How to clear a string for the previous value after using strcat()?

Question

I’m using the function strcat() to concatenate string and I’m having trouble returning the string to the previous state, what I’ve tried so far is this, but with nothing successful:

const char *get_route(const char *route)
{
    const char *final_str = strcat("https://discord.com/api/v9", route);
    return final_str;
}

I take the string that comes in the parameter route, as an example /users/@me and concateno with the discord_api_url, take for example https://discord.com/api/v9/users/@me, but when I use the function again, the new value that comes from route is not concatenated with the variable discord_api_url and yes with the previous one that was https://discord.com/api/v9/users/@me, I’ve tried to use malloc() in a new string and copy the https://discord.com/api/v9 for her, but when I give a free() the string goes away :p

How do I after concatenation, the URL string is not modified and I can reuse this function again?

Answer 1

I managed to find the following solution to work as I wanted:

char *discord_api_mutable = NULL;

const char *get_route(const char *route)
{
    static const char *discord_api_url = "https://discord.com/api/v9";
    const size_t str_size = strlen(discord_api_url) + strlen(route) + 1;

    if (discord_api_mutable == NULL)
        discord_api_mutable = malloc(str_size);
    else
        discord_api_mutable = realloc(discord_api_mutable, str_size);

    strcpy(discord_api_mutable, discord_api_url);
    strcat(discord_api_mutable, route);

    return discord_api_mutable;
}

I don’t know if it’s the right one, but it works, if anyone knows a better one, I’d be grateful