reduce the size of a string

Question

(disclaimer: this is not a complete exercise because I have to finish it, but error occurred in this part of code)

I did this exercise to practice memory allocation.

create a function that takes an url (a C string) and returns the name of the website (with "www." and with the extension). for example, given wikipedia's link, "http://www.wikipedia.org/", it has to return only "www.wikipedia.org" in another string (dynamically allocated in the heap).

this is what I did so far: do a for-loop, and when "i" is greater than 6, then start copying each character in another string until "/" is reached. I need to allocate the other string, and then reallocate that.

here's my attempt so far:

char *read_website(const char *url) {
    char *str = malloc(sizeof(char)); 
    if (str == NULL) {
        exit(1); 
    }
    for (unsigned int i = 0; url[i] != "/" && i > 6; ++i) {
        if (i <= 6) {
            continue; 
        }
        char* s = realloc(str, sizeof(char) + 1); 
        if (s == NULL) {
            exit(1); 
        }
        *str = *s; 
    }
    return str; 
}

int main(void) {
    char s[] = "http://www.wikipedia.org/"; 
    char *str = read_website(s); 
    return 0; 
}

(1) by debugging line-by-line, I've noticed that the program ends once for-loop is reached.

(2) another thing: I've chosen to create another pointer when I've used realloc, because I have to check if there's memory leak. Is it a good practice? Or should I've done something else?

Answer 1

There are multiple problems in your code:

• url[i] != "/" is incorrect, it is a type mismatch. You should compare the character url[i] with a character constant '/', not a string literal "/".

• char *s = realloc(str, sizeof(char) + 1); reallocates only to size 2, not the current length plus 1.

• you do not increase the pointers, neither do you use the index variable.

• instead of using malloc and realloc, you should first compute the length of the server name and allocate the array with the correct size directly.

Here is a modified version:

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

char *read_website(const char *url) {
    // skip the protocol part
    if (!strncmp(url, "http://", 7))
        url += 7;
    else if (!strncmp(url, "https://", 8))
        url += 8;
    // compute the length of the domain name, stop at ':' or '/'
    size_t n = strcspn(url, "/:");
    // return an allocated copy of the substring
    return strndup(url, n);
}

int main(void) {
    char s[] = "http://www.wikipedia.org/"; 
    char *str = read_website(s);
    printf("%s -> %s\n", s, str);
    free(str);
    return 0; 
}

strndup() is a POSIX function available on many systems and that will be part of the next version of the C Standard. If it is not available on your target, here is a simple implementation:

char *strndup(const char *s, size_t n) {
    char *p;
    size_t i;
    for (i = 0; i < n && s[i]; i++)
        continue;
    p = malloc(i + 1);
    if (p) {
        memcpy(p, s, i);
        p[i] = '\0';
    }
    return p;
}