Question

I am writing a C extension for PHP and there I have to build up a string based on some substrings. Currently it looks like this:

spprintf(
    &bucket_list_url, 
    strlen(host) + 1 + sizeof(port) + 1 + strlen(prefix) +strlen("?buckets=true"),
    "%s:%d/%s?buckets=true",
    host, 
    port,
    prefix
);

It works, but looks horrible and is possibly difficult to maintain. Is there a more cleaner way for safely concatenating these strings together?

I am a beginner in C, as you’ve probably noticed, so I couldn’t come up with any cleaner solution so far.

Answer 1

If you’re using the GNU C runtime (glibc), you can use the asprintf(3) function to format a string to a dynamically allocated buffer. That way, you don’t have to worry about having a large enough buffer. For example:

// Error checking omitted for expository purposes
char *bucket_list_url;
asprintf(&bucket_list_url, "%s:%d/%s?buckets=true", host, port, prefix);
...  // do stuff
free(bucket_list_url);

If you’re not using glibc, you can still use snprintf(3), but you have to guess at the buffer length. If you guess wrong, you have to allocate a bigger buffer and try again.