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       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 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

       To get 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-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 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

              $ 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 supported 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-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 could be either on or off on for different

              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 it.

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 differ. 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 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.

                     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 only needs to be done 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.

Two Interfaces

       libcurl first introduced the  so  called  easy  interface.  All  operations  in  the  easy
       interface  are  prefixed with 'curl_easy'. The easy interface lets you do single transfers
       with a synchronous and blocking function call.

       libcurl also offers another interface that allows multiple  simultaneous  transfers  in  a
       single  thread,  the so called multi interface. More about that interface is detailed in a
       separate chapter further down. You still need to understand the easy interface  first,  so
       please continue reading for better understanding.

Handle the Easy libcurl

       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

       You set properties and options for this handle using curl_easy_setopt(3). They control how
       the  subsequent transfer or transfers will be made. Options remain set in the handle until
       set again to something different. They are sticky. Multiple requests using the same handle
       will use the same options.

       If  you  at  any  point  would  like to blank all previously set options for a single easy
       handle, you can call curl_easy_reset(3) and you can also make a clone of  an  easy  handle
       (with all its set options) using curl_easy_duphandle(3).

       Many  of  the options you set in libcurl are "strings", pointers to data terminated with a
       zero byte. When you set strings with curl_easy_setopt(3), libcurl makes its  own  copy  so
       that they don't need to be kept around in your application after being set[4].

       One  of  the most basic properties to set in the handle is the URL. You set your preferred
       URL to transfer with CURLOPT_URL(3) in a manner similar to:

        curl_easy_setopt(handle, CURLOPT_URL, "");

       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

        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 callback function gets in the fourth  argument  by  setting
       another property:

        curl_easy_setopt(easyhandle, CURLOPT_WRITEDATA, &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

       libcurl  offers  its  own default internal callback that will take care of the data if you
       don't set the callback with CURLOPT_WRITEFUNCTION(3).  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(3) 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 an open file with CURLOPT_WRITEDATA(3), it will crash. You should therefore avoid  this
       to make your program run fine virtually everywhere.

       (CURLOPT_WRITEDATA(3) 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_WRITEFUNCTION(3)  if  you
       set CURLOPT_WRITEDATA(3) - 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

       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(3) 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.

       For  some  protocols,  downloading a file can involve a complicated process of logging in,
       setting the transfer mode, changing the current directory  and  finally  transferring  the
       file  data. libcurl takes care of all that complication for you. Given simply the URL to a
       file, libcurl will take care of all the details needed to get  the  file  moved  from  one
       machine to another.

Multi-threading Issues

       libcurl is thread safe but there are a few exceptions. Refer to libcurl-thread(3) for more

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

       There's  one  golden rule when these things occur: set the CURLOPT_VERBOSE(3) option to 1.
       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(3) set 1.

       Of course, there are bugs left. We need 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  many  details  as  you   possibly   can:   a   protocol   dump   that
       CURLOPT_VERBOSE(3)  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(3) is not enough, you increase the level of debug data your application
       receive by using the CURLOPT_DEBUGFUNCTION(3).

       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

       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

        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_READDATA, &filedata);

       Tell libcurl that we want to upload:

        curl_easy_setopt(easyhandle, CURLOPT_UPLOAD, 1L);

       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(3)
       for all known file sizes like this[1]:

        /* in this example, file_size must be an curl_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
       number 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 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:


       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(3)  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(3). It is used quite similar  to  the  CURLOPT_USERPWD(3)  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(3) option:

        curl_easy_setopt(easyhandle, CURLOPT_NETRC, 1L);

       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

       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  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 which  way  libcurl
       will  (attempt  to)  use  them.  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
       (SPNEGO). You can tell libcurl which one to use with CURLOPT_HTTPAUTH(3) 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(3):

        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

        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 different versions of HTTP POST that libcurl

       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  CURLOPT_POSTFIELDS(3),  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  prevent  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, 23L);

        /* 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 were first documented in the RFC1867
       (updated in RFC2388). They're called multi-part because they're built by a chain of parts,
       each part being a single unit of data. 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 a MIME API consisting in  several  functions:  using
       those,  you  can  create and fill a multi-part form.  Function curl_mime_init(3) creates a
       multi-part  body;  you  can  then  append  new  parts   to   a   multi-part   body   using
       curl_mime_addpart(3).   There  are  three  possible  data sources for a part: memory using
       curl_mime_data(3), file using curl_mime_filedata(3) and user-defined  data  read  callback
       using  curl_mime_data_cb(3).   curl_mime_name(3)  sets  a  part's (i.e.: form field) name,
       while curl_mime_filename(3) fills in the remote file name. With curl_mime_type(3), you can
       tell  the  MIME  type  of a part, curl_mime_headers(3) allows defining the part's headers.
       When a multi-part body is no longer needed, you can destroy it using curl_mime_free(3).

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

        curl_mime *multipart = curl_mime_init(easyhandle);
        curl_mimepart *part = curl_mime_addpart(mutipart);
        curl_mime_name(part, "name");
        curl_mime_data(part, "daniel", CURL_ZERO_TERMINATED);
        part = curl_mime_addpart(mutipart);
        curl_mime_name(part, "project");
        curl_mime_data(part, "curl", CURL_ZERO_TERMINATED);
        part = curl_mime_addpart(mutipart);
        curl_mime_name(part, "logotype-image");
        curl_mime_filedata(part, "curl.png");

        /* Set the form info */
        curl_easy_setopt(easyhandle, CURLOPT_MIMEPOST, multipart);

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

        /* free the post data again */

       To  post  multiple  files for a single form field, you must supply each file in a separate
       part, all with the same field name.  Although  function  curl_mime_subparts(3)  implements
       nested  muti-parts,  this way of multiple files posting is deprecated by RFC 7578, chapter

       To set the data source from an already opened FILE pointer, use:

        curl_mime_data_cb(part, filesize, (curl_read_callback) fread,
                          (curl_seek_callback) fseek, NULL, filepointer);

       A deprecated curl_formadd(3) function is still supported in libcurl.   It  should  however
       not  be  used  anymore  for new designs and programs using it ought to be converted to the
       MIME API. It is however described here as an aid to conversion.

       Using curl_formadd, you add parts to the form. When you're done adding parts, you post the
       whole form.

       The MIME API example above is expressed as follows using this function:

        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 */

       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 as you like, but this little
       example will show how you set headers to one specific part when you add that to  the  post

        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,

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

        curl_formfree(post); /* free post */
        curl_slist_free_all(headers); /* 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 one as your next request. You force an easyhandle to go back
       to GET by using the CURLOPT_HTTPGET(3) option:

        curl_easy_setopt(easyhandle, CURLOPT_HTTPGET, 1L);

       Just setting CURLOPT_POSTFIELDS(3) to "" or NULL will *not*  stop  libcurl  from  doing  a
       POST. It will just make it POST without any data to send!

Converting from deprecated form API to MIME API

       Four rules have to be respected in building the multi-part:
       - The easy handle must be created before building the multi-part.
       - The multi-part is always created by a call to curl_mime_init(easyhandle).
       - Each part is created by a call to curl_mime_addpart(multipart).
       - When complete, the multi-part must be bound to the easy handle using CURLOPT_MIMEPOST(3)
       instead of CURLOPT_HTTPPOST(3).

       Here are some example of curl_formadd calls to MIME API sequences:

        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "id",
                     CURLFORM_COPYCONTENTS, "daniel", CURLFORM_END);
                     CURLFORM_CONTENTHEADER, headers,
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "id");
        curl_mime_data(part, "daniel", CURL_ZERO_TERMINATED);
        curl_mime_headers(part, headers, FALSE);

       Setting the last curl_mime_headers argument to TRUE would have caused the  headers  to  be
       automatically  released  upon  destroyed  the  multi-part,  thus saving a clean-up call to

        curl_formadd(&post, &last,
                     CURLFORM_PTRNAME, "logotype-image",
                     CURLFORM_FILECONTENT, "-",
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "logotype-image");
        curl_mime_data_cb(part, (curl_off_t) -1, fread, fseek, NULL, stdin);

       curl_mime_name always copies the field name. The special file name "-" is not supported by
       curl_mime_file:  to  read  an open file, use a callback source using fread(). The transfer
       will be chunked since the data size is unknown.

        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "datafile[]",
                     CURLFORM_FILE, "file1",
                     CURLFORM_FILE, "file2",
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "datafile[]");
        curl_mime_filedata(part, "file1");
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "datafile[]");
        curl_mime_filedata(part, "file2");

       The deprecated multipart/mixed implementation of multiple files field is translated to two
       distinct parts with the same name.

        curl_easy_setopt(easyhandle, CURLOPT_READFUNCTION, myreadfunc);
        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "stream",
                     CURLFORM_STREAM, arg,
                     CURLFORM_CONTENTLEN, (curl_off_t) datasize,
                     CURLFORM_FILENAME, "",
                     CURLFORM_CONTENTTYPE, "application/zip",
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "stream");
        curl_mime_data_cb(part, (curl_off_t) datasize,
                          myreadfunc, NULL, NULL, arg);
        curl_mime_filename(part, "");
        curl_mime_type(part, "application/zip");

       CURLOPT_READFUNCTION  callback  is  not  used:  it is replace by directly setting the part
       source data from the callback read function.

        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "memfile",
                     CURLFORM_BUFFER, "memfile.bin",
                     CURLFORM_BUFFERPTR, databuffer,
                     CURLFORM_BUFFERLENGTH, (long) sizeof databuffer,
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "memfile");
        curl_mime_data(part, databuffer, (curl_off_t) sizeof databuffer);
        curl_mime_filename(part, "memfile.bin");

       curl_mime_data always copies the initial data: data buffer  is  thus  free  for  immediate

        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "message",
                     CURLFORM_FILECONTENT, "msg.txt",
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "message");
        curl_mime_filedata(part, "msg.txt");
        curl_mime_filename(part, NULL);

       Use  of  curl_mime_filedata  sets  the  remote file name as a side effect: it is therefore
       necessary to clear it for CURLFORM_FILECONTENT emulation.

Showing Progress

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

       Switch on the progress meter by, oddly enough, setting CURLOPT_NOPROGRESS(3) to zero. This
       option is set to 1 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

       Set  the  progress  callback by using CURLOPT_PROGRESSFUNCTION(3). 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(3). 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

       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 use 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, "");

              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(3) option,
              and set the port number separately with CURLOPT_PROXYPORT(3).

              Tell libcurl what kind of proxy it is with CURLOPT_PROXYTYPE(3) (if  not,  it  will
              default to assume a HTTP proxy):

               curl_easy_setopt(easyhandle, CURLOPT_PROXYTYPE, CURLPROXY_SOCKS4);

       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.

              To  explicitly  disable  libcurl's  checking  for  and  using the proxy environment
              variables, set the proxy name to "" - an empty string - with CURLOPT_PROXY(3).

       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

              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

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

              Tell libcurl to use proxy tunneling like this:

               curl_easy_setopt(easyhandle, CURLOPT_HTTPPROXYTUNNEL, 1L);

              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 interpreter; 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

       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 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 save 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(3)  option.  Default  is 5. There 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  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(3) to 1. 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(3) to 1.

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. You can replace or
       remove them by using the CURLOPT_HTTPHEADER(3) 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.

       Accept "*/*".

       Expect When doing POST requests, libcurl sets this header to  "100-continue"  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

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 has 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.

              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(3) is there for you.
              It is very simple to use:

               curl_easy_setopt(easyhandle, CURLOPT_CUSTOMREQUEST, "MYOWNREQUEST");

              When using the custom request, you change the request keyword of the actual request
              you are performing. Thus, by default you make a 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 is 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.  For  instance,  if  you  want to completely prevent the
              "Accept:" header from being 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

              All  HTTP  requests includes the version number to tell the server which version we
              support. libcurl speaks 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, CURL_HTTP_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 require data interchange and thus need  a  data-connection  must  be  left  to
              libcurl's own judgement. 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(3) would instead be
              called CURLOPT_POSTQUOTE(3) 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(3) to send commands
              before a transfer, no transfer will actually take place when a  quote  command  has

              If  you  set  the  CURLOPT_HEADER(3) to 1, 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(3). If this option is set, no actual file content transfer will
              be performed.

              If  you  do want to list the contents of a FTP directory using your own defined FTP
              command, CURLOPT_CUSTOMREQUEST(3) 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  ones  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(3) 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  used  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 the header file to read cookies from with CURLOPT_COOKIEFILE(3).

       The  CURLOPT_COOKIEFILE(3) 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(3)  doesn't
       have to exist to enable the parser, so a common way to just enable the parser and not read
       any cookies is to use the name of a file you know doesn't exist.

       If you would 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(3) is used for that too, as libcurl will automatically  find  out  what
       kind of file it is and act accordingly.

       Perhaps  the  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(3), that file name will be created
       and all received cookies will be stored in it when curl_easy_cleanup(3)  is  called.  This
       enables 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 customize 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(3) to zero.

       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(3) 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(3) to zero.

MIME API revisited for SMTP and IMAP

       In addition to support HTTP multi-part form fields, the MIME API  can  be  used  to  build
       structured  e-mail  messages  and  send  them  via  SMTP  or  append such messages to IMAP

       A structured e-mail message may contain several parts: some are displayed  inline  by  the
       MUA,  some  are  attachments.  Parts  can also be structured as multi-part, for example to
       include another e-mail message or to offer several text formats alternatives. This can  be
       nested to any level.

       To  build  such  a  message, you prepare the nth-level multi-part and then include it as a
       source to the parent multi-part using function curl_mime_subparts(3).  Once  it  has  been
       bound  to  its  parent  multi-part, a nth-level multi-part belongs to it and should not be
       freed explicitly.

       E-mail messages data is  not  supposed  to  be  non-ascii  and  line  length  is  limited:
       fortunately,  some  transfer  encodings  are  defined  by  the  standards  to  support the
       transmission of such incompatible data. Function curl_mime_encoder(3) tells  a  part  that
       its  source  data  must  be encoded before being sent. It also generates the corresponding
       header for that part.  If the part data you want to send is  already  encoded  in  such  a
       scheme,  do  not  use  this  function  (this would over-encode it), but explicitly set the
       corresponding part header.

       Upon sending  such  a  message,  libcurl  prepends  it  with  the  header  list  set  with
       CURLOPT_HTTPHEADER(3), as 0th-level mime part headers.

       Here  is  an example building an e-mail message with an inline plain/html text alternative
       and a file attachment encoded in base64:

        curl_mime *message = curl_mime_init(easyhandle);

        /* The inline part is an alternative proposing the html and the text
           versions of the e-mail. */
        curl_mime *alt = curl_mime_init(easyhandle);

        /* HTML message. */
        curl_mimepart *part = curl_mime_addpart(alt);
        curl_mime_data(part, "<html><body><p>This is HTML</p></body></html>",
        curl_mime_type(part, "text/html");

        /* Text message. */
        part = curl_mime_addpart(alt);
        curl_mime_data(part, "This is plain text message",

        /* Create the inline part. */
        part = curl_mime_addpart(message);
        curl_mime_subparts(part, alt);
        curl_mime_type(part, "multipart/alternative");
        struct curl_slist *headers = curl_slist_append(NULL,
                          "Content-Disposition: inline");
        curl_mime_headers(part, headers, TRUE);

        /* Add the attachment. */
        part = curl_mime_addpart(message);
        curl_mime_filedata(part, "manual.pdf");
        curl_mime_encoder(part, "base64");

        /* Build the mail headers. */
        headers = curl_slist_append(NULL, "From:");
        headers = curl_slist_append(headers, "To:");

        /* Set these into the easy handle. */
        curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers);
        curl_easy_setopt(easyhandle, CURLOPT_MIMEPOST, mime);

       It should be noted that appending a message to an IMAP directory requires the message size
       to  be known prior upload. It is therefore not possible to include parts with unknown data
       size in this context.

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(3) to 1.

       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_HEADERDATA(3).

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

       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

       The  libcurl  project  takes  security seriously.  The library is written with caution and
       precautions are taken to mitigate many kinds of risks  encountered  while  operating  with
       potentially  malicious  servers on the Internet.  It is a powerful library, however, which
       allows application writers to make trade offs between ease  of  writing  and  exposure  to
       potential  risky  operations.  If used the right way, you can use libcurl to transfer data
       pretty safely.

       Many applications are used in closed networks where users and servers can be trusted,  but
       many  others  are  used  on arbitrary servers and are fed input from potentially untrusted
       users.  Following is a discussion about some risks  in  the  ways  in  which  applications
       commonly  use  libcurl  and  potential  mitigations  of  those  risks.  It  is by no means
       comprehensive, but shows classes of attacks that robust applications should consider.  The
       Common Weakness Enumeration project at is a good reference for many
       of these and similar types of weaknesses of which application writers should be aware.

       Command Lines
              If you use a command line tool (such as curl)  that  uses  libcurl,  and  you  give
              options  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. Write them to a protected file and use the -K option to avoid this.

       .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

              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 Basic 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  an  authentication mechanism or other protocol that
              doesn't let snoopers see your password: Digest, CRAM-MD5, Kerberos, SPNEGO or  NTLM
              authentication, HTTPS, FTPS, SCP and SFTP are a few examples.

              The CURLOPT_FOLLOWLOCATION(3) option automatically follows HTTP redirects sent by a
              remote server.  These redirects can refer to any kind of URL,  not  just  HTTP.  By
              default  libcurl  will  allow all protocols on redirect except several disabled for
              security reasons: Since 7.19.4 FILE and SCP are disabled, and since 7.40.0 SMB  and
              SMBS are also disabled.

              A  redirect  to  a  file:  URL would cause the libcurl to read (or write) arbitrary
              files from the local filesystem.  If the application returns the data back  to  the
              user  (as  would  happen  in some kinds of CGI scripts), an attacker could leverage
              this to read otherwise forbidden data (e.g.  file://localhost/etc/passwd).

              If authentication credentials are stored in the ~/.netrc file, or  Kerberos  is  in
              use,  any  other  URL type (not just file:) that requires authentication is also at
              risk.  A redirect such as ftp://some-internal-server/private-file would then return
              data even when the server is password protected.

              In the same way, if an unencrypted SSH private key has been configured for the user
              running the libcurl application, SCP:  or  SFTP:  URLs  could  access  password  or
              private-key protected resources, e.g. sftp://user@some-internal-server/etc/passwd

              The CURLOPT_REDIR_PROTOCOLS(3) and CURLOPT_NETRC(3) options can be used to mitigate
              against this kind of attack.

              A redirect can also specify a  location  available  only  on  the  machine  running
              libcurl,  including  servers  hidden  behind  a  firewall  from the attacker.  e.g.
     or http://intranet/delete-stuff.cgi?delete=all  or  tftp://bootp-

              Apps  can mitigate against this by disabling CURLOPT_FOLLOWLOCATION(3) and handling
              redirects itself, sanitizing URLs as necessary. Alternately,  an  app  could  leave
              CURLOPT_FOLLOWLOCATION(3)  enabled but set CURLOPT_REDIR_PROTOCOLS(3) and install a
              CURLOPT_OPENSOCKETFUNCTION(3) callback function in which  addresses  are  sanitized
              before use.

       Private Resources
              A  user who can control the DNS server of a domain being passed in within a URL can
              change the address of the host to a local,  private  address  which  a  server-side
              libcurl-using    application    could    then   use.   e.g.   the   innocuous   URL
     could actually resolve to  the  IP  address  of  a
              server behind a firewall, such as or  Apps can mitigate against
              this by setting a CURLOPT_OPENSOCKETFUNCTION(3) and checking the address  before  a

              All  the  malicious  scenarios regarding redirected URLs apply just as well to non-
              redirected URLs, if the user is allowed to specify  an  arbitrary  URL  that  could
              point to a private resource. For example, a web app providing a translation service
              might happily translate file://localhost/etc/passwd and display the  result.   Apps
              can  mitigate  against  this  with  the  CURLOPT_PROTOCOLS(3)  option as well as by
              similar mitigation techniques for redirections.

              A malicious FTP server could in response to the PASV command return an  IP  address
              and  port  number  for  a  server  local  to  the  app running libcurl but behind a
              firewall.  Apps can mitigate against this by using the  CURLOPT_FTP_SKIP_PASV_IP(3)
              option or CURLOPT_FTPPORT(3).

       IPv6 Addresses
              libcurl  will  normally  handle  IPv6 addresses transparently and just as easily as
              IPv4 addresses. That means that a sanitizing function that  filters  out  addresses
              like isn't sufficient--the equivalent IPv6 addresses ::1, ::, 0:00::0:1,
              :: and ::ffff:7f00:1 supplied somehow by an attacker would  all  bypass  a
              naive  filter  and  could allow access to undesired local resources.  IPv6 also has
              special address blocks like link-local and site-local that generally  shouldn't  be
              accessed  by a server-side libcurl-using application.  A poorly-configured firewall
              installed in a data center, organization or server may also be configured to  limit
              IPv4  connections  but  leave  IPv6  connections  wide  open.   In  some cases, the
              CURL_IPRESOLVE_V4 option can be used to limit resolved addresses to IPv4  only  and
              bypass these issues.

              When  uploading,  a  redirect can cause a local (or remote) file to be overwritten.
              Apps must not allow any unsanitized  URL  to  be  passed  in  for  uploads.   Also,
              CURLOPT_FOLLOWLOCATION(3)  should  not be used on uploads.  Instead, the app should
              handle redirects itself, sanitizing each URL first.

              Use of CURLOPT_UNRESTRICTED_AUTH(3) could cause authentication  information  to  be
              sent  to  an  unknown  second  server.  Apps can mitigate against this by disabling
              CURLOPT_FOLLOWLOCATION(3)  and  handling   redirects   itself,   sanitizing   where

              Use of the CURLAUTH_ANY option to CURLOPT_HTTPAUTH(3) could result in user name and
              password being sent in clear text to an HTTP server.  Instead, use CURLAUTH_ANYSAFE
              which  ensures  that  the  password is encrypted over the network, or else fail the

              Use of the CURLUSESSL_TRY option to CURLOPT_USE_SSL(3) could result  in  user  name
              and   password   being  sent  in  clear  text  to  an  FTP  server.   Instead,  use
              CURLUSESSL_CONTROL to ensure that an encrypted connection is used or else fail  the

              If  cookies  are  enabled  and cached, then a user could craft a URL which performs
              some malicious action to a site whose authentication is already stored in a cookie.
              e.g. Apps can mitigate against
              this by disabling cookies or clearing them between requests.

       Dangerous URLs
              SCP URLs can contain raw commands within the scp: URL, which is a  side  effect  of
              how  the  SCP  protocol  is designed. e.g.  scp://user:pass@host/a;date >/tmp/test;
              Apps must not allow unsanitized SCP: URLs to be passed in for downloads.

       Denial of Service
              A malicious server could cause libcurl to effectively hang by sending a trickle  of
              data  through,  or  even  no  data at all but just keeping the TCP connection open.
              This could result in a  denial-of-service  attack.  The  CURLOPT_TIMEOUT(3)  and/or
              CURLOPT_LOW_SPEED_LIMIT(3) options can be used to mitigate against this.

              A  malicious  server  could  cause  libcurl to effectively hang by starting to send
              data, then severing the connection without cleanly closing the TCP connection.  The
              app  could  install  a CURLOPT_SOCKOPTFUNCTION(3) callback function and set the TCP
              SO_KEEPALIVE option to mitigate against this.  Setting one of the  timeout  options
              would also work against this attack.

              A  malicious  server  could  cause  libcurl to download an infinite amount of data,
              potentially  causing  all  of  memory  or  disk   to   be   filled.   Setting   the
              CURLOPT_MAXFILESIZE_LARGE(3)  option  is  not  sufficient  to  guard  against this.
              Instead, the app should monitor the amount of data received  within  the  write  or
              progress callback and abort once the limit is reached.

              A malicious HTTP server could cause an infinite redirection loop, causing a denial-
              of-service. This can be mitigated by using the CURLOPT_MAXREDIRS(3) option.

       Arbitrary Headers
              User-supplied   data   must   be   sanitized   when   used    in    options    like
              are used to generate structured data. Characters like embedded carriage returns  or
              ampersands  could  allow the user to create additional headers or fields that could
              cause malicious transactions.

       Server-supplied Names
              A server can supply data which the application may, in some cases, use  as  a  file
              name.  The  curl  command-line  tool does this with --remote-header-name, using the
              Content-disposition: header to generate a file name.  An application could also use
              CURLINFO_EFFECTIVE_URL to generate a file name from a server-supplied redirect URL.
              Special care must be taken to sanitize such names to avoid  the  possibility  of  a
              malicious  server supplying one like "/etc/passwd", "\autoexec.bat", "prn:" or even

       Server Certificates
              A secure application should  never  use  the  CURLOPT_SSL_VERIFYPEER(3)  option  to
              disable certificate validation. There are numerous attacks that are enabled by apps
              that fail to  properly  validate  server  TLS/SSL  certificates,  thus  enabling  a
              malicious server to spoof a legitimate one. HTTPS without validated certificates is
              potentially as insecure as a plain HTTP connection.

       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

              Be sure to limit access to application logs if they could hold private or security-
              related data.  Besides the obvious candidates like user names and passwords, things
              like URLs, cookies or even file names could also hold sensitive data.

              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.

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 it is 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.  The name
       might make it seem that the multi interface is for multi-threaded programs, but the  truth
       is  almost  the  reverse.   The  multi  interface  allows a single-threaded application to
       perform the same kinds of multiple, simultaneous transfers  that  multi-threaded  programs
       can  perform.   It  allows  many  of  the benefits of multi-threaded transfers without the
       complexity of managing and synchronizing many threads.

       To complicate matters somewhat more, there are even two versions of the  multi  interface.
       The event based one, also called multi_socket and the "normal one" designed for using with
       select(). See the libcurl-multi.3 man page for details on  the  multi_socket  event  based
       API, this description here is for the select() oriented one.

       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 into a "multi stack".

       You  create  the  easy handles you want, one for each concurrent transfer, and you set all
       the options just like you  learned  above,  and  then  you  create  a  multi  handle  with
       curl_multi_init(3)   and   add   all   those  easy  handles  to  that  multi  handle  with

       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 calling curl_multi_perform(3).

       curl_multi_perform(3)  is asynchronous. It will only perform what can be done now and then
       return back control to your program. It is designed to  never  block.  You  need  to  keep
       calling the function until all transfers are completed.

       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(3), 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(3) to allow libcurl to do what it wants to do. Take  note
       that  libcurl does also feature some time-out code so we advise you to never use very long
       timeouts on select() before you call curl_multi_perform(3) again. curl_multi_timeout(3) is
       provided to help you get a suitable timeout period.

       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 in  any  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 curl_easy_cleanup(3)ed.

       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

       You  can  share  some  data between easy handles when the easy interface is used, and some
       data is share automatically when you use the multi interface.

       When you add easy handles to a multi handle, these easy handles will automatically share a
       lot  of  the  data  that  otherwise would be kept on a per-easy handle basis when the easy
       interface is used.

       The DNS cache is shared between handles within a  multi  handle,  making  subsequent  name
       resolving  faster,  and  the  connection  pool  that  is  kept  to better allow persistent
       connections and connection re-use is also shared. If you're using the easy interface,  you
       can  still  share  these  between  specific easy handles by using the share interface, see

       Some things are never shared automatically, not within multi  handles,  like  for  example
       cookies so the only way to share that is with the share interface.


       [1]    libcurl  7.10.3  and  later  have  the  ability to switch over to chunked Transfer-
              Encoding in cases where 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.

       [4]    This  behavior was different in versions before 7.17.0, where strings had to remain
              valid past the end of the curl_easy_setopt(3) call.


       libcurl-errors(3), libcurl-multi(3), libcurl-easy(3)