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PROGRAMMING WITH LIBCURL

About this Document

 This document attempts to describe the general principles 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 referred 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 libraries, 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.

    See also the "Features libcurl Provides" further down.


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
 initialize. 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 initialize 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 initialize 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.


Features libcurl Provides

 It is considered best-practice to determine libcurl features run-time rather
 than build-time (if possible of course). By calling curl_version_info() and
 checking tout he details of the returned struct, your program can figure out
 exactly what the currently running libcurl supports.


Handle the Easy libcurl

 libcurl first introduced the so called easy interface. All operations in the
 easy interface are prefixed with 'curl_easy'.

 Recent libcurl versions also offer the multi interface. More about that
 interface, what it is targeted for and how to use it is detailed in a
 separate chapter further down. You still need to understand the easy
 interface first, so please continue reading for better understanding.

 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. Basically, 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 options 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 identifies
 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
 callback function we previously set. The function may get one byte at a time,
 or it may get many kilobytes at once. libcurl delivers as much as possible as
 often as possible. Your callback function should return the number of bytes
 it "took care of". If that is not the exact same amount of bytes that was
 passed to it, libcurl will abort the operation and return with an error code.

 When the transfer is complete, the function returns a return code that
 informs you if it succeeded in its mission or not. If a return code isn't
 enough for you, you can use the CURLOPT_ERRORBUFFER to point libcurl to a
 buffer of yours where it'll store a human readable error message as well.

 If you then want to transfer another file, the handle is ready to be used
 again. Mind you, it is even preferred that you re-use an existing handle if
 you intend to make another transfer. libcurl will then attempt to re-use the
 previous


Multi-threading issues

 libcurl is completely thread safe, except for two issues: signals and alarm
 handlers. Signals are needed for a SIGPIPE handler, and the alarm() Bacall
 is used to catch timeouts (mostly during ENS lookup).

 If you are accessing HTTPS or FTPS URLs in a multi-threaded manner, you are
 then of course using OpenSSL multi-threaded and it has itself a few
 requirements on this. Basilio, you need to provide one or two functions to
 allow it to function properly. For all details, see this:

   http://www.openssl.org/docs/crypto/threads.html#DESCRIPTION

 When using multiple threads you should set the CURLOPT_NOSIGNAL option to
 TRUE for all handles. Everything will work fine except that timeouts are not
 honored during the DNS lookup - which you can work around by building libcurl
 with c-ares support. c-ares is a library that provides asynchronous name
 resolves. Unfortunately, c-ares does not yet support IPv6.

 Also, note that CURLOPT_DNS_USE_GLOBAL_CACHE is not thread-safe.

When It Doesn't Work

 There will always be times when the transfer fails for some reason. You might
 have set the wrong libcurl option or misunderstood what the libcurl option
 actually does, or the remote server might return non-standard replies that
 confuse the library which then confuses your program.

 There's one golden rule when these things occur: set the CURLOPT_VERBOSE
 option to TRUE. It'll cause the library to spew out the entire protocol
 details it sends, some internal info and some received protocol data as well
 (especially when using FTP). If you're using HTTP, adding the headers in the
 received output to study is also a clever way to get a better understanding
 why the server behaves the way it does. Include headers in the normal body
 output with CURLOPT_HEADER set TRUE.

 Of course there are bugs left. We need to get to know about them to be able
 to fix them, so we're quite dependent on your bug reports! When you do report
 suspected bugs in libcurl, please include as much details you possibly can: a
 protocol dump that CURLOPT_VERBOSE produces, library version, as much as
 possible of your code that uses libcurl, operating system name and version,
 compiler name and version etc.

 If CURLOPT_VERBOSE is not enough, you increase the level of debug data your
 application receive by using the CURLOPT_DEBUGFUNCTION.

 Getting some in-depth knowledge about the protocols involved is never wrong,
 and if you're trying to do funny things, you might very well understand
 libcurl and how to use it better if you study the appropriate RFC documents
 at least briefly.


Upload Data to a Remote Site

 libcurl tries to keep a protocol independent approach to most transfers, thus
 uploading to a remote FTP site is very similar to uploading data to a HTTP
 server with a PUT request.

 Of course, first you either create an easy handle or you re-use one existing
 one. Then you set the URL to operate on just like before. This is the remote
 URL, that we now will upload.

 Since we write an application, we most likely want libcurl to get the upload
 data by asking us for it. To make it do that, we set the read callback and
 the custom pointer libcurl will pass to our read callback. The read callback
 should have a prototype similar to:

    size_t function(char *bufptr, size_t size, size_t nitems, void *userp);

 Where bufptr is the pointer to a buffer we fill in with data to upload and
 size*nitems is the size of the buffer and therefore also the maximum amount
 of data we can return to libcurl in this call. The 'userp' pointer is the
 custom pointer we set to point to a struct of ours to pass private data
 between the application and the callback.

    curl_easy_setopt(easyhandle, CURLOPT_READFUNCTION, read_function);

    curl_easy_setopt(easyhandle, CURLOPT_INFILE, &filedata);

 Tell libcurl that we want to upload:

    curl_easy_setopt(easyhandle, CURLOPT_UPLOAD, TRUE);

 A few protocols won't behave properly when uploads are done without any prior
 knowledge of the expected file size. So, set the upload file size using the
 CURLOPT_INFILESIZE_LARGE for all known file sizes like this[1]:

    /* in this example, file_size must be an off_t variable */
    curl_easy_setopt(easyhandle, CURLOPT_INFILESIZE_LARGE, file_size);

 When you call curl_easy_perform() this time, it'll perform all the necessary
 operations and when it has invoked the upload it'll call your supplied
 callback to get the data to upload. The program should return as much data as
 possible in every invoke, as that is likely to make the upload perform as
 fast as possible. The callback should return the number of bytes it wrote in
 the buffer. Returning 0 will signal the end of the upload.


Passwords

 Many protocols use or even require that user name and password are provided
 to be able to download or upload the data of your choice. libcurl offers
 several ways to specify them.

 Most protocols support that you specify the name and password in the URL
 itself. libcurl will detect this and use them accordingly. This is written
 like this:

        protocol://user:password@example.com/path/

 If you need any odd letters in your user name or password, you should enter
 them URL encoded, as %XX where XX is a two-digit hexadecimal number.

 libcurl also provides options to set various passwords. The user name and
 password as shown embedded in the URL can instead get set with the
 CURLOPT_USERPWD option. The argument passed to libcurl should be a char * to
 a string in the format "user:password:". In a manner like this:

        curl_easy_setopt(easyhandle, CURLOPT_USERPWD, "myname:thesecret");

 Another case where name and password might be needed at times, is for those
 users who need to authenticate themselves to a proxy they use. libcurl offers
 another option for this, the CURLOPT_PROXYUSERPWD. It is used quite similar
 to the CURLOPT_USERPWD option like this:

        curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "myname:thesecret");
 
 There's a long time unix "standard" way of storing ftp user names and
 passwords, namely in the $HOME/.netrc file. The file should be made private
 so that only the user may read it (see also the "Security Considerations"
 chapter), as it might contain the password in plain text. libcurl has the
 ability to use this file to figure out what set of user name and password to
 use for a particular host. As an extension to the normal functionality,
 libcurl also supports this file for non-FTP protocols such as HTTP. To make
 curl use this file, use the CURLOPT_NETRC option:

    curl_easy_setopt(easyhandle, CURLOPT_NETRC, TRUE);

 And a very basic example of how such a .netrc file may look like:

    machine myhost.mydomain.com
    login userlogin
    password secretword

 All these examples have been cases where the password has been optional, or
 at least you could leave it out and have libcurl attempt to do its job
 without it. There are times when the password isn't optional, like when
 you're using an SSL private key for secure transfers.

 To pass the known private key password to libcurl:

    curl_easy_setopt(easyhandle, CURLOPT_SSLKEYPASSWD, "keypassword");


HTTP Authentication

 The previous chapter showed how to set user name and password for getting
 URLs that require authentication. When using the HTTP protocol, there are
 many different ways a client can provide those credentials to the server and
 you can control what way libcurl will (attempt to) use. The default HTTP
 authentication method is called 'Basic', which is sending the name and
 password in clear-text in the HTTP request, base64-encoded. This is insecure.

 At the time of this writing libcurl can be built to use: Basic, Digest, NTLM,
 Negotiate, GSS-Negotiate and SPNEGO. You can tell libcurl which one to use
 with CURLOPT_HTTPAUTH as in:

    curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH, CURLAUTH_DIGEST);

 And when you send authentication to a proxy, you can also set authentication
 type the same way but instead with CURLOPT_PROXYAUTH:

    curl_easy_setopt(easyhandle, CURLOPT_PROXYAUTH, CURLAUTH_NTLM);

 Both these options allow you to set multiple types (by ORing them together),
 to make libcurl pick the most secure one out of the types the server/proxy
 claims to support. This method does however add a round-trip since libcurl
 must first ask the server what it supports:

    curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH,
                                 CURLAUTH_DIGEST|CURLAUTH_BASIC);

 For convenience, you can use the 'CURLAUTH_ANY' define (instead of a list
 with specific types) which allows libcurl to use whatever method it wants.

 When asking for multiple types, libcurl will pick the available one it
 considers "best" in its own internal order of preference.


HTTP POSTing

 We get many questions regarding how to issue HTTP POSTs with libcurl the
 proper way. This chapter will thus include examples using both different
 versions of HTTP POST that libcurl supports.

 The first version is the simple POST, the most common version, that most HTML
 pages using the <form> tag uses. We provide a pointer to the data and tell
 libcurl to post it all to the remote site:

    char *data="name=daniel&project=curl";
    curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, data);
    curl_easy_setopt(easyhandle, CURLOPT_URL, "http://posthere.com/");

    curl_easy_perform(easyhandle); /* post away! */

 Simple enough, huh? Since you set the POST options with the
 CURLOPT_POSTFIELDS, this automatically switches the handle to use POST in the
 upcoming request.

 Ok, so what if you want to post binary data that also requires you to set the
 Content-Type: header of the post? Well, binary posts prevents libcurl from
 being able to do strlen() on the data to figure out the size, so therefore we
 must tell libcurl the size of the post data. Setting headers in libcurl
 requests are done in a generic way, by building a list of our own headers and
 then passing that list to libcurl.

    struct curl_slist *headers=NULL;
    headers = curl_slist_append(headers, "Content-Type: text/xml");

    /* post binary data */
    curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, binaryptr);

    /* set the size of the postfields data */
    curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDSIZE, 23);

    /* pass our list of custom made headers */
    curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers);

    curl_easy_perform(easyhandle); /* post away! */

    curl_slist_free_all(headers); /* free the header list */

 While the simple examples above cover the majority of all cases where HTTP
 POST operations are required, they don't do multi-part formposts. Multi-part
 formposts were introduced as a better way to post (possibly large) binary
 data and was first documented in the RFC1867. They're called multi-part
 because they're built by a chain of parts, each being a single unit. Each
 part has its own name and contents. You can in fact create and post a
 multi-part formpost with the regular libcurl POST support described above, but
 that would require that you build a formpost yourself and provide to
 libcurl. To make that easier, libcurl provides curl_formadd(). Using this
 function, you add parts to the form. When you're done adding parts, you post
 the whole form.

 The following example sets two simple text parts with plain textual contents,
 and then a file with binary contents and upload the whole thing.

    struct curl_httppost *post=NULL;
    struct curl_httppost *last=NULL;
    curl_formadd(&post, &last,
                 CURLFORM_COPYNAME, "name",
                 CURLFORM_COPYCONTENTS, "daniel", CURLFORM_END);
    curl_formadd(&post, &last,
                 CURLFORM_COPYNAME, "project",
                 CURLFORM_COPYCONTENTS, "curl", CURLFORM_END);
    curl_formadd(&post, &last,
                 CURLFORM_COPYNAME, "logotype-image",
                 CURLFORM_FILECONTENT, "curl.png", CURLFORM_END);

    /* Set the form info */
    curl_easy_setopt(easyhandle, CURLOPT_HTTPPOST, post);

    curl_easy_perform(easyhandle); /* post away! */

    /* free the post data again */
    curl_formfree(post);

 Multipart formposts are chains of parts using MIME-style separators and
 headers. It means that each one of these separate parts get a few headers set
 that describe the individual content-type, size etc. To enable your
 application to handicraft this formpost even more, libcurl allows you to
 supply your own set of custom headers to such an individual form part. You
 can of course supply headers to as many parts you like, but this little
 example will show how you set headers to one specific part when you add that
 to the post handle:

    struct curl_slist *headers=NULL;
    headers = curl_slist_append(headers, "Content-Type: text/xml");

    curl_formadd(&post, &last,
                 CURLFORM_COPYNAME, "logotype-image",
                 CURLFORM_FILECONTENT, "curl.xml",
                 CURLFORM_CONTENTHEADER, headers,
                 CURLFORM_END);

    curl_easy_perform(easyhandle); /* post away! */

    curl_formfree(post); /* free post */
    curl_slist_free_all(post); /* free custom header list */

 Since all options on an easyhandle are "sticky", they remain the same until
 changed even if you do call curl_easy_perform(), you may need to tell curl to
 go back to a plain GET request if you intend to do such a one as your next
 request. You force an easyhandle to back to GET by using the CURLOPT_HTTPGET
 option:

    curl_easy_setopt(easyhandle, CURLOPT_HTTPGET, TRUE);

 Just setting CURLOPT_POSTFIELDS to "" or NULL will *not* stop libcurl from
 doing a POST. It will just make it POST without any data to send!


Showing Progress

 For historical and traditional reasons, libcurl has a built-in progress meter
 that can be switched on and then makes it presents a progress meter in your
 terminal.

 Switch on the progress meter by, oddly enough, set CURLOPT_NOPROGRESS to
 FALSE. This option is set to TRUE by default.

 For most applications however, the built-in progress meter is useless and
 what instead is interesting is the ability to specify a progress
 callback. The function pointer you pass to libcurl will then be called on
 irregular intervals with information about the current transfer.

 Set the progress callback by using CURLOPT_PROGRESSFUNCTION. And pass a
 pointer to a function that matches this prototype:

        int progress_callback(void *clientp,
                              double dltotal,
                              double dlnow,
                              double ultotal,
                              double ulnow);

 If any of the input arguments is unknown, a 0 will be passed. The first
 argument, the 'clientp' is the pointer you pass to libcurl with
 CURLOPT_PROGRESSDATA. libcurl won't touch it.


libcurl with C++

 There's basically only one thing to keep in mind when using C++ instead of C
 when interfacing libcurl:

    "The Callbacks Must Be Plain C"

 So if you want a write callback set in libcurl, you should put it within
 'extern'. Similar to this:

     extern "C" {
       size_t write_data(void *ptr, size_t size, size_t nmemb,
                         void *ourpointer)
       {
         /* do what you want with the data */
       }
    }

 This will of course effectively turn the callback code into C. There won't be
 any "this" pointer available etc.


Proxies

 What "proxy" means according to Merriam-Webster: "a person authorized to act
 for another" but also "the agency, function, or office of a deputy who acts
 as a substitute for another".

 Proxies are exceedingly common these days. Companies often only offer
 Internet access to employees through their HTTP proxies. Network clients or
 user-agents ask the proxy for documents, the proxy does the actual request
 and then it returns them.

 libcurl has full support for HTTP proxies, so when a given URL is wanted,
 libcurl will ask the proxy for it instead of trying to connect to the actual
 host identified in the URL.

 The fact that the proxy is a HTTP proxy puts certain restrictions on what can
 actually happen. A requested URL that might not be a HTTP URL will be still
 be passed to the HTTP proxy to deliver back to libcurl. This happens
 transparently, and an application may not need to know. I say "may", because
 at times it is very important to understand that all operations over a HTTP
 proxy is using the HTTP protocol. For example, you can't invoke your own
 custom FTP commands or even proper FTP directory listings.

  Proxy Options

    To tell libcurl to use a proxy at a given port number:

       curl_easy_setopt(easyhandle, CURLOPT_PROXY, "proxy-host.com:8080");

    Some proxies require user authentication before allowing a request, and
    you pass that information similar to this:

       curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "user:password");

    If you want to, you can specify the host name only in the CURLOPT_PROXY
    option, and set the port number separately with CURLOPT_PROXYPORT.

  Environment Variables

    libcurl automatically checks and uses a set of environment variables to
    know what proxies to use for certain protocols. The names of the variables
    are following an ancient de facto standard and are built up as
    "[protocol]_proxy" (note the lower casing). Which makes the variable
    'http_proxy' checked for a name of a proxy to use when the input URL is
    HTTP. Following the same rule, the variable named 'ftp_proxy' is checked
    for FTP URLs. Again, the proxies are always HTTP proxies, the different
    names of the variables simply allows different HTTP proxies to be used.

    The proxy environment variable contents should be in the format
    "[protocol://][user:password]machine[:port]". Where the protocol:// part
    is simply ignored if present (so http://proxy and bluerk://proxy will do
    the same) and the optional port number specifies on which port the proxy
    operates on the host. If not specified, the internal default port number
    will be used and that is most likely *not* the one you would like it to
    be.

    There are two special environment variables. 'all_proxy' is what sets
    proxy for any URL in case the protocol specific variable wasn't set, and
    'no_proxy' defines a list of hosts that should not use a proxy even though
    a variable may say so. If 'no_proxy' is a plain asterisk ("*") it matches
    all hosts.

  SSL and Proxies

    SSL is for secure point-to-point connections. This involves strong
    encryption and similar things, which effectively makes it impossible for a
    proxy to operate as a "man in between" which the proxy's task is, as
    previously discussed. Instead, the only way to have SSL work over a HTTP
    proxy is to ask the proxy to tunnel trough everything without being able
    to check or fiddle with the traffic.

    Opening an SSL connection over a HTTP proxy is therefor a matter of asking
    the proxy for a straight connection to the target host on a specified
    port. This is made with the HTTP request CONNECT. ("please mr proxy,
    connect me to that remote host").

    Because of the nature of this operation, where the proxy has no idea what
    kind of data that is passed in and out through this tunnel, this breaks
    some of the very few advantages that come from using a proxy, such as
    caching.  Many organizations prevent this kind of tunneling to other
    destination port numbers than 443 (which is the default HTTPS port
    number).

  Tunneling Through Proxy

    As explained above, tunneling is required for SSL to work and often even
    restricted to the operation intended for SSL; HTTPS.

    This is however not the only time proxy-tunneling might offer benefits to
    you or your application.

    As tunneling opens a direct connection from your application to the remote
    machine, it suddenly also re-introduces the ability to do non-HTTP
    operations over a HTTP proxy. You can in fact use things such as FTP
    upload or FTP custom commands this way.

    Again, this is often prevented by the administrators of proxies and is
    rarely allowed.

    Tell libcurl to use proxy tunneling like this:

       curl_easy_setopt(easyhandle, CURLOPT_HTTPPROXYTUNNEL, TRUE);

    In fact, there might even be times when you want to do plain HTTP
    operations using a tunnel like this, as it then enables you to operate on
    the remote server instead of asking the proxy to do so. libcurl will not
    stand in the way for such innovative actions either!

  Proxy Auto-Config

    Netscape first came up with this. It is basically a web page (usually
    using a .pac extension) with a javascript that when executed by the
    browser with the requested URL as input, returns information to the
    browser on how to connect to the URL. The returned information might be
    "DIRECT" (which means no proxy should be used), "PROXY host:port" (to tell
    the browser where the proxy for this particular URL is) or "SOCKS
    host:port" (to direct the browser to a SOCKS proxy).

    libcurl has no means to interpret or evaluate javascript and thus it
    doesn't support this. If you get yourself in a position where you face
    this nasty invention, the following advice have been mentioned and used in
    the past:

    - Depending on the javascript complexity, write up a script that
      translates it to another language and execute that.

    - Read the javascript code and rewrite the same logic in another language.

    - Implement a javascript interpreted, people have successfully used the
      Mozilla javascript engine in the past.

    - Ask your admins to stop this, for a static proxy setup or similar.


Persistence Is The Way to Happiness

 Re-cycling the same easy handle several times when doing multiple requests is
 the way to go.

 After each single curl_easy_perform() operation, libcurl will keep the
 connection alive and open. A subsequent request using the same easy handle to
 the same host might just be able to use the already open connection! This
 reduces network impact a lot.

 Even if the connection is dropped, all connections involving SSL to the same
 host again, will benefit from libcurl's session ID cache that drastically
 reduces re-connection time.

 FTP connections that are kept alive saves a lot of time, as the command-
 response round-trips are skipped, and also you don't risk getting blocked
 without permission to login again like on many FTP servers only allowing N
 persons to be logged in at the same time.

 libcurl caches DNS name resolving results, to make lookups of a previously
 looked up name a lot faster.

 Other interesting details that improve performance for subsequent requests
 may also be added in the future.

 Each easy handle will attempt to keep the last few connections alive for a
 while in case they are to be used again. You can set the size of this "cache"
 with the CURLOPT_MAXCONNECTS option. Default is 5. It is very seldom any
 point in changing this value, and if you think of changing this it is often
 just a matter of thinking again.

 When the connection cache gets filled, libcurl must close an existing
 connection in order to get room for the new one. To know which connection to
 close, libcurl uses a "close policy" that you can affect with the
 CURLOPT_CLOSEPOLICY option. There's only two polices implemented as of this
 writing (libcurl 7.9.4) and they are:

  CURLCLOSEPOLICY_LEAST_RECENTLY_USED simply close the one that hasn't been
  used for the longest time. This is the default behavior.

  CURLCLOSEPOLICY_OLDEST closes the oldest connection, the one that was
  created the longest time ago.

 There are, or at least were, plans to support a close policy that would call
 a user-specified callback to let the user be able to decide which connection
 to dump when this is necessary and therefor is the CURLOPT_CLOSEFUNCTION an
 existing option still today. Nothing ever uses this though and this will not
 be used within the foreseeable future either.

 To force your upcoming request to not use an already existing connection (it
 will even close one first if there happens to be one alive to the same host
 you're about to operate on), you can do that by setting CURLOPT_FRESH_CONNECT
 to TRUE. In a similar spirit, you can also forbid the upcoming request to be
 "lying" around and possibly get re-used after the request by setting
 CURLOPT_FORBID_REUSE to TRUE.


HTTP Headers Used by libcurl

 When you use libcurl to do HTTP requests, it'll pass along a series of
 headers automatically. It might be good for you to know and understand these
 ones.

  Host

    This header is required by HTTP 1.1 and even many 1.0 servers and should
    be the name of the server we want to talk to. This includes the port
    number if anything but default.

  Pragma

    "no-cache". Tells a possible proxy to not grab a copy from the cache but
    to fetch a fresh one.

  Accept:

    "image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, */*". Cloned from a
    browser once a hundred years ago.

  Expect:

    When doing multi-part formposts, libcurl will set this header to
    "100-continue" to ask the server for an "OK" message before it proceeds
    with sending the data part of the post.


Customizing Operations

 There is an ongoing development today where more and more protocols are built
 upon HTTP for transport. This has obvious benefits as HTTP is a tested and
 reliable protocol that is widely deployed and have excellent proxy-support.

 When you use one of these protocols, and even when doing other kinds of
 programming you may need to change the traditional HTTP (or FTP or...)
 manners. You may need to change words, headers or various data.

 libcurl is your friend here too.

  CUSTOMREQUEST

    If just changing the actual HTTP request keyword is what you want, like
    when GET, HEAD or POST is not good enough for you, CURLOPT_CUSTOMREQUEST
    is there for you. It is very simple to use:

       curl_easy_setopt(easyhandle, CURLOPT_CUSTOMREQUEST, "MYOWNRUQUEST");

    When using the custom request, you change the request keyword of the
    actual request you are performing. Thus, by default you make GET request
    but you can also make a POST operation (as described before) and then
    replace the POST keyword if you want to. You're the boss.

  Modify Headers

    HTTP-like protocols pass a series of headers to the server when doing the
    request, and you're free to pass any amount of extra headers that you
    think fit. Adding headers are this easy:

       struct curl_slist *headers=NULL; /* init to NULL is important */

       headers = curl_slist_append(headers, "Hey-server-hey: how are you?");
       headers = curl_slist_append(headers, "X-silly-content: yes");

       /* pass our list of custom made headers */
       curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers);

       curl_easy_perform(easyhandle); /* transfer http */

       curl_slist_free_all(headers); /* free the header list */

   ... and if you think some of the internally generated headers, such as
   Accept: or Host: don't contain the data you want them to contain, you can
   replace them by simply setting them too:

       headers = curl_slist_append(headers, "Accept: Agent-007");
       headers = curl_slist_append(headers, "Host: munged.host.line");

  Delete Headers

    If you replace an existing header with one with no contents, you will
    prevent the header from being sent. Like if you want to completely prevent
    the "Accept:" header to be sent, you can disable it with code similar to
    this:

       headers = curl_slist_append(headers, "Accept:");

    Both replacing and canceling internal headers should be done with careful
    consideration and you should be aware that you may violate the HTTP
    protocol when doing so.

  Enforcing chunked transfer-encoding

    By making sure a request uses the custom header "Transfer-Encoding:
    chunked" when doing a non-GET HTTP operation, libcurl will switch over to
    "chunked" upload, even though the size of the data to upload might be
    known. By default, libcurl usually switches over to chunked upload
    automatically if the upload data size is unknown.

  HTTP Version

    There's only one aspect left in the HTTP requests that we haven't yet
    mentioned how to modify: the version field. All HTTP requests includes the
    version number to tell the server which version we support. libcurl speak
    HTTP 1.1 by default. Some very old servers don't like getting 1.1-requests
    and when dealing with stubborn old things like that, you can tell libcurl
    to use 1.0 instead by doing something like this:

       curl_easy_setopt(easyhandle, CURLOPT_HTTP_VERSION,
                                    CURLHTTP_VERSION_1_0);

  FTP Custom Commands

    Not all protocols are HTTP-like, and thus the above may not help you when
    you want to make for example your FTP transfers to behave differently.

    Sending custom commands to a FTP server means that you need to send the
    commands exactly as the FTP server expects them (RFC959 is a good guide
    here), and you can only use commands that work on the control-connection
    alone. All kinds of commands that requires data interchange and thus needs
    a data-connection must be left to libcurl's own judgment. Also be aware
    that libcurl will do its very best to change directory to the target
    directory before doing any transfer, so if you change directory (with CWD
    or similar) you might confuse libcurl and then it might not attempt to
    transfer the file in the correct remote directory.

    A little example that deletes a given file before an operation:

      headers = curl_slist_append(headers, "DELE file-to-remove");

      /* pass the list of custom commands to the handle */
      curl_easy_setopt(easyhandle, CURLOPT_QUOTE, headers);

      curl_easy_perform(easyhandle); /* transfer ftp data! */

      curl_slist_free_all(headers); /* free the header list */

    If you would instead want this operation (or chain of operations) to
    happen _after_ the data transfer took place the option to
    curl_easy_setopt() would instead be called CURLOPT_POSTQUOTE and used the
    exact same way.

    The custom FTP command will be issued to the server in the same order they
    are added to the list, and if a command gets an error code returned back
    from the server, no more commands will be issued and libcurl will bail out
    with an error code (CURLE_FTP_QUOTE_ERROR). Note that if you use
    CURLOPT_QUOTE to send commands before a transfer, no transfer will
    actually take place when a quote command has failed.

    If you set the CURLOPT_HEADER to true, you will tell libcurl to get
    information about the target file and output "headers" about it. The
    headers will be in "HTTP-style", looking like they do in HTTP.

    The option to enable headers or to run custom FTP commands may be useful
    to combine with CURLOPT_NOBODY. If this option is set, no actual file
    content transfer will be performed.

  FTP Custom CUSTOMREQUEST

    If you do what list the contents of a FTP directory using your own defined
    FTP command, CURLOPT_CUSTOMREQUEST will do just that. "NLST" is the
    default one for listing directories but you're free to pass in your idea
    of a good alternative.


Cookies Without Chocolate Chips

 In the HTTP sense, a cookie is a name with an associated value. A server
 sends the name and value to the client, and expects it to get sent back on
 every subsequent request to the server that matches the particular conditions
 set. The conditions include that the domain name and path match and that the
 cookie hasn't become too old.

 In real-world cases, servers send new cookies to replace existing one to
 update them. Server use cookies to "track" users and to keep "sessions".

 Cookies are sent from server to clients with the header Set-Cookie: and
 they're sent from clients to servers with the Cookie: header.

 To just send whatever cookie you want to a server, you can use CURLOPT_COOKIE
 to set a cookie string like this:

    curl_easy_setopt(easyhandle, CURLOPT_COOKIE, "name1=var1; name2=var2;");

 In many cases, that is not enough. You might want to dynamically save
 whatever cookies the remote server passes to you, and make sure those cookies
 are then use accordingly on later requests.

 One way to do this, is to save all headers you receive in a plain file and
 when you make a request, you tell libcurl to read the previous headers to
 figure out which cookies to use. Set header file to read cookies from with
 CURLOPT_COOKIEFILE.

 The CURLOPT_COOKIEFILE option also automatically enables the cookie parser in
 libcurl. Until the cookie parser is enabled, libcurl will not parse or
 understand incoming cookies and they will just be ignored. However, when the
 parser is enabled the cookies will be understood and the cookies will be kept
 in memory and used properly in subsequent requests when the same handle is
 used. Many times this is enough, and you may not have to save the cookies to
 disk at all. Note that the file you specify to CURLOPT_COOKIEFILE doesn't
 have to exist to enable the parser, so a common way to just enable the parser
 and not read able might be to use a file name you know doesn't exist.

 If you rather use existing cookies that you've previously received with your
 Netscape or Mozilla browsers, you can make libcurl use that cookie file as
 input. The CURLOPT_COOKIEFILE is used for that too, as libcurl will
 automatically find out what kind of file it is and act accordingly.

 The perhaps most advanced cookie operation libcurl offers, is saving the
 entire internal cookie state back into a Netscape/Mozilla formatted cookie
 file. We call that the cookie-jar. When you set a file name with
 CURLOPT_COOKIEJAR, that file name will be created and all received cookies
 will be stored in it when curl_easy_cleanup() is called. This enabled cookies
 to get passed on properly between multiple handles without any information
 getting lost.


FTP Peculiarities We Need

 FTP transfers use a second TCP/IP connection for the data transfer. This is
 usually a fact you can forget and ignore but at times this fact will come
 back to haunt you. libcurl offers several different ways to custom how the
 second connection is being made.

 libcurl can either connect to the server a second time or tell the server to
 connect back to it. The first option is the default and it is also what works
 best for all the people behind firewalls, NATs or IP-masquerading setups.
 libcurl then tells the server to open up a new port and wait for a second
 connection. This is by default attempted with EPSV first, and if that doesn't
 work it tries PASV instead. (EPSV is an extension to the original FTP spec
 and does not exist nor work on all FTP servers.)

 You can prevent libcurl from first trying the EPSV command by setting
 CURLOPT_FTP_USE_EPSV to FALSE.

 In some cases, you will prefer to have the server connect back to you for the
 second connection. This might be when the server is perhaps behind a firewall
 or something and only allows connections on a single port. libcurl then
 informs the remote server which IP address and port number to connect to.
 This is made with the CURLOPT_FTPPORT option. If you set it to "-", libcurl
 will use your system's "default IP address". If you want to use a particular
 IP, you can set the full IP address, a host name to resolve to an IP address
 or even a local network interface name that libcurl will get the IP address
 from.

 When doing the "PORT" approach, libcurl will attempt to use the EPRT and the
 LPRT before trying PORT, as they work with more protocols. You can disable
 this behavior by setting CURLOPT_FTP_USE_EPRT to FALSE.


Headers Equal Fun

 Some protocols provide "headers", meta-data separated from the normal
 data. These headers are by default not included in the normal data stream,
 but you can make them appear in the data stream by setting CURLOPT_HEADER to
 TRUE.

 What might be even more useful, is libcurl's ability to separate the headers
 from the data and thus make the callbacks differ. You can for example set a
 different pointer to pass to the ordinary write callback by setting
 CURLOPT_WRITEHEADER.

 Or, you can set an entirely separate function to receive the headers, by
 using CURLOPT_HEADERFUNCTION.

 The headers are passed to the callback function one by one, and you can
 depend on that fact. It makes it easier for you to add custom header parsers
 etc.

 "Headers" for FTP transfers equal all the FTP server responses. They aren't
 actually true headers, but in this case we pretend they are! ;-)


Post Transfer Information

 [ curl_easy_getinfo ]


Security Considerations

 libcurl is in itself not insecure. If used the right way, you can use libcurl
 to transfer data pretty safely.

 There are of course many things to consider that may loosen up this
 situation:

  Command Lines

    If you use a command line tool (such as curl) that uses libcurl, and you
    give option to the tool on the command line those options can very likely
    get read by other users of your system when they use 'ps' or other tools
    to list currently running processes.

    To avoid this problem, never feed sensitive things to programs using
    command line options.

  .netrc

    .netrc is a pretty handy file/feature that allows you to login quickly and
    automatically to frequently visited sites. The file contains passwords in
    clear text and is a real security risk. In some cases, your .netrc is also
    stored in a home directory that is NFS mounted or used on another network
    based file system, so the clear text password will fly through your
    network every time anyone reads that file!

    To avoid this problem, don't use .netrc files and never store passwords in
    plain text anywhere.

  Clear Text Passwords

    Many of the protocols libcurl supports send name and password unencrypted
    as clear text (HTTP Basic authentication, FTP, TELNET etc). It is very
    easy for anyone on your network or a network nearby yours, to just fire up
    a network analyzer tool and eavesdrop on your passwords. Don't let the
    fact that HTTP uses base64 encoded passwords fool you. They may not look
    readable at a first glance, but they very easily "deciphered" by anyone
    within seconds.

    To avoid this problem, use protocols that don't let snoopers see your
    password: HTTPS, FTPS and FTP-kerberos are a few examples. HTTP Digest
    authentication allows this too, but isn't supported by libcurl as of this
    writing.

  Showing What You Do

    On a related issue, be aware that even in situations like when you have
    problems with libcurl and ask someone for help, everything you reveal in
    order to get best possible help might also impose certain security related
    risks. Host names, user names, paths, operating system specifics etc (not
    to mention passwords of course) may in fact be used by intruders to gain
    additional information of a potential target.

    To avoid this problem, you must of course use your common sense. Often,
    you can just edit out the sensitive data or just search/replace your true
    information with faked data.


Multiple Transfers Using the multi Interface

 The easy interface as described in detail in this document is a synchronous
 interface that transfers one file at a time and doesn't return until its
 done.

 The multi interface on the other hand, allows your program to transfer
 multiple files in both directions at the same time, without forcing you to
 use multiple threads.

 To use this interface, you are better off if you first understand the basics
 of how to use the easy interface. The multi interface is simply a way to make
 multiple transfers at the same time, by adding up multiple easy handles in to
 a "multi stack".

 You create the easy handles you want and you set all the options just like
 you have been told above, and then you create a multi handle with
 curl_multi_init() and add all those easy handles to that multi handle with
 curl_multi_add_handle().

 When you've added the handles you have for the moment (you can still add new
 ones at any time), you start the transfers by call curl_multi_perform().

 curl_multi_perform() is asynchronous. It will only execute as little as
 possible and then return back control to your program. It is designed to
 never block. If it returns CURLM_CALL_MULTI_PERFORM you better call it again
 soon, as that is a signal that it still has local data to send or remote data
 to receive.

 The best usage of this interface is when you do a select() on all possible
 file descriptors or sockets to know when to call libcurl again. This also
 makes it easy for you to wait and respond to actions on your own
 application's sockets/handles. You figure out what to select() for by using
 curl_multi_fdset(), that fills in a set of fd_set variables for you with the
 particular file descriptors libcurl uses for the moment.

 When you then call select(), it'll return when one of the file handles signal
 action and you then call curl_multi_perform() to allow libcurl to do what it
 wants to do. Take note that libcurl does also feature some time-out code so
 we advice you to never use very long timeouts on select() before you call
 curl_multi_perform(), which thus should be called unconditionally every now
 and then even if none of its file descriptors have signaled ready. Another
 precaution you should use: always call curl_multi_fdset() immediately before
 the select() call since the current set of file descriptors may change when
 calling a curl function.

 If you want to stop the transfer of one of the easy handles in the stack, you
 can use curl_multi_remove_handle() to remove individual easy
 handles. Remember that easy handles should be curl_easy_cleanup()ed.

 When a transfer within the multi stack has finished, the counter of running
 transfers (as filled in by curl_multi_perform()) will decrease. When the
 number reaches zero, all transfers are done.

 curl_multi_info_read() can be used to get information about completed
 transfers. It then returns the CURLcode for each easy transfer, to allow you
 to figure out success on each individual transfer.


SSL, Certificates and Other Tricks

 [ seeding, passwords, keys, certificates, ENGINE, ca certs ]


Sharing Data Between Easy Handles

 [ fill in ]

-----
Footnotes:

[1] = libcurl 7.10.3 and later have the ability to switch over to chunked
      Transfer-Encoding in cases were HTTP uploads are done with data of an
      unknown size.

[2] = This happens on Windows machines when libcurl is built and used as a
      DLL. However, you can still do this on Windows if you link with a static
      library.

[3] = The curl-config tool is generated at build-time (on unix-like systems)
      and should be installed with the 'make install' or similar instruction
      that installs the library, header files, man pages etc.