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libcurl-tutorial(3)	      libcurl programming	   libcurl-tutorial(3)

       libcurl-tutorial - libcurl programming tutorial

       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

       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

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

       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-con‐
	      fig'[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 the 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 a sup‐
	      ported SSL library 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-con‐
	      fig' 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.

       autoconf macro
	      When you write your configure script to detect libcurl and setup
	      variables accordingly, we offer a prewritten macro that probably
	      does     everything    you    need    in	  this	  area.	   See
	      docs/libcurl/libcurl.m4 file - it includes docs on  how  to  use

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 envi‐

       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


       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:

		     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 prop‐
		     erly,  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.

		     which only does anything on libcurls compiled  and	 built
		     SSL-enabled.  On  these  systems,	this will make libcurl
		     initialize the SSL library 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(3) hasn't been called by the time curl_easy_perform(3)
       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(3), which is the opposite of the init call. It will
       then   do   the	reversed  operations  to  cleanup  the	resources  the
       curl_global_init(3) call initialized.

       Repeated calls to curl_global_init(3) and curl_global_cleanup(3) should
       be avoided. They should only be called once each.

Features libcurl Provides
       It  is  considered  best-practice to determine libcurl features at run-
       time rather than at build-time (if  possible  of	 course).  By  calling
       curl_version_info(3)  and  checking  out	 the  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 han‐
       dle. 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

       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(3). They control how the subsequent transfer or trans‐
       fers  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(3),  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

	curl_easy_setopt(handle, CURLOPT_URL, "");

       Let's assume for a while that you want to receive data as the URL iden‐
       tifies  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

	size_t	write_data(void	 *buffer,  size_t  size,  size_t  nmemb,  void

       You tell libcurl to pass all data to this function by issuing  a	 func‐
       tion 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_WRITEDATA, &internal_struct);

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

       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_WRITEDATA

       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	 plat‐
       forms[2],  libcurl won't be able to operate on files opened by the pro‐
       gram. Thus, if you use the default callback and pass in	an  open  file
       with  CURLOPT_WRITEDATA, it will crash. You should therefore avoid this
       to make your program run fine virtually everywhere.

       (CURLOPT_WRITEDATA was formerly known as CURLOPT_FILE. Both names still
       work and do the same thing).

       If you're using libcurl as a win32 DLL, you MUST use the CURLOPT_WRITE‐
       FUNCTION if you set CURLOPT_WRITEDATA - or you will experience crashes.

       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);

       curl_easy_perform(3)  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 connection.

Multi-threading Issues
       The  first basic rule is that you must never share a libcurl handle (be
       it easy or multi or whatever) between multiple threads.	Only  use  one
       handle in one thread at a time.

       libcurl	is  completely thread safe, except for two issues: signals and
       SSL/TLS handlers. Signals are used timeouting name resolves (during DNS
       lookup) - when built without c-ares support and not on Windows..

       If you are accessing HTTPS or FTPS URLs in a multi-threaded manner, you
       are then of course using the underlying SSL library multi-threaded  and
       those  libs might have their own requirements on this issue. Basically,
       you need to provide one or two functions to allow it to function	 prop‐
       erly. For all details, see this:




	is claimed to be thread-safe already without anything required


	Required actions unknown

       When using multiple threads you should set the CURLOPT_NOSIGNAL	option
       to TRUE for all handles. Everything will or might 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 fully support IPv6. On some platforms, libcurl simply will not
       function properly multi-threaded unless this option is set.

       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 pro‐

       There's one golden rule when these things occur: set  the  CURLOPT_VER‐
       BOSE  option  to	 TRUE.	It'll cause the library to spew out the entire
       protocol details it sends, some internal info and some received	proto‐
       col  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  appropri‐
       ate RFC documents at least briefly.

Upload Data to a Remote Site
       libcurl	tries  to  keep a protocol independent approach to most trans‐
       fers, 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

       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	 maxi‐
       mum  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

	/* 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(3) 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 num‐
       ber of bytes it wrote in the buffer. Returning 0 will signal the end of
       the upload.

       Many protocols use or even require that user name and password are pro‐
       vided 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:


       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 num‐

       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:these‐

       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 Con‐
       siderations" 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 pro‐
       tocols such as HTTP. To make curl use this file, use the	 CURLOPT_NETRC

	curl_easy_setopt(easyhandle, CURLOPT_NETRC, TRUE);

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

	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	trans‐

       To pass the known private key password to libcurl:

	curl_easy_setopt(easyhandle, CURLOPT_KEYPASSWD, "keypassword");

HTTP Authentication
       The  previous chapter showed how to set user name and password for get‐
       ting 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 authenti‐
       cation 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,

       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.

       We get many questions regarding how to issue HTTP  POSTs	 with  libcurl
       the proper way. This chapter will thus include examples using both dif‐
       ferent 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, "");

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

       Simple enough, huh? Since you  set  the	POST  options  with  the  CUR‐
       LOPT_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. Set‐
       ting 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	 form‐
       post yourself and provide to libcurl. To make that easier, libcurl pro‐
       vides curl_formadd(3). 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

	struct curl_httppost *post=NULL;
	struct curl_httppost *last=NULL;
	curl_formadd(&post, &last,
		     CURLFORM_COPYNAME, "name",
	curl_formadd(&post, &last,
		     CURLFORM_COPYNAME, "project",
	curl_formadd(&post, &last,
		     CURLFORM_COPYNAME, "logotype-image",

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

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

	/* free the post data again */

       Multipart formposts are chains of parts using MIME-style separators and
       headers. It means that each one of these separate parts get a few head‐
       ers  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",

	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(3), 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 CUR‐
       LOPT_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 CANNOT be non-static class member functions

       Example C++ code:

       class AClass {
	   static size_t write_data(void *ptr, size_t size, size_t nmemb,
				    void *ourpointer)
	     /* do what you want with the data */

       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 proxies. Network clients  or
       user-agents  ask	 the  proxy  for  documents, the proxy does the actual
       request and then it returns them.

       libcurl supports SOCKS and HTTP proxies. 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.

       If you're using a SOCKS proxy, you may find that libcurl doesn't	 quite
       support all operations through it.

       For  HTTP proxies: 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 pro‐
       tocol.  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-");

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

	       curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD,  "user:pass‐

	      If  you  want to, you can specify the host name only in the CUR‐
	      LOPT_PROXY option, and set the port number separately with  CUR‐

	      Tell libcurl what kind of proxy it is with CURLOPT_PROXYTYPE (if
	      not, it will default to assume a HTTP proxy):

	       curl_easy_setopt(easyhandle,	 CURLOPT_PROXYTYPE,	 CURL‐

       Environment Variables

	      libcurl automatically checks and uses a set of environment vari‐
	      ables to know what proxies to use	 for  certain  protocols.  The
	      names  of	 the variables are following an ancient de facto stan‐
	      dard and are built up as "[protocol]_proxy" (note the lower cas‐
	      ing).  Which  makes  the variable 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 proto‐
	      col:// 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.

	      To explicitly disable libcurl's checking for and using the proxy
	      environment  variables,  set  the	 proxy	name  to "" - an empty
	      string - with CURLOPT_PROXY.

       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

	      - 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 sim‐

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(3) 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 con‐
       nection! 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  com‐
       mand- 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 previ‐
       ously 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.

       To  force  your upcoming request to not use an already existing connec‐
       tion (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. You can replace or remove them by using  the	 CURLOPT_HTTP‐
       HEADER option.

       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 "*/*".

       Expect When doing POST requests, libcurl sets this header to  "100-con‐
	      tinue"  to ask the server for an "OK" message before it proceeds
	      with sending the data part of  the  post.	 If  the  POSTed  data
	      amount is deemed "small", libcurl will not use this header.

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

       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.

	      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

	       curl_easy_setopt(easyhandle,   CURLOPT_CUSTOMREQUEST,   "MYOWN‐

	      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:");

       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 dis‐
	      able 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-Encod‐
	      ing: 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

	      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,

       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 direc‐
	      tory 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	opera‐
	      tions) to happen _after_ the data transfer took place the option
	      to curl_easy_setopt(3) 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_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

	      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.

	      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	 "ses‐

       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  CUR‐
       LOPT_COOKIE to set a cookie string like this:

	curl_easy_setopt(easyhandle,	    CURLOPT_COOKIE,	  "name1=var1;

       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 accord‐

       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(3) 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

       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 CUR‐

       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,

       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

       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 pro‐

	      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,  TEL‐
	      NET  etc).  It is very easy for anyone on your network or a net‐
	      work 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,	every‐
	      thing  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 syn‐
       chronous 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 multi‐
       ple 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(3) and add all those easy handles to that multi	handle
       with curl_multi_add_handle(3).

       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(3)  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 pos‐
       sible 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(3), that fills in a set of fd_set variables
       for you with the particular  file  descriptors  libcurl	uses  for  the

       When  you then call select(), it'll return when one of the file handles
       signal action and you then call curl_multi_perform(3) 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(3), 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(3) 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(3) to remove individual
       easy    handles.	   Remember    that    easy    handles	  should    be

       When  a	transfer  within  the multi stack has finished, the counter of
       running	transfers  (as	filled	in  by	 curl_multi_perform(3))	  will
       decrease. When the number reaches zero, all transfers are done.

       curl_multi_info_read(3)	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 ]

       [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 sim‐
	      ilar  instruction	 that  installs the library, header files, man
	      pages etc.

libcurl				  27 Feb 2007		   libcurl-tutorial(3)

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