How to check if a character is a digit

main.c

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

int main(int argc, char* argv[]) {
    char c = 't';

    if (c >= 48 && c <= 57) {
        printf("This is a digit: %c\n", c);
    } else {
        printf("This is not a digit: %c\n", c);
    }
    
    if (c >= '0' && c <= '9') {
        printf("This is a digit: %c\n", c);
    } else {
        printf("This is not a digit: %c\n", c);
    }
    return 0;
}