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Updated: Thursday, June 13, 2019
 
 

libcurl-tutorial (3)

Name

libcurl-tutorial - libcurl programming tutorial

Synopsis

Please see following description for synopsis

Description

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



NAME
       libcurl-tutorial - libcurl programming tutorial

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

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

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


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

       Compiling the Program
              Your compiler needs  to  know  where  the  libcurl  headers  are
              located.  Therefore you must set your compiler's include path to
              point to the directory where you installed them. The  'curl-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  could
              be either on or off on for 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
              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 envi-
       ronments.

       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

        curl_global_init()

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

              CURL_GLOBAL_WIN32
                     which only does anything on Windows machines.  When  used
                     on  a  Windows machine, it'll make libcurl initialize the
                     win32 socket stuff. Without having that initialized 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.

              CURL_GLOBAL_SSL
                     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  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
       threads.

       Get an easy handle with

        easyhandle = curl_easy_init();

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

       You   set   properties   and   options   for    this    handle    using
       curl_easy_setopt(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. 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 sim-
       ilar to:

        curl_easy_setopt(handle, CURLOPT_URL, "http://domain.com/");

       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
       prototype:

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

       You tell libcurl to pass all data to this function by issuing  a  func-
       tion 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 appli-
       cation  and  the  function that gets invoked by libcurl. libcurl itself
       won't touch the data you pass with CURLOPT_WRITEDATA(3).

       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 pass-
       ing a 'FILE *' to a file opened for  writing  with  the  CURLOPT_WRITE-
       DATA(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  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(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_WRITE-
       FUNCTION(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 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(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 information.


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-
       gram.

       There's one golden rule when these things occur: set  the  CURLOPT_VER-
       BOSE(3)  option  to  1.  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(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  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 an HTTP server with a PUT request.

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

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

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

       Where  bufptr is the pointer to a buffer we fill in with data to upload
       and size*nitems is the size of the buffer and therefore also the  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_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 num-
       ber of bytes it wrote in the buffer. Returning 0 will signal the end of
       the upload.


Passwords
       Many protocols use or even require that user name and password are 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:

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

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

       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:these-
       cret");

       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 CUR-
       LOPT_NETRC(3) option:

        curl_easy_setopt(easyhandle, CURLOPT_NETRC, 1L);

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

        machine myhost.mydomain.com
        login userlogin
        password secretword

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

       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 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 authenti-
       cation 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 supports:

        curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH,
        CURLAUTH_DIGEST|CURLAUTH_BASIC);

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

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


HTTP POSTing
       We  get  many  questions regarding how to issue HTTP POSTs with libcurl
       the proper way. This chapter will thus include examples using both 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, "http://posthere.com/");

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

       Simple  enough,  huh?  Since  you  set  the  POST options with the CUR-
       LOPT_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. 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, 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  form-
       post  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  head-
       ers.  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 */
        curl_mime_free(multipart);

       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 func-
       tion curl_mime_subparts(3) implements nested multi-parts, this  way  of
       multiple files posting is deprecated by RFC 7578, chapter 4.3.

       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 */
        curl_formfree(post);

       Multipart formposts are chains of parts using MIME-style separators and
       headers. It means that each one of these separate parts get a few 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  as  you  like,
       but  this  little example will show how you set headers to one specific
       part when you add that to the post handle:

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

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

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

        curl_formfree(post); /* free post */
        curl_slist_free_all(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(easyhan-
       dle).
       - 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,
                     CURLFORM_END);
       becomes:
        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_slist_free_all(3).

        curl_formadd(&post, &last,
                     CURLFORM_PTRNAME, "logotype-image",
                     CURLFORM_FILECONTENT, "-",
                     CURLFORM_END);
       becomes:
        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 call-
       back 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",
                     CURLFORM_END);
       becomes:
        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, "archive.zip",
                     CURLFORM_CONTENTTYPE, "application/zip",
                     CURLFORM_END);
       becomes:
        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, "archive.zip");
        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,
                     CURLFORM_END);
       becomes:
        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 reuse.

        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "message",
                     CURLFORM_FILECONTENT, "msg.txt",
                     CURLFORM_END);
       becomes:
        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 emula-
       tion.


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   CUR-
       LOPT_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 transfer.

       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  CUR-
       LOPT_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 */
           }
        }


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

       Proxies are exceedingly common these days. Companies often  only  offer
       Internet  access to employees through their 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 an 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 an HTTP proxy use the HTTP proto-
       col. For example, you can't invoke your own custom FTP commands or even
       proper FTP directory listings.


       Proxy Options

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

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

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

               curl_easy_setopt(easyhandle,  CURLOPT_PROXYUSERPWD, "user:pass-
              word");

              If you want to, you can specify the host name only in  the  CUR-
              LOPT_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 an HTTP proxy):

               curl_easy_setopt(easyhandle,      CURLOPT_PROXYTYPE,      CURL-
              PROXY_SOCKS4);


       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_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  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(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 an 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 an HTTP proxy is therefore a mat-
              ter 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 an 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, 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 sim-
              ilar.


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 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  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(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  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(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_HTTP-
       HEADER(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-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 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.


       CUSTOMREQUEST
              If just changing the actual HTTP request  keyword  is  what  you
              want,  like  when  GET, HEAD or POST is not good enough for you,
              CURLOPT_CUSTOMREQUEST(3) is there for you. It is very simple  to
              use:

               curl_easy_setopt(easyhandle,  CURLOPT_CUSTOMREQUEST,  "MYOWNRE-
              QUEST");

              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: munged.host.line");


       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-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 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 an 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  opera-
              tions) to happen _after_ the data transfer took place the option
              to   curl_easy_setopt(3)   would   instead   be   called    CUR-
              LOPT_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 failed.

              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.


       FTP Custom CUSTOMREQUEST
              If you do want to list the contents of an  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  "ses-
       sions".

       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(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  net-
       work 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 directories.

       A  structured  e-mail  message may contain several parts: some are dis-
       played 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  explic-
       itly.

       E-mail messages data is not supposed to be non-ascii and line length is
       limited: fortunately, some transfer encodings are defined by the  stan-
       dards  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_ZERO_TERMINATED);
        curl_mime_type(part, "text/html");

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

        /* 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: me@example.com");
        headers = curl_slist_append(headers, "To: you@example.com");

        /* 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 CUR-
       LOPT_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 CURLOPT_HEADERFUNCTION(3).

       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
       See curl_easy_getinfo(3).

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 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 syn-
       chronizing 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 descrip-
       tion 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 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 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  trans-
       fers are completed.

       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
       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_time-
       out(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 invoke.

       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
       libcurl-share(3).

       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.

Footnotes
       [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 sim-
              ilar 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.


ATTRIBUTES
       See attributes(7) for descriptions of the following attributes:


       +---------------+------------------+
       |ATTRIBUTE TYPE | ATTRIBUTE VALUE  |
       +---------------+------------------+
       |Availability   | web/curl         |
       +---------------+------------------+
       |Stability      | Uncommitted      |
       +---------------+------------------+
SEE ALSO
       libcurl-errors(3), libcurl-multi(3), libcurl-easy(3)



NOTES
       This     software     was    built    from    source    available    at
       https://github.com/oracle/solaris-userland.   The  original   community
       source    was    downloaded    from    https://github.com/curl/curl/ar-
       chive/curl-7_64_0.zip

       Further information about this software can be found on the open source
       community website at http://curl.haxx.se/.



libcurl                           19 Sep 2014              libcurl-tutorial(3)