docs/FAQ

Updated: June 17, 2003 (http://curl.haxx.se/docs/faq.html)
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FAQ

 1. Philosophy
  1.1 What is cURL?
  1.2 What is libcurl?
  1.3 What is cURL not?
  1.4 When will you make curl do XXXX ?
  1.5 Who makes cURL?
  1.6 What do you get for making cURL?
  1.7 What about CURL from curl.com?
  1.8 I have a problem who do I mail?

 2. Install Related Problems
  2.1 configure doesn't find OpenSSL even when it is installed
   2.1.1. native linker doesn't find openssl
   2.1.2. only the libssl lib is missing
  2.2 Does curl work/build with other SSL libraries?
  2.3 Where can I find a copy of LIBEAY32.DLL?
  2.4 Does cURL support Socks (RFC 1928) ?

 3. Usage Problems
  3.1 curl: (1) SSL is disabled, https: not supported
  3.2 How do I tell curl to resume a transfer?
  3.3 Why doesn't my posting using -F work?
  3.4 How do I tell curl to run custom FTP commands?
  3.5 How can I disable the Pragma: nocache header?
  3.6 Does curl support ASP, XML, XHTML or HTML version Y?
  3.7 Can I use curl to delete/rename a file through FTP?
  3.8 How do I tell curl to follow HTTP redirects?
  3.9 How do I use curl in my favorite programming language?
  3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
  3.11 How do I POST with a different Content-Type?
  3.12 Why do FTP specific features over HTTP proxy fail?
  3.13 Why does my single/double quotes fail?
  3.14 Does curl support javascript or pac (automated proxy config)?

 4. Running Problems
  4.1 Problems connecting to SSL servers.
  4.2 Why do I get problems when I use & or % in the URL?
  4.3 How can I use {, }, [ or ] to specify multiple URLs?
  4.4 Why do I get downloaded data even though the web page doesn't exist?
  4.5 Why do I get return code XXX from a HTTP server?
   4.5.1 "400 Bad Request"
   4.5.2 "401 Unauthorized"
   4.5.3 "403 Forbidden"
   4.5.4 "404 Not Found"
   4.5.5 "405 Method Not Allowed"
   4.5.6 "301 Moved Permanently"
  4.6 Can you tell me what error code 142 means?
  4.7 How do I keep user names and passwords secret in Curl command lines?
  4.8 I found a bug!
  4.9 Curl can't authenticate to the server that requires NTLM?
  4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
  4.11 Why does my HTTP range requests return the full document?
  4.12 Why do I get "certificate verify failed" ?

 5. libcurl Issues
  5.1 Is libcurl thread-safe?
  5.2 How can I receive all data into a large memory chunk?
  5.3 How do I fetch multiple files with libcurl?
  5.4 Does libcurl do Winsock initing on win32 systems?
  5.5 Does CURLOPT_FILE and CURLOPT_INFILE work on win32 ?
  5.6 What about Keep-Alive or persistent connections?
  5.7 Link errors when building libcurl on Windows!

 6. License Issues
  6.1 I have a GPL program, can I use the libcurl library?
  6.2 I have a closed-source program, can I use the libcurl library?
  6.3 I have a BSD licensed program, can I use the libcurl library?
  6.4 I have a program that uses LGPL libraries, can I use libcurl?
  6.5 Can I modify curl/libcurl for my program and keep the changes secret?
  6.6 Can you please change the curl/libcurl license to XXXX?

==============================================================================

1. Philosophy

  1.1 What is cURL?

  cURL (or simply just 'curl') is a command line tool for getting or sending
  files using URL syntax. The name is a play on 'Client for URLs', originally
  with URL spelled in uppercase to make it obvious it deals with URLs. The
  fact it can also be pronounced 'see URL' also helped, it works as an
  abbrivation for "Client URL Request Library" or why not the recursive
  version: "Curl is a URL Request Library".

  Curl supports a range of common Internet protocols, currently including
  HTTP, HTTPS, FTP, FTPS, GOPHER, LDAP, DICT, TELNET and FILE.

  We spell it cURL or just curl. We pronounce it with an initial k sound:
  [kurl].

  NOTE: there are numerous sub-projects and related projects that also use the
  word curl in the project names in various combinations, but you should take
  notice that this FAQ is directed at the command-line tool named curl (and
  libcurl the library), and may therefore not be valid for other curl
  projects.

  1.2 What is libcurl?

  libcurl is a reliable and portable library which provides you with an easy
  interface to a range of common Internet protocols.

  You can use libcurl for free in your application even if it is commercial
  or closed-source.

  1.3 What is cURL not?
  
  Curl is *not* a wget clone even though that is a very common misconception.
  Never, during curl's development, have we intended curl to replace wget or
  compete on its market. Curl is targeted at single-shot file transfers.

  Curl is not a web site mirroring program. If you wanna use curl to mirror
  something: fine, go ahead and write a script that wraps around curl to make
  it reality (like curlmirror.pl does).

  Curl is not an FTP site mirroring program. Sure, get and send FTP with curl
  but if you want systematic and sequential behavior you should write a
  script (or write a new program that interfaces libcurl) and do it.

  Curl is not a PHP tool, even though it works perfectly well when used from
  or with PHP.

  Curl is not a single-OS program. Curl exists, compiles, builds and runs
  under a wide range of operating systems, including all modern Unixes (and a
  bunch of older ones too), Windows, Amiga, BeOS, OS/2, OS X, QNX etc.

  1.4 When will you make curl do XXXX ?

  We love suggestions of what to change in order to make curl and libcurl
  better. We do however believe in a few rules when it comes to the future of
  curl:

  * Curl -- the command line tool -- is to remain a non-graphical command line
    tool. If you want GUIs or fancy scripting capabilities, you should look
    for another tool that uses libcurl.

  * We do not add things to curl that other small and available tools already
    do very fine at the side. Curl's output is fine to pipe into another
    program or redirect to another file for the next program to interpret.

  * We focus on protocol related issues and improvements. If you wanna do more
    magic with the supported protocols than curl currently does, chances are
    big we will agree. If you wanna add more protocols, we may very well
    agree.

  * If you want someone else to make all the work while you wait for us to
    implement it for you, that is not a very friendly attitude. We spend a
    considerable time already on maintaining and developing curl. In order to
    get more out of us, you should consider trading in some of your time and
    efforts in return.

  * If you write the code, chances are bigger that it will get into curl
    faster.

  1.5 Who makes cURL?

  cURL and libcurl are not made by any single individual. Sure, Daniel
  Stenberg writes the major parts, but other persons' submissions are
  important and crucial. Anyone can contribute and post their changes and
  improvements and have them inserted in the main sources (of course on the
  condition that developers agree on that the fixes are good).

  The list of contributors in the docs/THANKS file is only a small part of all
  the people that every day provide us with bug reports, suggestions, ideas
  and source code.

  curl is developed by a community, with Daniel at the wheel.

  1.6 What do you get for making cURL?

  Project cURL is entirely free and open. No person gets paid for developing
  curl. We do this voluntarily on our spare time.

  We get some help from companies. Contactor Data hosts the curl web site,
  Haxx owns the curl web site's domain and sourceforge.net hosts several
  project services we take advantage from, like the bug tracker, mailing lists
  and more.

  If you want to support our project with a donation or similar, one way of
  doing that would be to buy "gift certificates" at useful online shopping
  sites, such as amazon.com or thinkgeek.com. Another way would be to sponsor
  us through a banner-program or even better: by helping us coding,
  documenting, testing etc. You're welcome to send us a buck using paypal, as
  described here: http://curl.haxx.se/donation.html

  1.7 What about CURL from curl.com?

  During the summer 2001, curl.com was busy advertising their client-side
  programming language for the web, named CURL.

  We are in no way associated with curl.com or their CURL programming
  language. 

  Our project name curl has been in effective use since 1998. We were not the
  first computer related project to use the name "curl" and do not claim any
  first-hand rights to the name.

  We recognize that we will be living in parallel with curl.com and wish them
  every success.

  1.8 I have a problem who do I mail?

  Please do not mail any single individual unless you really need to. Keep
  curl-related questions on a suitable mailing list. All available mailing
  lists are listed in the MANUAL document and online at
  http://curl.haxx.se/mail/

  Keeping curl-related questions and discussions on mailing lists allows
  others to join in and help, to share their ideas, contribute their
  suggestions and spread their wisdom. Keeping discussions on public mailing
  lists also allows for others to learn from this (both current and future
  users thanks to the web based archives of the mailing lists), thus saving us
  from having to repeat ourselves even more. Thanks for respecting this.


2. Install Related Problems

  2.1. configure doesn't find OpenSSL even when it is installed

  This may be because of several reasons.

    2.1.1. native linker doesn't find openssl

    Affected platforms:
      Solaris (native cc compiler)
      HPUX (native cc compiler)
      SGI IRIX (native cc compiler)
      SCO UNIX (native cc compiler)

    When configuring curl, I specify --with-ssl. OpenSSL is installed in
    /usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
    CRYPTO_lock in -lcrypto

    Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
    -lcrypto, so ld can't find the library. This is due to a bug in the GNU
    autoconf tool.

    Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
    ./configure places the -L/usr/local/ssl/lib early enough in the command
    line to make things work

    Solution submitted by: Bob Allison <allisonb@users.sourceforge.net>

    2.1.2. only the libssl lib is missing

    If all include files and the libcrypto lib is present, with only the
    libssl being missing according to configure, this is mostly likely because
    a few functions are left out from the libssl.

    If the function names missing include RSA or RSAREF you can be certain
    that this is because libssl requires the RSA and RSAREF libs to build.

    See the INSTALL file section that explains how to add those libs to
    configure. Make sure that you remove the config.cache file before you
    rerun configure with the new flags.

  2.2. Does curl work/build with other SSL libraries?

  Curl has been written to use OpenSSL, although there should not be much
  problems using a different library. If anyone does "port" curl to use a
  different SSL library, we are of course very interested in getting the
  patch!

  2.3. Where can I find a copy of LIBEAY32.DLL?

  That is an OpenSSL binary built for Windows.

  Curl uses OpenSSL to do the SSL stuff. The LIBEAY32.DLL is what curl needs
  on a windows machine to do https://. Check out the curl web site to find
  accurate and up-to-date pointers to recent OpenSSL DLLs and other binary
  packages.

  2.4. Does cURL support Socks (RFC 1928) ?

  There is limited support for SOCKS5 for curl built with IPv6 support
  disabled.


3. Usage problems

  3.1. curl: (1) SSL is disabled, https: not supported

  If you get this output when trying to get anything from a https:// server,
  it means that the configure script couldn't find all libs and include files
  it requires for SSL to work. If the configure script fails to find them,
  curl is simply built without SSL support.

  To get the https:// support into a curl that was previously built but that
  reports that https:// is not supported, you should dig through the document
  and logs and check out why the configure script doesn't find the SSL libs
  and/or include files.

  Also, check out the other paragraph in this FAQ labeled "configure doesn't
  find OpenSSL even when it is installed".

  3.2. How do I tell curl to resume a transfer?

  Curl supports resumed transfers both ways on both FTP and HTTP.

  Try the -C option.

  3.3. Why doesn't my posting using -F work?

  You can't simply use -F or -d at your choice. The web server that will
  receive your post assumes one of the formats. If the form you're trying to
  "fake" sets the type to 'multipart/form-data', then and only then you must
  use the -F type. In all the most common cases, you should use -d which then
  causes a posting with the type 'application/x-www-form-urlencoded'.

  This is described in some detail in the MANUAL and TheArtOfHttpScripting
  documents, and if you don't understand it the first time, read it again
  before you post questions about this to the mailing list. Also, try reading
  through the mailing list archives for old postings and questions regarding
  this.

  3.4. How do I tell curl to run custom FTP commands?

  You can tell curl to perform optional commands both before and/or after a
  file transfer. Study the -Q/--quote option.

  Since curl is used for file transfers, you don't use curl to just perform
  FTP commands without transferring anything. Therefore you must always specify
  a URL to transfer to/from even when doing custom FTP commands.

  3.5. How can I disable the Pragma: nocache header?

  You can change all internally generated headers by adding a replacement with
  the -H/--header option. By adding a header with empty contents you safely
  disable that one. Use -H "Pragma:" to disable that specific header.

  3.6. Does curl support ASP, XML, XHTML or HTML version Y?

  To curl, all contents are alike. It doesn't matter how the page was
  generated. It may be ASP, PHP, Perl, shell-script, SSI or plain
  HTML-files. There's no difference to curl and it doesn't even know what kind
  of language that generated the page.

  See also item 3.14 regarding javascript.

  3.7. Can I use curl to delete/rename a file through FTP?

  Yes. You specify custom FTP commands with -Q/--quote.

  One example would be to delete a file after you have downloaded it:

     curl -O ftp://download.com/coolfile -Q '-DELE coolfile'

  3.8 How do I tell curl to follow HTTP redirects?

  Curl does not follow so-called redirects by default. The Location: header
  that informs the client about this is only interpreted if you're using the
  -L/--location option. As in:

     curl -L http://redirector.com

  3.9 How do I use curl in my favorite programming language?

  There exist many language interfaces/bindings for curl that integrates it
  better with various languages. If you are fluid in a script language, you
  may very well opt to use such an interface instead of using the command line
  tool.

  Find out more about which languages that support curl directly, and how to
  install and use them, in the libcurl section of the curl web site:

        http://curl.haxx.se/libcurl/

  In February 2003, there are interfaces available for the following
  languages: Basic, C, C++, Cocoa, Dylan, Euphoria, Java, Lua, Object-Pascal,
  Pascal, Perl, PHP, PostgreSQL, Python, Rexx, Ruby, Scheme and Tcl. By the
  time you read this, additional ones may have appeared!

  3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?

  Curl adheres to the HTTP spec, which basically means you can play with *any*
  protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and
  XML-RPC are all such ones. You can use -X to set custom requests and -H to
  set custom headers (or replace internally generated ones).

  Using libcurl is of course just as fine and you'd just use the proper
  library options to do the same.

  3.11 How do I POST with a different Content-Type?

  You can always replace the internally generated headers with -H/--header.
  To make a simple HTTP POST with text/xml as content-type, do something like:

        curl -d "datatopost" -H "Content-Type: text/xml" [URL]

  3.12 Why do FTP specific features over HTTP proxy fail?

  Because when you use a HTTP proxy, the protocol spoken on the network will
  be HTTP, even if you specify a FTP URL. This effectively means that you
  normally can't use FTP specific features such as FTP upload and FTP quote
  etc.

  There is one exception to this rule, and that is if you can "tunnel through"
  the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
  and is generally not available as proxy admins usually disable tunneling to
  other ports than 443 (which is used for HTTPS access through proxies).

  3.13 Why does my single/double quotes fail?

  To specify a command line option that includes spaces, you might need to
  put the entire option within quotes. Like in:

   curl -d " with spaces " url.com
  
  or perhaps

   curl -d ' with spaces ' url.com

  Exactly what kind of quotes and how to do this is entirely up to the shell
  or command line interpreter that you are using. For most unix shells, you
  can more or less pick either single (') or double (") quotes. For
  Windows/DOS prompts I believe you're forced to use double (") quotes.

  Please study the documentation for your particular environment. Examples in
  the curl docs will use a mix of both these ones as shown above. You must
  adjust them to work in your environment.

  Remember that curl works and runs on more operating systems than most single
  individuals have ever tried.

  3.14 Does curl support javascript or pac (automated proxy config)?

  Many web pages do magic stuff using embedded javascript. Curl and libcurl
  have no built-in support for that, so it will be treated just like any other
  contents.

  .pac files are a netscape invention and are sometimes used by organizations
  to allow them to differentiate which proxies to use. The .pac contents is
  just a javascript program that gets invoked by the browser and that returns
  the name of the proxy to connect to. Since curl doesn't support javascript,
  it can't support .pac proxy configuration either.

  Some work-arounds usually suggested to overcome this javascript dependency:

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

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

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

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


4. Running Problems

  4.1. Problems connecting to SSL servers.

  It took a very long time before we could sort out why curl had problems to
  connect to certain SSL servers when using SSLeay or OpenSSL v0.9+.  The
  error sometimes showed up similar to:

  16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:

  It turned out to be because many older SSL servers don't deal with SSLv3
  requests properly. To correct this problem, tell curl to select SSLv2 from
  the command line (-2/--sslv2).

  There has also been examples where the remote server didn't like the SSLv2
  request and instead you had to force curl to use SSLv3 with -3/--sslv3.

  4.2. Why do I get problems when I use & or % in the URL?

  In general unix shells, the & letter is treated special and when used, it
  runs the specified command in the background. To safely send the & as a part
  of a URL, you should quote the entire URL by using single (') or double (")
  quotes around it.

  An example that would invoke a remote CGI that uses &-letters could be:

     curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'

  In Windows, the standard DOS shell treats the %-letter specially and you
  need to use TWO %-letters for each single one you want to use in the URL.

  Also note that if you want the literal %-letter to be part of the data you
  pass in a POST using -d/--data you must encode it as '%25' (which then also
  needs the %-letter doubled on Windows machines).

  4.3. How can I use {, }, [ or ] to specify multiple URLs?

  Because those letters have a special meaning to the shell, and to be used in
  a URL specified to curl you must quote them.

  An example that downloads two URLs (sequentially) would do:

    curl '{curl,www}.haxx.se'

  To be able to use those letters as actual parts of the URL (without using
  them for the curl URL "globbing" system), use the -g/--globoff option:

    curl -g 'www.site.com/weirdname[].html'

  4.4. Why do I get downloaded data even though the web page doesn't exist?

  Curl asks remote servers for the page you specify. If the page doesn't exist
  at the server, the HTTP protocol defines how the server should respond and
  that means that headers and a "page" will be returned. That's simply how
  HTTP works.

  By using the --fail option you can tell curl explicitly to not get any data
  if the HTTP return code doesn't say success.

  4.5 Why do I get return code XXX from a HTTP server?

  RFC2616 clearly explains the return codes. This is a short transcript. Go
  read the RFC for exact details:

    4.5.1 "400 Bad Request"

    The request could not be understood by the server due to malformed
    syntax. The client SHOULD NOT repeat the request without modifications.

    4.5.2 "401 Unauthorized"

    The request requires user authentication.

    4.5.3 "403 Forbidden"

    The server understood the request, but is refusing to fulfill it.
    Authorization will not help and the request SHOULD NOT be repeated.

    4.5.4 "404 Not Found"

    The server has not found anything matching the Request-URI. No indication
    is given of whether the condition is temporary or permanent.

    4.5.5 "405 Method Not Allowed"

    The method specified in the Request-Line is not allowed for the resource
    identified by the Request-URI. The response MUST include an Allow header
    containing a list of valid methods for the requested resource.

    4.5.6 "301 Moved Permanently"
 
    If you get this return code and an HTML output similar to this:

       <H1>Moved Permanently</H1> The document has moved <A
       HREF="http://same_url_now_with_a_trailing_slash/">here</A>.

    it might be because you request a directory URL but without the trailing
    slash. Try the same operation again _with_ the trailing URL, or use the
    -L/--location option to follow the redirection.

  4.6. Can you tell me what error code 142 means?

  All error codes that are larger than the highest documented error code means
  that curl has exited due to a crash. This is a serious error, and we
  appreciate a detailed bug report from you that describes how we could go
  ahead and repeat this!

  4.7. How do I keep user names and passwords secret in Curl command lines?

  This problem has two sides:

  The first part is to avoid having clear-text passwords in the command line
  so that they don't appear in 'ps' outputs and similar. That is easily
  avoided by using the "-K" option to tell curl to read parameters from a
  file or stdin to which you can pass the secret info.

  To keep the passwords in your account secret from the rest of the world is
  not a task that curl addresses. You could of course encrypt them somehow to
  at least hide them from being read by human eyes, but that is not what
  anyone would call security.

  Also note that regular HTTP and FTP passwords are sent in clear across the
  network. All it takes for anyone to fetch them is to listen on the network.
  Eavesdropping is very easy.

  4.8 I found a bug!

  It is not a bug if the behavior is documented. Read the docs first.

  If it is a problem with a binary you've downloaded or a package for your
  particular platform, try contacting the person who built the package/archive
  you have.

  If there is a bug, read the BUGS document first. Then report it as described
  in there.

  4.9. Curl can't authenticate to the server that requires NTLM?

  This is supported in curl 7.10.6 or later. No earlier curl version knows
  of this magic.

  NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You
  should not use such ones.

  4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!

  Many web servers allow or demand that the administrator configures the
  server properly for these requests to work on the web server.

  Some servers seem to support HEAD only on certain kinds of URLs.

  To fully grasp this, try the documentation for the particular server
  software you're trying to interact with. This is not anything curl can do
  anything about.

  4.11 Why does my HTTP range requests return the full document?

  Because the range may not be supported by the server, or the server may
  choose to ignore it and return the full document anyway.

  4.12 Why do I get "certificate verify failed" ?

  You invoke curl 7.10 or later to communicate on a https:// URL and get an
  error back looking something similar to this:

      curl: (35) SSL: error:14090086:SSL routines: 
      SSL3_GET_SERVER_CERTIFICATE:certificate verify failed

  Then it means that curl couldn't verify that the server's certificate was
  good. Curl verifies the certificate using the CA cert bundle that comes with
  the curl installation.

  To disable the verification (which makes it act like curl did before 7.10),
  use -k. This does however enable man-in-the-middle attacks.

  If you get this failure but are having a CA cert bundle installed and used,
  the server's certificate is not signed by one of the CA's in the bundle. It
  might for example be self-signed. You then correct this problem by obtaining
  a valid CA cert for the server. Or again, decrease the security by disabling
  this check.

  Details are also in the SSLCERTS file in the release archives, found online
  here: http://curl.haxx.se/docs/sslcerts.html

5. libcurl Issues

  5.1. Is libcurl thread-safe?

  Yes.

  We have written the libcurl code specificly adjusted for multi-threaded
  programs. libcurl will use thread-safe functions instead of non-safe ones if
  your system has such.

  We would appreciate some kind of report or README file from those who have
  used libcurl in a threaded environment.

  5.2 How can I receive all data into a large memory chunk?

  [ See also the examples/getinmemory.c source ]

  You are in full control of the callback function that gets called every time
  there is data received from the remote server. You can make that callback do
  whatever you want. You do not have to write the received data to a file.

  One solution to this problem could be to have a pointer to a struct that you
  pass to the callback function. You set the pointer using the
  curl_easy_setopt(CURLOPT_FILE) function. Then that pointer will be passed to
  the callback instead of a FILE * to a file:

        /* imaginary struct */
        struct MemoryStruct {
          char *memory;
          size_t size;
        };

        /* imaginary callback function */
        size_t
        WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
        {
          register int realsize = size * nmemb;
          struct MemoryStruct *mem = (struct MemoryStruct *)data;
        
          mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
          if (mem->memory) {
            memcpy(&(mem->memory[mem->size]), ptr, realsize);
            mem->size += realsize;
            mem->memory[mem->size] = 0;
          }
          return realsize;
        }

  5.3 How do I fetch multiple files with libcurl?

  libcurl has excellent support for transferring multiple files. You should
  just repeatedly set new URLs with curl_easy_setopt() and then transfer it
  with curl_easy_perform(). The handle you get from curl_easy_init() is not
  only reusable, but you're even encouraged to reuse it if you can, as that
  will enable libcurl to use persistent connections.

  5.4 Does libcurl do Winsock initialization on win32 systems?

  Yes, if told to in the curl_global_init() call.

  5.5 Does CURLOPT_FILE and CURLOPT_INFILE work on win32 ?

  Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
  that DLL use the FILE *. If you set CURLOPT_FILE you must also use
  CURLOPT_WRITEFUNCTION as well to set a function that writes the file, even
  if that simply writes the data to the specified FILE*. Similarly, if you use
  CURLOPT_INFILE you must also specify CURLOPT_READFUNCTION.

  (Provided by Joel DeYoung and Bob Schader)

  5.6 What about Keep-Alive or persistent connections?

  curl and libcurl have excellent support for persistent connections when
  transferring several files from the same server.  Curl will attempt to reuse
  connections for all URLs specified on the same command line/config file, and
  libcurl will reuse connections for all transfers that are made using the
  same libcurl handle.

  5.7 Link errors when building libcurl on Windows!

  You need to make sure that your project, and all the libraries (both static
  and dynamic) that it links against, are compiled/linked against the same run
  time library.

  This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
  options to the command line compiler. /MD (linking against MSVCRT dll) seems
  to be the most commonly used option.

  (Provided by Andrew Francis)


6. License Issues

  Curl and libcurl are released under a MIT/X derivate license. The license is
  very liberal and should not impose a problem for your project. This section
  is just a brief summary for the cases we get the most questions. (Parts of
  this section was much enhanced by Bjorn Reese.)

  6.1. I have a GPL program, can I use the libcurl library?

  Yes!

  Since libcurl may be distributed under the MIT/X derivate license, it can be
  used together with GPL in any software.

  6.2. I have a closed-source program, can I use the libcurl library?

  Yes!

  libcurl does not put any restrictions on the program that uses the library.

  6.3. I have a BSD licensed program, can I use the libcurl library?

  Yes!

  libcurl does not put any restrictions on the program that uses the library.

  6.4. I have a program that uses LGPL libraries, can I use libcurl?

  Yes!

  The LGPL license doesn't clash with other licenses.

  6.5. Can I modify curl/libcurl for my program and keep the changes secret?

  Yes!

  The MIT/X derivate license practically allows you to do almost anything with
  the sources, on the condition that the copyright texts in the sources are
  left intact.

  6.6. Can you please change the curl/libcurl license to XXXX?

  No.

  We have carefully picked this license after years of development and
  discussions and a large amount of people have contributed with source code
  knowing that this is the license we use. This license puts the restrictions
  we want on curl/libcurl and it does not spread to other programs or
  libraries that use it. It should be possible for everyone to use libcurl or
  curl in their projects, no matter what license they already have in use.