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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-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 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 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 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 environments.

       You program libcurl the same way on all platforms that libcurl runs  on.  There  are  only  a  few  minor
       details  that differ. If you just make sure to write your code portable enough, you can create a portable
       program. libcurl should not 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 will make
                     libcurl initialize the win32 socket stuff. Without having that initialized  properly,  your
                     program  cannot use sockets properly. You should only do this once for each application, so
                     if your program already does this or of another library in use does it, you should not tell
                     libcurl to do this as well.

              CURL_GLOBAL_SSL
                     which only does anything on libcurls compiled and built SSL-enabled. On these systems, this
                     will make libcurl initialize 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) has not 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 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 transfers.

       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 do not 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, "http://domain.com/");

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

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

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

        curl_easy_setopt(easyhandle, CURLOPT_WRITEFUNCTION, write_data);

       You can control what data your 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 will not 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 do  not  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 will not 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  are  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 will 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
       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 is not enough for you, you can use the CURLOPT_ERRORBUFFER(3) to point
       libcurl to a buffer of yours where it will 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 does not Work

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

       There's  one  golden  rule when these things occur: set the CURLOPT_VERBOSE(3) option to 1. it will 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 are 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 are 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 are trying to  do
       funny  things,  you  might  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 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 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 will not 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 will perform all the necessary operations and when it
       has invoked the upload it will call your supplied callback to get the data to upload. The program  should
       return  as much data as possible in every invoke, as that is likely to make the upload perform as fast as
       possible. The callback should return the number of bytes it wrote in the buffer. Returning 0 will  signal
       the end of the upload.

Passwords

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

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

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

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

       libcurl  also  provides options to set various passwords. The user name and password as shown embedded in
       the URL can instead get set with the CURLOPT_USERPWD(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 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 is not optional,
       like when you are using an SSL private key for secure transfers.

       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 supports:

        curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH,
        CURLAUTH_DIGEST|CURLAUTH_BASIC);

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

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

HTTP POSTing

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

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

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

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

       Simple enough, huh? Since you set the POST options with  the  CURLOPT_POSTFIELDS(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 do not 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 are
       called multi-part because they are 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(multipart);
        curl_mime_name(part, "name");
        curl_mime_data(part, "daniel", CURL_ZERO_TERMINATED);
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "project");
        curl_mime_data(part, "curl", CURL_ZERO_TERMINATED);
        part = curl_mime_addpart(multipart);
        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 function 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 are 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 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 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(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,
                     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 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",
                     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 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 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 CURLOPT_PROGRESSDATA(3). libcurl will not 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  are using a SOCKS proxy, you may find that libcurl does not 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  important  to  understand that all operations over an HTTP proxy use the HTTP
       protocol. For example, you cannot invoke your own custom  FTP  commands  or  even  proper  FTP  directory
       listings.

       Proxy Options

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

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

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

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

              If you want to, you can specify the host name only in the CURLOPT_PROXY(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, 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 was not 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 matter of asking the proxy for a
              straight connection to the target host on a specified port. This is made  with  the  HTTP  request
              CONNECT. ("please mr proxy, connect me to that remote host").

              Because  of  the  nature  of this operation, where the proxy has no idea what kind of data that is
              passed in and out through this tunnel, this breaks some of the 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 does not 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 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
       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 do not 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 rarely 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 are 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 will 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 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 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 are the boss.

       Modify Headers
              HTTP-like  protocols  pass  a  series of headers to the server when doing the request, and you are
              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:  do  not
              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-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 old servers do not 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
              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 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 are 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
       has not 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 are 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) does not 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 does not exist.

       If  you would rather use existing cookies that you have 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  does  not  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 email
       messages and send them via SMTP or append such messages to IMAP directories.

       A structured email 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 email 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.

       Email 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 email 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 email. */
        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
       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 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 are not 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 synchronous interface that transfers  one
       file at a time and does not 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 curl_multi_add_handle(3).

       When  you  have  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  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 will 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 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 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 are 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 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.

SEE ALSO

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

libcurl 7.81.0                                  December 20, 2021                            libcurl-tutorial(3)