libcurl-the-guide

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PROGRAMMING WITH LIBCURL

About this Document

 This document will attempt to describe the general principle and some basic
 approaches to consider when programming with libcurl. The text will focus
 mainly on the C interface but might apply fairly well on other interfaces as
 well as they usually follow the C one pretty closely.

 This document will refer to 'the user' as the person writing the source code
 that uses libcurl. That would probably be you or someone in your position.
 What will be generally refered to as 'the program' will be the collected
 source code that you write that is using libcurl for transfers. The program
 is outside libcurl and libcurl is outside of the program.

 To get the more details on all options and functions described herein, please
 refer to their respective man pages.

Building

 There are many different ways to build C programs. This chapter will assume a
 unix-style build process. If you use a different build system, you can still
 read this to get general information that may apply to your environment as
 well.

  Compiling the Program

    Your compiler needs to know where the libcurl headers are
    located. Therefore you must set your compiler's include path to point to
    the directory where you installed them. The 'curl-config'[3] tool can be
    used to get this information:

        $ curl-config --cflags

  Linking the Program with libcurl

    When having compiled the program, you need to link your object files to
    create a single executable. For that to succeed, you need to link with
    libcurl and possibly also with other libraries that libcurl itself depends
    on. Like OpenSSL librararies, but even some standard OS libraries may be
    needed on the command line. To figure out which flags to use, once again
    the 'curl-config' tool comes to the rescue:

        $ curl-config --libs

  SSL or Not

    libcurl can be built and customized in many ways. One of the things that
    varies from different libraries and builds is the support for SSL-based
    transfers, like HTTPS and FTPS. If OpenSSL was detected properly at
    build-time, libcurl will be built with SSL support. To figure out if an
    installed libcurl has been built with SSL support enabled, use
    'curl-config' like this:

        $ curl-config --feature

    And if SSL is supported, the keyword 'SSL' will be written to stdout,
    possibly together with a few other features that can be on and off on
    different libcurls.


Portable Code in a Portable World

 The people behind libcurl have put a considerable effort to make libcurl work
 on a large amount of different operating systems and environments.

 You program libcurl the same way on all platforms that libcurl runs on. There
 are only very few minor considerations that differs. If you just make sure to
 write your code portable enough, you may very well create yourself a very
 portable program. libcurl shouldn't stop you from that.


Global Preparation

 The program must initialize some of the libcurl functionality globally. That
 means it should be done exactly once, no matter how many times you intend to
 use the library. Once for your program's entire life time. This is done using

    curl_global_init()

 and it takes one parameter which is a bit pattern that tells libcurl what to
 intialize. Using CURL_GLOBAL_ALL will make it initialize all known internal
 sub modules, and might be a good default option. The current two bits that
 are specified are:

  CURL_GLOBAL_WIN32 which only does anything on Windows machines. When used on
  a Windows machine, it'll make libcurl intialize the win32 socket
  stuff. Without having that initialized properly, your program cannot use
  sockets properly. You should only do this once for each application, so if
  your program already does this or of another library in use does it, you
  should not tell libcurl to do this as well.

  CURL_GLOBAL_SSL which only does anything on libcurls compiled and built
  SSL-enabled. On these systems, this will make libcurl init OpenSSL properly
  for this application. This is only needed to do once for each application so
  if your program or another library already does this, this bit should not be
  needed.

 libcurl has a default protection mechanism that detects if curl_global_init()
 hasn't been called by the time curl_easy_perform() is called and if that is
 the case, libcurl runs the function itself with a guessed bit pattern. Please
 note that depending solely on this is not considered nice nor very good.

 When the program no longer uses libcurl, it should call
 curl_global_cleanup(), which is the opposite of the init call. It will then
 do the reversed operations to cleanup the resources the curl_global_init()
 call initialized.

 Repeated calls to curl_global_init() and curl_global_cleanup() should be
 avoided. They should only be called once each.

Handle the Easy libcurl

 libcurl version 7 is oriented around the so called easy interface. All
 operations in the easy interface are prefixed with 'curl_easy'.

 Future libcurls will also offer the multi interface. More about that
 interface, what it is targeted for and how to use it is still only debated on
 the libcurl mailing list and developer web pages. Join up to discuss and
 figure out!

 To use the easy interface, you must first create yourself an easy handle. You
 need one handle for each easy session you want to perform. Basicly, you
 should use one handle for every thread you plan to use for transferring. You
 must never share the same handle in multiple threads.

 Get an easy handle with

    easyhandle = curl_easy_init();

 It returns an easy handle. Using that you proceed to the next step: setting
 up your preferred actions. A handle is just a logic entity for the upcoming
 transfer or series of transfers.

 You set properties and options for this handle using curl_easy_setopt(). They
 control how the subsequent transfer or transfers will be made. Options remain
 set in the handle until set again to something different. Alas, multiple
 requests using the same handle will use the same options.

 Many of the informationals you set in libcurl are "strings", pointers to data
 terminated with a zero byte. Keep in mind that when you set strings with
 curl_easy_setopt(), libcurl will not copy the data. It will merely point to
 the data. You MUST make sure that the data remains available for libcurl to
 use until finished or until you use the same option again to point to
 something else.

 One of the most basic properties to set in the handle is the URL. You set
 your preferred URL to transfer with CURLOPT_URL in a manner similar to:

    curl_easy_setopt(easyhandle, CURLOPT_URL, "http://curl.haxx.se/");

 Let's assume for a while that you want to receive data as the URL indentifies
 a remote resource you want to get here. Since you write a sort of application
 that needs this transfer, I assume that you would like to get the data passed
 to you directly instead of simply getting it passed to stdout. So, you write
 your own function that matches this prototype:

    size_t write_data(void *buffer, size_t size, size_t nmemb, void *userp);

 You tell libcurl to pass all data to this function by issuing a function
 similar to this:

    curl_easy_setopt(easyhandle, CURLOPT_WRITEFUNCTION, write_data);

 You can control what data your function get in the forth argument by setting
 another property:

    curl_easy_setopt(easyhandle, CURLOPT_FILE, &internal_struct);

 Using that property, you can easily pass local data between your application
 and the function that gets invoked by libcurl. libcurl itself won't touch the
 data you pass with CURLOPT_FILE.

 libcurl offers its own default internal callback that'll take care of the
 data if you don't set the callback with CURLOPT_WRITEFUNCTION. It will then
 simply output the received data to stdout. You can have the default callback
 write the data to a different file handle by passing a 'FILE *' to a file
 opened for writing with the CURLOPT_FILE option.

 Now, we need to take a step back and have a deep breath. Here's one of those
 rare platform-dependent nitpicks. Did you spot it? On some platforms[2],
 libcurl won't be able to operate on files opened by the program. Thus, if you
 use the default callback and pass in a an open file with CURLOPT_FILE, it
 will crash. You should therefore avoid this to make your program run fine
 virtually everywhere.

 There are of course many more options you can set, and we'll get back to a
 few of them later. Let's instead continue to the actual transfer:

    success = curl_easy_perform(easyhandle);

 The curl_easy_perform() will connect to the remote site, do the necessary
 commands and receive the transfer. Whenever it receives data, it calls the