Write a C program to read an IPv4 address and to check its validity

Question

I know my error is within the arrangement of the {} but can't tell where. I've been trying to fix it for an hour. I'm not sure if there's something wrong with my code. Also, I'm new in coding so please baby steps if you can.

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

int main (void)
{
    int ch, dots, bytes, temp;
    dots = bytes = temp = 0;
    printf ("Enter IP address (x.x.x.x): ");
    while ((ch = getchar ()) != '\n' && ch != EOF) {
        if (ch < '0' || ch != EOF) {
            if (ch == '.') {
                dots++;
            }
            if (temp != -1) {
                if (temp > 255) {
                    printf
                        ("Error: The value of each byte should be in [0,255]\n");
                    return 0;
                }

                bytes++;
                temp = -1;      //The value -1 means current address byte is checked
            } else {
                printf ("Error: Acceptable chars are only digits and dots\n");
                return 0;
            }
        } else {
            if (temp == -1)
                temp = 0;       //Make it 0, to start checking the next address byte
            temp = 10 * temp + (ch - '0');
        }
        if (temp != -1)         //Check the value of the last address byte
        {
            if (temp > 255) {
                printf
                    ("Error: The value of each byte should be in [0, 255]\n");
                return 0;
            }
            bytes++;
        }
    }
    if (dots != 3 || bytes != 4) {
        printf ("Error: The IP format should be x.x.x.x\n");
    }
    return 0;
}

Answer 1

Your code actually compiles cleanly, you have an error in logic. You will need to spend a bit of time with a debugger, See How to debug small programs -- OR -- you can make life quite a bit easier on yourself...

Your task is not a difficult one, if you just approach it in a different way. You are only validating whether there are periods '.' between 4 octets of 3 numbers each. (an IPv4 address affectionately referred to as a dotted-quad). If these are the only tests you are making, you can simply let sscanf do it for you. You then only need check that the values are between 0 and 255.

(you can also grab the 1st character after the last quad and validate it is the '\n' character to insure there are no extraneous characters tacked onto the end of the IP)

This greatly simplifies the logic (and the amount of time you will spend with the debugger). Implementing the above logic, you could whittle your code down to the following and still use a proper fgets followed by sscanf, e.g.

#include <stdio.h>

#define QUAD  4     /* if you need a constant define one (or more) */
#define MAXC 32     /* don't skimp on buffer size */

int main (void)
{
    char buf[MAXC] = "",    /* buf to hold IP read by fgets */
        nl = 0;             /* charater to validate newline following IP */
    int ip[QUAD] = {0};     /* array to store each quad for testing */

    printf ("Enter IP address (x.x.x.x): ");
    if (fgets (buf, MAXC, stdin)) {         /* read IP into buf */
        if (sscanf (buf, "%d.%d.%d.%d%c",   /* parse with sscanf */
                    &ip[0], &ip[1], &ip[2], &ip[3], &nl) != 5) {
            fprintf (stderr, "error: invalid IPv4 format.\n");
            return 1;
        }
        for (int i = 0; i < QUAD; i++)      /* loop over each quad */
            if (ip[i] < 0 || 255 < ip[i]) { /* validate 0-255 */
                fprintf (stderr, "error: invalid quad '%d'.\n", ip[i]);
                return 1;
            }
    }

    if (nl != '\n') {   /* validate following char is '\n' */
        fprintf (stderr, "error: additional characters following IP.\n");
        return 1;
    }

    printf ("valid IP: %s", buf);   /* all tests passed - good address */

    return 0;
}

Example Use/Output

$ ./bin/chkip
Enter IP address (x.x.x.x): 12.23.34.45
valid IP: 12.23.34.45

$ ./bin/chkip
Enter IP address (x.x.x.x): 12.23.34.45a
error: additional characters following IP.

$ ./bin/chkip
Enter IP address (x.x.x.x): 12.23.34.450
error: invalid quad '450'.

$ ./bin/chkip
Enter IP address (x.x.x.x): 12.32.foo
error: invalid IPv4 format.

Look things over and let me know if you have further questions. If you must use getchar(), it is doable, but I'll need to find some aspirin before wading through each bit of your loop logic with you. Let me know if that is the case.