我一直在用C编写这段代码，它有很多漏洞，我不知道如何清理。
当我在main上调用get_next_line时，我希望它返回.txt文件的下一行。
有人能帮帮我吗？

size_t  ft_strlen(char *s)
{
    size_t  i;

    if (!s)
        return (0);
    i = 0;
    while (s[i] != '\0')
        i++;
    return (i);
}

char    *ft_strchr(char *s, int c)
{
    size_t  i;

    i = 0;
    if (!s)
        return (0);
    while ((s[i] != '\0') && (s[i] != (unsigned char)c))
        i++;
    if ((s[i] == (unsigned char)c) || (c == '\0'))
        return ((char *)&s[i]);
    return (0);
}

char    *ft_substr(char *s, unsigned int start, size_t len)
{
    size_t  i;
    size_t  j;
    char    *sub;

    i = 0;
    j = 0;
    if (start >= ft_strlen(s) || !s || !len)
    {
        sub = malloc(1 * sizeof(char));
        sub[0] = '\0';
        return (sub);
    }
    while (s[start + j] != '\0' && j < len)
        j++;
    if (s[start + j] != '\0')
        j++;
    sub = malloc((j + 1) * sizeof(char));
    if (!sub)
    {
        free(s);
        return (0); 
    }
    while (s[start] != '\0' && j > i)
        sub[i++] = s[start++];
    sub[i] = '\0';
    //free(s);
    return (sub);
}

char    *ft_strjoin(char *s1, char *s2)
{
    char    *cat;
    size_t  i;
    size_t  j;

    i = 0;
    j = 0;
    if (!s1)
    {
        s1 = malloc(sizeof(char) * 1);
        s1[0] = '\0';
    }
    cat = (char *)malloc((ft_strlen(s1) + ft_strlen(s2) + 1) * sizeof(char));
    if (!cat)
        return (NULL);
    while (s1[i] != '\0')
    {
        cat[i] = s1[i];
        i++;
    }
    while (s2[j] != '\0')
        cat[i++] = s2[j++];
    free(s1);
    cat[i] = '\0';
    return (cat);
}

char    *ft_content(int fd, char *content)
{
    char    *buf;
    int     buf_nb;

    buf_nb = 1;
    buf = (char *)malloc((BUFFER_SIZE + 1) * sizeof(char));
    if (!buf)
        return (0);
    while ((!ft_strchr(content, '\n')) && buf_nb != 0)
    {
        buf_nb = read(fd, buf, BUFFER_SIZE);
        if (buf_nb == -1)
        {
            free(buf);
            free(content);
            return (NULL);
        }
        buf[buf_nb] = '\0';
        content = ft_strjoin(content, buf);
    }
    free(buf);
    return (content);
}

char    *get_next_line(int fd)
{
    static char *content;
    char        *sub;
    int         i;
    int         len;

    i = 0;
    if (fd < 0 || BUFFER_SIZE <= 0)
        return (NULL);
    content = ft_content(fd, content);
    if (content == NULL)
        return (NULL);
    if (content[0] == '\0')
        return (NULL);
    while (content[i] != '\n' && content[i] != '\0')
        i++;
    sub = ft_substr(content, 0, i);
    len = ft_strlen(content);
    i++;
    content = ft_substr(content, i, len);
    return (sub);
}
int    main(void)
{
    int    a = open("exemplo.txt", O_RDONLY);
    printf("%s", get_next_line(a));
    printf("%s", get_next_line(a));
    printf("%s", get_next_line(a));
    printf("%s", get_next_line(a));
    return (0);
}

我已经做了几个变化，但我总是得到内存泄漏和分段故障。
我只想看看文件。