Arduino HTTP Client Library

Accessing HTTP Servers with Arduino HTTPClient Library

About HTTPClient

HTTPClient is a Arduino Library to easily post normal HTTP Requests using the Arduino Ethernet Shield (a WiFly Version is on the way). Instead of manually implementing the HTTP protocol for each and every web page you want to read from or post to you can simply use the HTTP Client library.

HTTPClient was implemented as memory aware as possible. E.g. all default HTTP information is directly read from flash memory to save up precious RAM.

Getting HTTP Client

You can download the latest version of HTTP Client from GitHub. Unzip it in your  Arduino Libraries Folder and include ‘HTTPClient.h’ in your Arduino sketch. Arduino.cc has some description how to do this.

If you encounter any problems there is an issue system on GitHub were you can post your issues.

Creating a HTTP Client

HTTP Client works with the Arduino Ethernet Library. Before you can create a HTTP client you must initialize your ethernet connection with something like:

Ethernet.begin(mac, ip);

For details on this see Server.begin() for the Arduino Ethernet Library.
To create a client (in this example for pachube) you can simply call one of
the constructors:

// The address of the server you want to connect to (pachube.com):
byte server[] = { 173,203,98,29 };
HTTPClient client("api.pachube.com",server);

which is equivalent to

HTTPClient client("api.pachube.com",server,80);

Now you are ready to go.

Posting request

HTTP client supports three types of requests:

  • normal GET request to get some data from a URL.
  • POST requests to transfer bigger amount of data to a server.
  • PUT request as used by REST APIs.
  • currently there is no need for DELETE requests – so they do not exist yet. But this wil change as soon as this kind of requests is needed.

Reading the Response Data

The result of a request is a stream (aka FILE*) by that you can read the data without the need to keep the whole answer in memory – which would fail for most HTML pages. FILE* streams are a bit more unusual the normal Arduino streams. They have been  choosen since you can use all the nice fprintff and fscanf routines of avr-libc.
After reading the response from the stream it has to be closed with the method

closeStream(stream)

Do not forget it! Each stream has some data attached and if you forget to close the stream you get a memory leak, slowly filling up the precious memory of the Arduino.
The result code of a HTTP request can be read with getLastReturnCode(). It returns a integer containing the return code. 200 indicates that everything was ok. For further details of HTTP return codes refer to RFC2616 Section 10.
The HTTPClient has also a debug mode which can be witched on and off by using debug() with parameter 0 as no debug and anything else ? e.g. -1 ? as enabling debug output.
By default the debug code is disabled. If debug is enabled the complete request and response is printed out on the serial connection. Very useful if your request do not work.

Passing Additional Parameters

All request take a number of parameters (depending on the request type):

  • The URI – a string (char*) containing the uri – which is normally everything following the hostname of a URL for http://arduino.cc/en/Reference/HomePage it would be /Reference/HomePage.
  • Optional parameters as key value pairs. Parameters are appended to a URL like http://myhost/the/uri?parameter-name=parameter-value&other=parameter parameters are values of the struct http_client_parameter. It is easiest to do this like

    http_client_parameter parameters[] = {
    { "key","afad32216dd2aa83c768ce51eef041d69a90a6737b2187dada3bb301e4c48841" }
    ,{ NULL,NULL }
    };
  • For POST and PUT request a string with additional data can be passed as a string. The data has to be in memory. Future Versions may have future features.
  • For all requests additional headers can be specified. It works exactly the same was as uri parameters:

    http_client_parameter pachube_api_header[] = {
    { "X-PachubeApiKey","afad32216dd2aa83c768ce51eef041d69a90a6737b2187dada3bb301e4c48841" }
    ,{ NULL,NULL }
    };

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