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This document describes details on using the evhtp API

Required Dependencies

Optional Dependencies


Libevhtp was created as a replacement API for Libevent's current HTTP API. The reality of libevent's http interface is that it was created as a JIT server, meaning the developer never thought of it being used for creating a full-fledged HTTP service. Infact I am under the impression that the libevent http API was designed almost as an example of what you can do with libevent. It's not Apache in a box, but more and more developers are attempting to use it as so.

Libevent's HTTP pitfalls

  • It was not designed to be a fully functional HTTP server.
  • The code is messy, abstractions are almost non-existent, and feature-creep has made long-term maintainability very hard.
  • The parsing code is slow and requires data to be buffered before a full parse can be completed. This results in extranious memory usage and lots of string comparison functions.
  • There is no method for a user to access various parts of the request processing cycle. For example if the "Content-Length" header has a value of 50000, your callback is not executed until all 50000 bytes have been read.
  • Setting callback URI's do exact matches; meaning if you set a callback for "/foo/", requests for "/foo/bar/" are ignored.
  • Creating an HTTPS server is hard, it requires a bunch of work to be done on the underlying bufferevents.
  • As far as I know, streaming data back to a client is hard, if not impossible without messing with underlying bufferevents.
  • It's confusing to work with, this is probably due to the lack of proper documentation.

Libevhtp attempts to address these problems along with a wide variety of cool mechanisms allowing a developer to have complete control over your server operations. This is not to say the API cannot be used in a very simplistic manner - a developer can easily create a backwards compatible version of libevent's HTTP server to libevhtp.

A bit about the architecture of libevhtp


  1. Create a parent evhtp_t structure.
  2. Assign callbacks to the parent for specific URIs or posix-regex based URI's
  3. Optionally assign per-connection hooks (see hooks) to the callbacks.
  4. Optionally assign pre-accept and post-accept callbacks for incoming connections.
  5. Optionally enable built-in threadpool for connection handling (lock-free, and non-blocking).
  6. Optionally morph your server to HTTPS.
  7. Start the evhtp listener.

Request handling.

  1. Optionally deal with pre-accept and post-accept callbacks if they exist, allowing for a connection to be rejected if the function deems it as unacceptable.
  2. Optionally assign per-request hooks (see hooks) for a request (the most optimal place for setting these hooks is on a post-accept callback).
  3. Deal with either per-connection or per-request hook callbacks if they exist.
  4. Once the request has been fully processed, inform evhtp to send a reply.
A very basic example with no optional conditions.
#include <stdio.h>
#include <evhtp.h>

testcb(evhtp_request_t * req, void * a) {
    evbuffer_add_reference(req->buffer_out, "foobar", 6, NULL, NULL);
    evhtp_send_reply(req, EVHTP_RES_OK);

main(int argc, char ** argv) {
    evbase_t * evbase = event_base_new();
    evhtp_t  * htp    = evhtp_new(evbase, NULL);

    evhtp_set_cb(htp, "/test", testcb, NULL);
    evhtp_bind_socket(htp, "", 8080, 1024);
    event_base_loop(evbase, 0);
    return 0;
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