from a string

if (sscanf (argv[1], "%f", &floatvar)!=1) {
    fprintf(stderr, "error - not a float");
}

To parse a large unsigned hexadecimal integer "0x332561" from a string

if (sscanf (argv[1], "%xu", &uintvar)!=1) {
    fprintf(stderr, "error - not a hex integer");
}

If you need more error-handling than that, use a regex library.