Question

Background:

I’m working on my first C program with the library and I need to gather responses from each command sent to a SMTP server.

I’ve gotten as far as sending commands to the SMTP server and printing the response headers using curl_easy_setopt(curl_handle, CURLOPT_HEADERFUNCTION, parse_head), but I’m using multi threaded options. It is not at all clear when I get a response which command it was caused by. I am assuming that they will not necessarily be received in the same order sent. Is that correct?

Making it more difficult, since the library handles some calls (like setting up the initial connection) without my explicit request, I would need to handle more headers than explicit requests. That would be predictable and repeatable, but definitely adds an extra level of complexity.

Question:

Is there a “good” way to determine exactly which command resulted in which response header using multi thread?

Also, moderately related, does the library support returning the numeric return code or do I have to manually parse that out? Looking through the library, it seems that it doesn’t. I just want to be sure.

Answer 1

I am assuming that they will not necessarily be received in the same order sent. Is that correct?

Yes, it is. That’s how multithreading works.

Is there a “good” way to determine exactly which command resulted in which response header using multi thread?

Yes. You can set user data (context info, whatever you call it) using the CURLOPT_HEADERDATA option – this will be passed in as the 4th argument of your header function. So you can write code like this:

CURL *hndl = curl_easy_init();
// ... 
curl_easy_setopt(hndl, CURLOPT_HEADERFUNCTION, parse_head);
curl_easy_setopt(hndl, CURLOPT_HEADERDATA, some_pointer_that_identifies_the_thread);
// ...

size_t parse_head(void *buf, void *size_t sz, size_t nmemb, void *context)
{
    // context will be the pointer identifying the thread
}

does the library support returning the numeric return code or do I have to manually parse that out?

Yes, it does:

long httpStatus;
curl_easy_getinfo(hndl, CURLINFO_RESPONSE_CODE, &httpStatus);
if (200 <= httpStatus && httpStatus < 300) {
    // HTTP 2XX OK
} else {
    // Error (4XX, 5XX) or redirect (3XX)
}