Regex and strncpy not working as I understood them

I'm running into multiple issues with the following code:

1. When I'm running this code on a file containing multiple URLs per line, the regex while loop runs indefinitely.
2. On a file containing only one HTTP URL on each line, valgrind complains that matches[count] = malloc(sizeof(char) * match_length); and matches[count][match_length] = 0; are Invalid size 1 write. I think my allocation code sucks (sizeof(matches) / sizeof(matches[0]) after my while loops compute to 1).
3. I thought strncpy would add a null byte at the end of my string automatically but it appears it doesn't 🤷
4. Might be related to 3. : After those while loops, if I print each element of the array, they contains a newline, which shouldn't be the case, only way to get rid of those is to run matches[count][match_length] = 0;

I think I misunderstood lots of part about regex, strncpy and malloc. I haven't done C programming since college and wanted to get back at it.

    char **matches = (char **)malloc(sizeof(char *));
    char *line = NULL;
    size_t l = 0, count = 0, size = 1;
    ssize_t read = 0;
    
    regex_t rgx;
    if (regcomp(&rgx, "((gopher|https?)://[[:alnum:][:punct:]]+)", REG_ICASE | REG_EXTENDED)) {
        return 0;
    }
    
    while ((read = getline(&line, &l, file)) != -1) {
        regmatch_t match = { 0, 0 };
        int match_length, error;
        if ((error = regexec(&rgx, line + match.rm_eo, 1, &match, 0)) != 0)
            continue;
    
        do {
            printf("Error: %d, Line: %s\n", error, line + match.rm_eo);
            match_length = match.rm_eo - match.rm_so;
            matches[count] = malloc(sizeof(char) * match_length);
            strncpy(matches[count], line + match.rm_so, match_length);
            matches[count][match_length] = 0;
    
            if (++count == size) {
                size *= 2;
                matches = realloc(matches, sizeof(char *) * size);
                if (!matches) {
                    regfree(&rgx);
                    free(line);
                    return 0;
                }
            }
        } while ((error = regexec(&rgx, line + match.rm_eo + 1, 1, &match, REG_NOTBOL)) == 0);
    }

There are multiple problems:

• you should allocate one more byte in matches[count] = malloc(sizeof(char) * match_length); for the null terminator.

• strncpy does not set a null byte at the end of the destination array if the source string has a length greater or equal to the third length argument. This function semantics are confusing and error prone, learn why you should stop using strncpy already!

• instead of strncpy(matches[count], line + match.rm_so, match_length); you should use:

  memcpy(matches[count], line + match.rm_so, match_length);
  matches[count][match_length] = '\0';
  

  Note that allocating and copying can be achieved with:

  matches[count] = strndup(line + match.rm_so, match_length);
  

• matches[count][match_length] = 0; is causing the valgrind error message as you write beyond the allocation length of match_length.

• matches = realloc(matches, sizeof(char *) * size); causes a memory leak if realloc fails: the original pointer matches is lost and the memory it pointed to is still allocated.

• you can strip the newline at the end of line before applying the regex matcher.

• the positions stored in the match structure are relative to the beginning of the string argument to regexec, but you pass line + match.rm_so and line + match.rm_eo + 1 so the offsets are not relative to the line array and cannot be used.

• after the loop, you cannot use (sizeof(matches) / sizeof(matches[0]) to determine the number of entries in the array. Both matches and matches[0] are pointers, hence the value 1. You must pass count back to the caller along with the matches pointer.

Here is a modified version:

char **filter_urls(FILE *file, size_t *countp) {
    size_t count = 0, size = 1;
    char **matches = (char **)malloc(sizeof(char *) * size);
    char *line = NULL;
    size_t line_size = 0;
    int fail = 0;
    ssize_t len;
    regex_t rgx;

    if (matches == NULL)
        return NULL;

    if (regcomp(&rgx, "((gopher|https?)://[[:alnum:][:punct:]]+)", REG_ICASE | REG_EXTENDED)) {
        free(matches);
        return NULL;
    }
    
    while ((len = getline(&line, &line_size, file)) != -1) {
        // strip the trailing newline if any (optional)
        //if (len > 0 && line[len - 1] == '\n') {
        //    line[--len] == '\0'
        //}
        regmatch_t match = { 0, 0 };
        int flags = 0;
        char *ptr = line;
        while (regexec(&rgx, ptr, 1, &match, flags) != 0) {
            size_t match_length = match.rm_eo - match.rm_so;
            matches[count] = strndup(ptr + match.rm_so, match_length);
            if (matches[count] == NULL) {
                fail = 1;
                break;
            }
            if (++count == size) {
                size_t new_size = size * 2;
                char **new_matches = realloc(matches, sizeof(char *) * new_size);
                if (new_matches == NULL) {
                    fail = 1;
                    break;
                }
                size = new_size;
                matches = new_matches;
            }
            ptr += match.rm_eo;
            flags = REG_NOTBOL;
        }
    }
    regfree(&rgx);
    free(line);
    if (fail) {
        // allocation failure: free all
        while (count > 0) {
            free(matches[--count]);
        }
        free(matches);
        return NULL;
    }
    matches[count] = NULL;
    *countp = count;
    return matches;
}