When using printf for char only outputs the dots

Question

I'm currently working on a homework and can't get the result I want from the code below.

#include <stdio.h>

int main() {
    int base, num, x, y, z, t, choice;
    char hexa = 0, a, b, c, d, k, l, m, n, h, g, f, v;

    switch (hexa) {
    case 'A':
        num = 10;
        break;
    case 'B':
        num = 11;
        break;
    case 'C':
        num = 12;
        break;
    case 'D':
        num = 13;
        break;
    case 'E':
        num = 14;
        break;
    case 'F':
        num = 15;
        break;
    }

    printf("Please enter the base:\n");
    scanf("%d", &base);

    if (base == 10) {
        printf("Please enter your ip address:\n");
        scanf("%c.%c.%c.%c", &a, &b, &c, &d);

        //if((a<0 || a>255) && (b<0 || b>255) && (c<0 || c>255) && (d<0 || d>255)){ //FIX THIS
        //  printf("Sorry, that is not a valid address!");}
        printf("Please enter the sub-net mask\n"); //Skips this line
        scanf("%d.%d.%d.%d", &x, &y, &z, &t);
    }
    else if (base == 16) {
        printf("Enter your ip address\n");
        scanf("%c.%c.%c.%c", &k, &l, &m, &n);
        printf("Enter the subnet mask\n");
        scanf("%c.%c.%c.%c", &h, &g, &f, &v);
    }

    scanf("%d", &choice);

    if (choice == 1) { //there is a part that asks which 
        printf("%c.%c.%c.%c", a, b, c, d); //prints only dots
    }
    else if (choice == 2) {}
    else if (choice == 3) {}
    else if (choice == 4) {}
    else if (choice == 5) {}
    else if (choice == 6) {}

    return 0;
}

So basically I'm trying to get an IP and subnet and convert it to binary but as instructed I can't use bitwise shift and %x to get the IP address as base 16. The problem is when I try to print the IP address given printf only prints dots and nothing else. And also one of the skips the subnet mask scanf. I would be really glad if I can get some help from you guys as I don't have many friends to ask questions. Thanks.

EDIT: I don't know if anyone is going to face this problem but my solution was rather silly but it works. You basically take the input as scanf("%c%c.%c%c.%c%c.%c%c%c"), the last %c is for the enter as char takes enter as input.

Answer 1

Hi and welcome to stackoverflow !

My answer will only help you about the base 16 part because you can easily do the base 10 based on what you already achieved and some hints i'm giving to you.

Parsing using base 16

Like you mentioned, you can't use %x to retrieve the IP address so another possibility is to use strings. An IP address is digits from 0 to 255 so you can expect hex characters from 0x00 to 0xFF.

To me, the best way to achieve that is to use a scanset with the scanf function:

scanf("%2[0-9A-F]", str);

Pattern %2[0-9A-F] means scanf is expecting at most two characters that must be any digits or any capital letters between A and F (so any hex character). Note, str length must be 3 to store 2 characters + the null terminator \0.

Then, str buffer will be filled with ASCII characters being a hex representation of the IP address. You can use the strtol function to convert a hex-representation-string to an integer.

The full code is below:

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

#define DIGIT_SIZE 3 // 2 hex digit + null character

int main(void) {

    char d1[DIGIT_SIZE];
    char d2[DIGIT_SIZE];
    char d3[DIGIT_SIZE];
    char d4[DIGIT_SIZE];

    scanf("%2[0-9A-F].%2[0-9A-F].%2[0-9A-F].%2[0-9A-F]", d1, d2, d3, d4);

    printf("hex ip:[%s.%s.%s.%s]\n", d1, d2, d3, d4);

    int i1 = (int)strtol(d1, NULL, 16);
    int i2 = (int)strtol(d2, NULL, 16);
    int i3 = (int)strtol(d3, NULL, 16);
    int i4 = (int)strtol(d4, NULL, 16);

    printf("dec ip:[%d.%d.%d.%d]\n", i1, i2, i3, i4);

    return 0;
}

Few tries on my side:

$ 8.8.8.8  
$ hex ip:[8.8.8.8]
$ dec ip:[8.8.8.8]

$ A.B.C.D
$ hex ip:[A.B.C.D]
$ dec ip:[10.11.12.13]

$ FF.FF.FF.FF
$ hex ip:[FF.FF.FF.FF]
$ dec ip:[255.255.255.255]

$ 0.0.0.0
$ hex ip:[0.0.0.0]
$ dec ip:[0.0.0.0]

Answer 2

There are multiple problems in your code:

• The switch statement is useless as hexa has a constant value and num is not used anyway.

• You should parse decimal numbers with %d, %u or %i and hexadecimal numbers with %x.

• If you insist on storing le IP selectors as bytes, you should use unsigned char instead of plain char and use %hhu and %hhx respectively.

• The test for number validity is incorrect, it is an error to have any number outside the range 0..255, so || is required between all tests.

• you should print the IP components with %d, not %c: %c output characters, not numbers. These characters probably are not printable, so only the dots appear.

Here is a modified version:

#include <stdio.h>

int main() {
    int base, a, b, c, d, x, y, z, t, n, choice;

    printf("Please enter the base: ");
    if (scanf("%d", &base) != 1)
        return 1;

    if (base != 10 && base != 16) {
        printf("invalid base\n");
        return 1;
    }

    printf("Please enter your IP address: ");
    if (base == 10) {
        n = scanf("%d.%d.%d.%d", &a, &b, &c, &d);
    } else {
        n = scanf("%x.%x.%x.%x", &a, &b, &c, &d);
    }
    if (n != 4) {
        printf("invalid input\n");
        return 1;
    }
    if (a < 0 || a > 255 || b < 0 || b > 255 || c < 0 || c > 255 || d < 0 || d > 255) {
        printf("invalid IP address\n");
        return 1;
    }
    printf("Please enter the sub-net mask: ");
    if (base == 10) {
        n = scanf("%d.%d.%d.%d", &x, &y, &z, &t);
    } else {
        n = scanf("%x.%x.%x.%x", &x, &y, &z, &t);
    }
    if (n != 4) {
        printf("invalid input\n");
        return 1;
    }
    if (x < 0 || x > 255 || y < 0 || y > 255 || z < 0 || z > 255 || t < 0 || t > 255) {
        printf("invalid sub-net mask\n");
        return 1;
    }

    if (scanf("%d", &choice) != 1) {
        printf("invalid input\n");
        return 1;
    }
    switch (choice) {
      case 1:
        printf("%d.%d.%d.%d\n", a, b, c, d);
        break;
      case 2:
      case 3:
      case 4:
      case 5:
      case 6:
        break;
    }
    return 0;
}

Note that there is a way to compact the validity test:

    if (((a | b | c | d) & ~255U) {
        printf("invalid IP address\n");
        return 1;
    }