负输入使程序崩溃

Question

I have the following function:

char    *ft_substr(char const *s, unsigned int start, size_t len)
{
    char    *substr;

    if (!s)
        return (NULL);
    if (start > ft_strlen(s))
        len = 0;
    if (start <= ft_strlen(s) && s[start] != '\0')
    {   
        if (ft_strlen(s + start) < len)
            len = ft_strlen(s + start);
    }
    substr = (char *)malloc(sizeof(*s) * (len + 1));
    if (!substr)
        return (NULL);
    if (len == 0)
        return (substr);
    ft_strlcpy(substr, s + start, len + 1); 
    return (substr);
}

Some of the functions are self-made (strlen = ft_strlen etc).

The problem: if I pass in -1 for "start", my program segfaults because it is converted to 4.294.967.295. So it crashes when passed in that is my function body does not even execute making all protective measures useless. Even stranger, if I execute this on my Mac everything works fine, on Ubuntu 22.04 it does not.

Any suggestions? I am not allowed to change the function prototype.

Edit: I have to pass a series of tests and the following one I am failing constantly:

Error in test 4: ft_substr("hola", 4294967295, 0): Segmentation fault!

(The test actually inputs -1).

Also, I am not allowed to include a main that tests user input.

Edit 2: My main is simple and just calls the function with input "hola", -1, 3

Answer 1

You need to terminate the string you've copied with ft_strlcpy with '\0' . Your bounds checking could also be made a little simpler.

Example:

char *ft_substr(char const *s, unsigned int start, size_t len)
{
    if (!s) return NULL;

    size_t slen = ft_strlen(s);
    
    if (start > slen) {                 // start out of bounds, make empty string
        start = 0;
        len = 0;
    } else if(start + len > slen) {     // len is too large, shrink it
        len = slen - start;
    }

    char *substr = malloc(len + 1);
    if (!substr) return NULL;
    
    ft_strlcpy(substr, s + start, len); // note: copy len, not len+1
    substr[len] = '\0';                 // terminate the string
    
    return substr;
}