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1                                  _   _ ____  _
2                              ___| | | |  _ \| |
3                             / __| | | | |_) | |
4                            | (__| |_| |  _ <| |___
5                             \___|\___/|_| \_\_____|
6
7FAQ
8
9 1. Philosophy
10  1.1 What is cURL?
11  1.2 What is libcurl?
12  1.3 What is curl not?
13  1.4 When will you make curl do XXXX ?
14  1.5 Who makes curl?
15  1.6 What do you get for making curl?
16  1.7 What about CURL from curl.com?
17  1.8 I have a problem who do I mail?
18  1.9 Where do I buy commercial support for curl?
19  1.10 How many are using curl?
20  1.11 Why don't you update ca-bundle.crt
21  1.12 I have a problem who can I chat with?
22  1.13 curl's ECCN number?
23  1.14 How do I submit my patch?
24  1.15 How do I port libcurl to my OS?
25
26 2. Install Related Problems
27  2.1 configure doesn't find OpenSSL even when it is installed
28   2.1.1 native linker doesn't find OpenSSL
29   2.1.2 only the libssl lib is missing
30  2.2 Does curl work/build with other SSL libraries?
31  2.3 Where can I find a copy of LIBEAY32.DLL?
32  2.4 Does curl support SOCKS (RFC 1928) ?
33
34 3. Usage Problems
35  3.1 curl: (1) SSL is disabled, https: not supported
36  3.2 How do I tell curl to resume a transfer?
37  3.3 Why doesn't my posting using -F work?
38  3.4 How do I tell curl to run custom FTP commands?
39  3.5 How can I disable the Accept: */* header?
40  3.6 Does curl support ASP, XML, XHTML or HTML version Y?
41  3.7 Can I use curl to delete/rename a file through FTP?
42  3.8 How do I tell curl to follow HTTP redirects?
43  3.9 How do I use curl in my favorite programming language?
44  3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
45  3.11 How do I POST with a different Content-Type?
46  3.12 Why do FTP specific features over HTTP proxy fail?
47  3.13 Why does my single/double quotes fail?
48  3.14 Does curl support Javascript or PAC (automated proxy config)?
49  3.15 Can I do recursive fetches with curl?
50  3.16 What certificates do I need when I use SSL?
51  3.17 How do I list the root dir of an FTP server?
52  3.18 Can I use curl to send a POST/PUT and not wait for a response?
53  3.19 How do I get HTTP from a host using a specific IP address?
54  3.20 How to SFTP from my user's home directory?
55  3.21 Protocol xxx not supported or disabled in libcurl
56  3.22 curl -X gives me HTTP problems
57
58 4. Running Problems
59  4.1 Problems connecting to SSL servers.
60  4.2 Why do I get problems when I use & or % in the URL?
61  4.3 How can I use {, }, [ or ] to specify multiple URLs?
62  4.4 Why do I get downloaded data even though the web page doesn't exist?
63  4.5 Why do I get return code XXX from a HTTP server?
64   4.5.1 "400 Bad Request"
65   4.5.2 "401 Unauthorized"
66   4.5.3 "403 Forbidden"
67   4.5.4 "404 Not Found"
68   4.5.5 "405 Method Not Allowed"
69   4.5.6 "301 Moved Permanently"
70  4.6 Can you tell me what error code 142 means?
71  4.7 How do I keep user names and passwords secret in Curl command lines?
72  4.8 I found a bug!
73  4.9 Curl can't authenticate to the server that requires NTLM?
74  4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
75  4.11 Why does my HTTP range requests return the full document?
76  4.12 Why do I get "certificate verify failed" ?
77  4.13 Why is curl -R on Windows one hour off?
78  4.14 Redirects work in browser but not with curl!
79  4.15 FTPS doesn't work
80  4.16 My HTTP POST or PUT requests are slow!
81  4.17 Non-functional connect timeouts on Windows
82  4.18 file:// URLs containing drive letters (Windows, NetWare)
83  4.19 Why doesn't cURL return an error when the network cable is unplugged?
84  4.20 curl doesn't return error for HTTP non-200 responses!
85  4.21 Why is there a HTTP/1.1 in my HTTP/2 request?
86
87 5. libcurl Issues
88  5.1 Is libcurl thread-safe?
89  5.2 How can I receive all data into a large memory chunk?
90  5.3 How do I fetch multiple files with libcurl?
91  5.4 Does libcurl do Winsock initing on win32 systems?
92  5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
93  5.6 What about Keep-Alive or persistent connections?
94  5.7 Link errors when building libcurl on Windows!
95  5.8 libcurl.so.X: open failed: No such file or directory
96  5.9 How does libcurl resolve host names?
97  5.10 How do I prevent libcurl from writing the response to stdout?
98  5.11 How do I make libcurl not receive the whole HTTP response?
99  5.12 Can I make libcurl fake or hide my real IP address?
100  5.13 How do I stop an ongoing transfer?
101  5.14 Using C++ non-static functions for callbacks?
102  5.15 How do I get an FTP directory listing?
103  5.16 I want a different time-out!
104  5.17 Can I write a server with libcurl?
105  5.18 Does libcurl use threads?
106
107 6. License Issues
108  6.1 I have a GPL program, can I use the libcurl library?
109  6.2 I have a closed-source program, can I use the libcurl library?
110  6.3 I have a BSD licensed program, can I use the libcurl library?
111  6.4 I have a program that uses LGPL libraries, can I use libcurl?
112  6.5 Can I modify curl/libcurl for my program and keep the changes secret?
113  6.6 Can you please change the curl/libcurl license to XXXX?
114  6.7 What are my obligations when using libcurl in my commercial apps?
115
116 7. PHP/CURL Issues
117  7.1 What is PHP/CURL?
118  7.2 Who wrote PHP/CURL?
119  7.3 Can I perform multiple requests using the same handle?
120
121==============================================================================
122
1231. Philosophy
124
125  1.1 What is cURL?
126
127  cURL is the name of the project. The name is a play on 'Client for URLs',
128  originally with URL spelled in uppercase to make it obvious it deals with
129  URLs. The fact it can also be pronounced 'see URL' also helped, it works as
130  an abbreviation for "Client URL Request Library" or why not the recursive
131  version: "Curl URL Request Library".
132
133  The cURL project produces two products:
134
135  libcurl
136
137    A free and easy-to-use client-side URL transfer library, supporting DICT,
138    FILE, FTP, FTPS, GOPHER, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, POP3,
139    POP3S, RTMP, RTSP, SCP, SFTP, SMB, SMBS, SMTP, SMTPS, TELNET and TFTP.
140
141    libcurl supports HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading,
142    Kerberos, SPNEGO, HTTP form based upload, proxies, cookies, user+password
143    authentication, file transfer resume, http proxy tunneling and more!
144
145    libcurl is highly portable, it builds and works identically on numerous
146    platforms, including Solaris, NetBSD, FreeBSD, OpenBSD, Darwin, HP-UX,
147    IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, BeOS, Mac
148    OS X, Ultrix, QNX, OpenVMS, RISC OS, Novell NetWare, DOS, Symbian, OSF,
149    Android, Minix, IBM TPF and more...
150
151    libcurl is free, thread-safe, IPv6 compatible, feature rich, well
152    supported and fast.
153
154  curl
155
156    A command line tool for getting or sending files using URL syntax.
157
158    Since curl uses libcurl, curl supports the same wide range of common
159    Internet protocols that libcurl does.
160
161  We pronounce curl with an initial k sound. It rhymes with words like girl
162  and earl. This is a short WAV file to help you:
163
164     http://media.merriam-webster.com/soundc11/c/curl0001.wav
165
166  There are numerous sub-projects and related projects that also use the word
167  curl in the project names in various combinations, but you should take
168  notice that this FAQ is directed at the command-line tool named curl (and
169  libcurl the library), and may therefore not be valid for other curl-related
170  projects. (There is however a small section for the PHP/CURL in this FAQ.)
171
172  1.2 What is libcurl?
173
174  libcurl is a reliable and portable library which provides you with an easy
175  interface to a range of common Internet protocols.
176
177  You can use libcurl for free in your application, be it open source,
178  commercial or closed-source.
179
180  libcurl is most probably the most portable, most powerful and most often
181  used C-based multi-platform file transfer library on this planet - be it
182  open source or commercial.
183
184  1.3 What is curl not?
185
186  Curl is not a wget clone. That is a common misconception.  Never, during
187  curl's development, have we intended curl to replace wget or compete on its
188  market. Curl is targeted at single-shot file transfers.
189
190  Curl is not a web site mirroring program. If you want to use curl to mirror
191  something: fine, go ahead and write a script that wraps around curl to make
192  it reality (like curlmirror.pl does).
193
194  Curl is not an FTP site mirroring program. Sure, get and send FTP with curl
195  but if you want systematic and sequential behavior you should write a
196  script (or write a new program that interfaces libcurl) and do it.
197
198  Curl is not a PHP tool, even though it works perfectly well when used from
199  or with PHP (when using the PHP/CURL module).
200
201  Curl is not a program for a single operating system. Curl exists, compiles,
202  builds and runs under a wide range of operating systems, including all
203  modern Unixes (and a bunch of older ones too), Windows, Amiga, BeOS, OS/2,
204  OS X, QNX etc.
205
206  1.4 When will you make curl do XXXX ?
207
208  We love suggestions of what to change in order to make curl and libcurl
209  better. We do however believe in a few rules when it comes to the future of
210  curl:
211
212  Curl -- the command line tool -- is to remain a non-graphical command line
213  tool. If you want GUIs or fancy scripting capabilities, you should look for
214  another tool that uses libcurl.
215
216  We do not add things to curl that other small and available tools already do
217  very fine at the side. Curl's output is fine to pipe into another program or
218  redirect to another file for the next program to interpret.
219
220  We focus on protocol related issues and improvements. If you wanna do more
221  magic with the supported protocols than curl currently does, chances are big
222  we will agree. If you wanna add more protocols, we may very well agree.
223
224  If you want someone else to make all the work while you wait for us to
225  implement it for you, that is not a very friendly attitude. We spend a
226  considerable time already on maintaining and developing curl. In order to
227  get more out of us, you should consider trading in some of your time and
228  efforts in return.
229
230  If you write the code, chances are bigger that it will get into curl faster.
231
232  1.5 Who makes curl?
233
234  curl and libcurl are not made by any single individual. Daniel Stenberg is
235  project leader and main developer, but other persons' submissions are
236  important and crucial. Anyone can contribute and post their changes and
237  improvements and have them inserted in the main sources (of course on the
238  condition that developers agree on that the fixes are good).
239
240  The full list of all contributors is found in the docs/THANKS file.
241
242  curl is developed by a community, with Daniel at the wheel.
243
244  1.6 What do you get for making curl?
245
246  Project cURL is entirely free and open. No person gets paid for developing
247  curl on full time. We do this voluntarily, mostly on spare time.
248  Occasionally companies pay individual developers to work on curl, but that's
249  up to each company and developer. It is not controlled by nor supervised in
250  any way by the project.
251
252  We still get help from companies. Haxx provides web site, bandwidth, mailing
253  lists etc, sourceforge.net hosts project services we take advantage from,
254  like the bug tracker and github hosts the primary git repository. Also
255  again, some companies have sponsored certain parts of the development in the
256  past and I hope some will continue to do so in the future.
257
258  If you want to support our project, consider a donation or a banner-program
259  or even better: by helping us coding, documenting, testing etc.
260
261  1.7 What about CURL from curl.com?
262
263  During the summer 2001, curl.com was busy advertising their client-side
264  programming language for the web, named CURL.
265
266  We are in no way associated with curl.com or their CURL programming
267  language.
268
269  Our project name curl has been in effective use since 1998. We were not the
270  first computer related project to use the name "curl" and do not claim any
271  rights to the name.
272
273  We recognize that we will be living in parallel with curl.com and wish them
274  every success.
275
276  1.8 I have a problem who do I mail?
277
278  Please do not mail any single individual unless you really need to. Keep
279  curl-related questions on a suitable mailing list. All available mailing
280  lists are listed in the MANUAL document and online at
281  http://curl.haxx.se/mail/
282
283  Keeping curl-related questions and discussions on mailing lists allows
284  others to join in and help, to share their ideas, contribute their
285  suggestions and spread their wisdom. Keeping discussions on public mailing
286  lists also allows for others to learn from this (both current and future
287  users thanks to the web based archives of the mailing lists), thus saving us
288  from having to repeat ourselves even more. Thanks for respecting this.
289
290  If you have found or simply suspect a security problem in curl or libcurl,
291  mail curl-security at haxx.se (closed list of receivers, mails are not
292  disclosed) and tell. Then we can produce a fix in a timely manner before the
293  flaw is announced to the world, thus lessen the impact the problem will have
294  on existing users.
295
296  1.9 Where do I buy commercial support for curl?
297
298  curl is fully open source. It means you can hire any skilled engineer to fix
299  your curl-related problems.
300
301  We list available alternatives on the curl web site:
302  http://curl.haxx.se/support.html
303
304  1.10 How many are using curl?
305
306  It is impossible to tell.
307
308  We don't know how many users that knowingly have installed and use curl.
309
310  We don't know how many users that use curl without knowing that they are in
311  fact using it.
312
313  We don't know how many users that downloaded or installed curl and then
314  never use it.
315
316  In May 2012 Daniel did a counting game and came up with a number that may
317  be completely wrong or somewhat accurate. Over 500 million!
318
319  See http://daniel.haxx.se/blog/2012/05/16/300m-users/
320
321  1.11 Why don't you update ca-bundle.crt
322
323  The ca cert bundle that used to shipped with curl was very outdated and must
324  be replaced with an up-to-date version by anyone who wants to verify
325  peers. It is no longer provided by curl. The last curl release ever that
326  shipped a ca cert bundle was curl 7.18.0.
327
328  In the cURL project we've decided not to attempt to keep this file updated
329  (or even present anymore) since deciding what to add to a ca cert bundle is
330  an undertaking we've not been ready to accept, and the one we can get from
331  Mozilla is perfectly fine so there's no need to duplicate that work.
332
333  Today, with many services performed over HTTPS, every operating system
334  should come with a default ca cert bundle that can be deemed somewhat
335  trustworthy and that collection (if reasonably updated) should be deemed to
336  be a lot better than a private curl version.
337
338  If you want the most recent collection of ca certs that Mozilla Firefox
339  uses, we recommend that you extract the collection yourself from Mozilla
340  Firefox (by running 'make ca-bundle), or by using our online service setup
341  for this purpose: http://curl.haxx.se/docs/caextract.html
342
343  1.12 I have a problem who can I chat with?
344
345  There's a bunch of friendly people hanging out in the #curl channel on the
346  IRC network irc.freenode.net. If you're polite and nice, chances are big
347  that you can get -- or provide -- help instantly.
348
349  1.13 curl's ECCN number?
350
351  The US government restricts exports of software that contains or uses
352  cryptography. When doing so, the Export Control Classification Number (ECCN)
353  is used to identify the level of export control etc.
354
355  ASF gives a good explanation at https://www.apache.org/dev/crypto.html
356
357  We believe curl's number might be ECCN 5D002, another possibility is
358  5D992. It seems necessary to write them, asking to confirm.
359
360  Comprehensible explanations of the meaning of such numbers and how to
361  obtain them (resp.) are here
362
363  http://www.bis.doc.gov/licensing/exportingbasics.htm
364  http://www.bis.doc.gov/licensing/do_i_needaneccn.html
365
366  An incomprehensible description of the two numbers above is here
367  http://www.access.gpo.gov/bis/ear/pdf/ccl5-pt2.pdf
368
369  1.14 How do I submit my patch?
370
371  When you have made a patch or a change of whatever sort, and want to submit
372  that to the project, there are a few different ways we prefer:
373
374  o send a patch to the curl-library mailing list. We're many subscribers
375    there and there are lots of people who can review patches, comment on them
376    and "receive" them properly.
377
378  o if your patch changes or fixes a bug, you can also opt to submit a bug
379    report in the bug tracker and attach your patch there. There are less
380    people involved there.
381
382  Lots of more details are found in the CONTRIBUTE and INTERNALS docs.
383
384  1.15 How do I port libcurl to my OS?
385
386  Here's a rough step-by-step:
387
388  1. copy a suitable lib/config-*.h file as a start to lib/config-[youros].h
389
390  2. edit lib/config-[youros].h to match your OS and setup
391
392  3. edit lib/curl_setup.h to include config-[youros].h when your OS is
393     detected by the preprocessor, in the style others already exist
394
395  4. compile lib/*.c and make them into a library
396
397
3982. Install Related Problems
399
400  2.1 configure doesn't find OpenSSL even when it is installed
401
402  This may be because of several reasons.
403
404    2.1.1 native linker doesn't find openssl
405
406    Affected platforms:
407      Solaris (native cc compiler)
408      HPUX (native cc compiler)
409      SGI IRIX (native cc compiler)
410      SCO UNIX (native cc compiler)
411
412    When configuring curl, I specify --with-ssl. OpenSSL is installed in
413    /usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
414    CRYPTO_lock in -lcrypto
415
416    Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
417    -lcrypto, so ld can't find the library. This is due to a bug in the GNU
418    autoconf tool.
419
420    Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
421    ./configure places the -L/usr/local/ssl/lib early enough in the command
422    line to make things work
423
424    2.1.2 only the libssl lib is missing
425
426    If all include files and the libcrypto lib is present, with only the
427    libssl being missing according to configure, this is mostly likely because
428    a few functions are left out from the libssl.
429
430    If the function names missing include RSA or RSAREF you can be certain
431    that this is because libssl requires the RSA and RSAREF libs to build.
432
433    See the INSTALL file section that explains how to add those libs to
434    configure. Make sure that you remove the config.cache file before you
435    rerun configure with the new flags.
436
437  2.2 Does curl work/build with other SSL libraries?
438
439  Curl has been written to use a generic SSL function layer internally, and
440  that SSL functionality can then be provided by one out of many different SSL
441  backends.
442
443  curl can be built to use one of the following SSL alternatives: OpenSSL,
444  GnuTLS, yassl, NSS, PolarSSL, axTLS, Secure Transport (native iOS/OS X),
445  WinSSL (native Windows) or GSKit (native IBM i). They all have their pros
446  and cons, and we try to maintain a comparison of them here:
447  http://curl.haxx.se/docs/ssl-compared.html
448
449  2.3 Where can I find a copy of LIBEAY32.DLL?
450
451  That is an OpenSSL binary built for Windows.
452
453  Curl can be built with OpenSSL to do the SSL stuff. The LIBEAY32.DLL is then
454  what curl needs on a windows machine to do https:// etc. Check out the curl
455  web site to find accurate and up-to-date pointers to recent OpenSSL DLLs and
456  other binary packages.
457
458  2.4 Does curl support SOCKS (RFC 1928) ?
459
460  Yes, SOCKS 4 and 5 are supported.
461
462
4633. Usage problems
464
465  3.1 curl: (1) SSL is disabled, https: not supported
466
467  If you get this output when trying to get anything from a https:// server,
468  it means that the instance of curl/libcurl that you're using was built
469  without support for this protocol.
470
471  This could've happened if the configure script that was run at build time
472  couldn't find all libs and include files curl requires for SSL to work. If
473  the configure script fails to find them, curl is simply built without SSL
474  support.
475
476  To get the https:// support into a curl that was previously built but that
477  reports that https:// is not supported, you should dig through the document
478  and logs and check out why the configure script doesn't find the SSL libs
479  and/or include files.
480
481  Also, check out the other paragraph in this FAQ labelled "configure doesn't
482  find OpenSSL even when it is installed".
483
484  3.2 How do I tell curl to resume a transfer?
485
486  Curl supports resumed transfers both ways on both FTP and HTTP.
487  Try the -C option.
488
489  3.3 Why doesn't my posting using -F work?
490
491  You can't simply use -F or -d at your choice. The web server that will
492  receive your post expects one of the formats. If the form you're trying to
493  submit uses the type 'multipart/form-data', then and only then you must use
494  the -F type. In all the most common cases, you should use -d which then
495  causes a posting with the type 'application/x-www-form-urlencoded'.
496
497  This is described in some detail in the MANUAL and TheArtOfHttpScripting
498  documents, and if you don't understand it the first time, read it again
499  before you post questions about this to the mailing list. Also, try reading
500  through the mailing list archives for old postings and questions regarding
501  this.
502
503  3.4 How do I tell curl to run custom FTP commands?
504
505  You can tell curl to perform optional commands both before and/or after a
506  file transfer. Study the -Q/--quote option.
507
508  Since curl is used for file transfers, you don't normally use curl to
509  perform FTP commands without transferring anything. Therefore you must
510  always specify a URL to transfer to/from even when doing custom FTP
511  commands, or use -I which implies the "no body" option sent to libcurl.
512
513  3.5 How can I disable the Accept: */* header?
514
515  You can change all internally generated headers by adding a replacement with
516  the -H/--header option. By adding a header with empty contents you safely
517  disable that one. Use -H "Accept:" to disable that specific header.
518
519  3.6 Does curl support ASP, XML, XHTML or HTML version Y?
520
521  To curl, all contents are alike. It doesn't matter how the page was
522  generated. It may be ASP, PHP, Perl, shell-script, SSI or plain HTML
523  files. There's no difference to curl and it doesn't even know what kind of
524  language that generated the page.
525
526  See also item 3.14 regarding javascript.
527
528  3.7 Can I use curl to delete/rename a file through FTP?
529
530  Yes. You specify custom FTP commands with -Q/--quote.
531
532  One example would be to delete a file after you have downloaded it:
533
534     curl -O ftp://download.com/coolfile -Q '-DELE coolfile'
535
536  or rename a file after upload:
537
538     curl -T infile ftp://upload.com/dir/ -Q "-RNFR infile" -Q "-RNTO newname"
539
540  3.8 How do I tell curl to follow HTTP redirects?
541
542  Curl does not follow so-called redirects by default. The Location: header
543  that informs the client about this is only interpreted if you're using the
544  -L/--location option. As in:
545
546     curl -L http://redirector.com
547
548  Not all redirects are HTTP ones, see 4.14
549
550  3.9 How do I use curl in my favorite programming language?
551
552  There exist many language interfaces/bindings for curl that integrates it
553  better with various languages. If you are fluid in a script language, you
554  may very well opt to use such an interface instead of using the command line
555  tool.
556
557  Find out more about which languages that support curl directly, and how to
558  install and use them, in the libcurl section of the curl web site:
559  http://curl.haxx.se/libcurl/
560
561  All the various bindings to libcurl are made by other projects and people,
562  outside of the cURL project. The cURL project itself only produces libcurl
563  with its plain C API. If you don't find anywhere else to ask you can ask
564  about bindings on the curl-library list too, but be prepared that people on
565  that list may not know anything about bindings.
566
567  In October 2009, there were interfaces available for the following
568  languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Dylan, Eiffel, Euphoria,
569  Ferite, Gambas, glib/GTK+, Haskell, ILE/RPG, Java, Lisp, Lua, Mono, .NET,
570  Object-Pascal, O'Caml, Pascal, Perl, PHP, PostgreSQL, Python, R, Rexx, Ruby,
571  Scheme, S-Lang, Smalltalk, SP-Forth, SPL, Tcl, Visual Basic, Visual FoxPro,
572  Q, wxwidgets and XBLite. By the time you read this, additional ones may have
573  appeared!
574
575  3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
576
577  Curl adheres to the HTTP spec, which basically means you can play with *any*
578  protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and
579  XML-RPC are all such ones. You can use -X to set custom requests and -H to
580  set custom headers (or replace internally generated ones).
581
582  Using libcurl is of course just as fine and you'd just use the proper
583  library options to do the same.
584
585  3.11 How do I POST with a different Content-Type?
586
587  You can always replace the internally generated headers with -H/--header.
588  To make a simple HTTP POST with text/xml as content-type, do something like:
589
590        curl -d "datatopost" -H "Content-Type: text/xml" [URL]
591
592  3.12 Why do FTP specific features over HTTP proxy fail?
593
594  Because when you use a HTTP proxy, the protocol spoken on the network will
595  be HTTP, even if you specify a FTP URL. This effectively means that you
596  normally can't use FTP specific features such as FTP upload and FTP quote
597  etc.
598
599  There is one exception to this rule, and that is if you can "tunnel through"
600  the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
601  and is generally not available as proxy admins usually disable tunneling to
602  other ports than 443 (which is used for HTTPS access through proxies).
603
604  3.13 Why does my single/double quotes fail?
605
606  To specify a command line option that includes spaces, you might need to
607  put the entire option within quotes. Like in:
608
609   curl -d " with spaces " url.com
610
611  or perhaps
612
613   curl -d ' with spaces ' url.com
614
615  Exactly what kind of quotes and how to do this is entirely up to the shell
616  or command line interpreter that you are using. For most unix shells, you
617  can more or less pick either single (') or double (") quotes. For
618  Windows/DOS prompts I believe you're forced to use double (") quotes.
619
620  Please study the documentation for your particular environment. Examples in
621  the curl docs will use a mix of both these ones as shown above. You must
622  adjust them to work in your environment.
623
624  Remember that curl works and runs on more operating systems than most single
625  individuals have ever tried.
626
627  3.14 Does curl support Javascript or PAC (automated proxy config)?
628
629  Many web pages do magic stuff using embedded Javascript. Curl and libcurl
630  have no built-in support for that, so it will be treated just like any other
631  contents.
632
633  .pac files are a netscape invention and are sometimes used by organizations
634  to allow them to differentiate which proxies to use. The .pac contents is
635  just a Javascript program that gets invoked by the browser and that returns
636  the name of the proxy to connect to. Since curl doesn't support Javascript,
637  it can't support .pac proxy configuration either.
638
639  Some workarounds usually suggested to overcome this Javascript dependency:
640
641  Depending on the Javascript complexity, write up a script that translates it
642  to another language and execute that.
643
644  Read the Javascript code and rewrite the same logic in another language.
645
646  Implement a Javascript interpreter, people have successfully used the
647  Mozilla Javascript engine in the past.
648
649  Ask your admins to stop this, for a static proxy setup or similar.
650
651  3.15 Can I do recursive fetches with curl?
652
653  No. curl itself has no code that performs recursive operations, such as
654  those performed by wget and similar tools.
655
656  There exist wrapper scripts with that functionality (for example the
657  curlmirror perl script), and you can write programs based on libcurl to do
658  it, but the command line tool curl itself cannot.
659
660  3.16 What certificates do I need when I use SSL?
661
662  There are three different kinds of "certificates" to keep track of when we
663  talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.
664
665  CLIENT CERTIFICATE
666
667  The server you communicate may require that you can provide this in order to
668  prove that you actually are who you claim to be.  If the server doesn't
669  require this, you don't need a client certificate.
670
671  A client certificate is always used together with a private key, and the
672  private key has a pass phrase that protects it.
673
674  SERVER CERTIFICATE
675
676  The server you communicate with has a server certificate. You can and should
677  verify this certificate to make sure that you are truly talking to the real
678  server and not a server impersonating it.
679
680  CERTIFICATE AUTHORITY CERTIFICATE ("CA cert")
681
682  You often have several CA certs in a CA cert bundle that can be used to
683  verify a server certificate that was signed by one of the authorities in the
684  bundle. curl does not come with a CA cert bundle but most curl installs
685  provide one. You can also override the default.
686
687  The server certificate verification process is made by using a Certificate
688  Authority certificate ("CA cert") that was used to sign the server
689  certificate. Server certificate verification is enabled by default in curl
690  and libcurl and is often the reason for problems as explained in FAQ entry
691  4.12 and the SSLCERTS document
692  (http://curl.haxx.se/docs/sslcerts.html). Server certificates that are
693  "self-signed" or otherwise signed by a CA that you do not have a CA cert
694  for, cannot be verified. If the verification during a connect fails, you are
695  refused access. You then need to explicitly disable the verification to
696  connect to the server.
697
698  3.17 How do I list the root dir of an FTP server?
699
700  There are two ways. The way defined in the RFC is to use an encoded slash
701  in the first path part. List the "/tmp" dir like this:
702
703     curl ftp://ftp.sunet.se/%2ftmp/
704
705  or the not-quite-kosher-but-more-readable way, by simply starting the path
706  section of the URL with a slash:
707
708     curl ftp://ftp.sunet.se//tmp/
709
710  3.18 Can I use curl to send a POST/PUT and not wait for a response?
711
712  No.
713
714  But you could easily write your own program using libcurl to do such stunts.
715
716  3.19 How do I get HTTP from a host using a specific IP address?
717
718  For example, you may be trying out a web site installation that isn't yet in
719  the DNS. Or you have a site using multiple IP addresses for a given host
720  name and you want to address a specific one out of the set.
721
722  Set a custom Host: header that identifies the server name you want to reach
723  but use the target IP address in the URL:
724
725    curl --header "Host: www.example.com" http://127.0.0.1/
726
727  You can also opt to add faked host name entries to curl with the --resolve
728  option. That has the added benefit that things like redirects will also work
729  properly. The above operation would instead be done as:
730
731    curl --resolve www.example.com:80:127.0.0.1 http://www.example.com/
732
733  3.20 How to SFTP from my user's home directory?
734
735  Contrary to how FTP works, SFTP and SCP URLs specify the exact directory to
736  work with. It means that if you don't specify that you want the user's home
737  directory, you get the actual root directory.
738
739  To specify a file in your user's home directory, you need to use the correct
740  URL syntax which for sftp might look similar to:
741
742    curl -O -u user:password sftp://example.com/~/file.txt
743
744  and for SCP it is just a different protocol prefix:
745
746    curl -O -u user:password scp://example.com/~/file.txt
747
748  3.21 Protocol xxx not supported or disabled in libcurl
749
750  When passing on a URL to curl to use, it may respond that the particular
751  protocol is not supported or disabled. The particular way this error message
752  is phrased is because curl doesn't make a distinction internally of whether
753  a particular protocol is not supported (i.e. never got any code added that
754  knows how to speak that protocol) or if it was explicitly disabled. curl can
755  be built to only support a given set of protocols, and the rest would then
756  be disabled or not supported.
757
758  Note that this error will also occur if you pass a wrongly spelled protocol
759  part as in "htpt://example.com" or as in the less evident case if you prefix
760  the protocol part with a space as in " http://example.com/".
761
762  3.22 curl -X gives me HTTP problems
763
764  In normal circumstances, -X should hardly ever be used.
765
766  By default you use curl without explicitly saying which request method to
767  use when the URL identifies a HTTP transfer. If you just pass in a URL like
768  "curl http://example.com" it will use GET. If you use -d or -F curl will use
769  POST, -I will cause a HEAD and -T will make it a PUT.
770
771  If for whatever reason you're not happy with these default choices that curl
772  does for you, you can override those request methods by specifying -X
773  [WHATEVER]. This way you can for example send a DELETE by doing "curl -X
774  DELETE [URL]".
775
776  It is thus pointless to do "curl -XGET [URL]" as GET would be used
777  anyway. In the same vein it is pointless to do "curl -X POST -d data
778  [URL]"... But you can make a fun and somewhat rare request that sends a
779  request-body in a GET request with something like "curl -X GET -d data
780  [URL]"
781
782  Note that -X doesn't actually change curl's behavior as it only modifies the
783  actual string sent in the request, but that may of course trigger a
784  different set of events.
785
786  Accordingly, by using -XPOST on a command line that for example would follow
787  a 303 redirect, you will effectively prevent curl from behaving
788  correctly. Be aware.
789
790
7914. Running Problems
792
793  4.1 Problems connecting to SSL servers.
794
795  It took a very long time before we could sort out why curl had problems to
796  connect to certain SSL servers when using SSLeay or OpenSSL v0.9+.  The
797  error sometimes showed up similar to:
798
799  16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:
800
801  It turned out to be because many older SSL servers don't deal with SSLv3
802  requests properly. To correct this problem, tell curl to select SSLv2 from
803  the command line (-2/--sslv2).
804
805  There have also been examples where the remote server didn't like the SSLv2
806  request and instead you had to force curl to use SSLv3 with -3/--sslv3.
807
808  4.2 Why do I get problems when I use & or % in the URL?
809
810  In general unix shells, the & symbol is treated specially and when used, it
811  runs the specified command in the background. To safely send the & as a part
812  of a URL, you should quote the entire URL by using single (') or double (")
813  quotes around it. Similar problems can also occur on some shells with other
814  characters, including ?*!$~(){}<>\|;`.  When in doubt, quote the URL.
815
816  An example that would invoke a remote CGI that uses &-symbols could be:
817
818     curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
819
820  In Windows, the standard DOS shell treats the percent sign specially and you
821  need to use TWO percent signs for each single one you want to use in the
822  URL.
823
824  If you want a literal percent sign to be part of the data you pass in a POST
825  using -d/--data you must encode it as '%25' (which then also needs the
826  percent sign doubled on Windows machines).
827
828  4.3 How can I use {, }, [ or ] to specify multiple URLs?
829
830  Because those letters have a special meaning to the shell, and to be used in
831  a URL specified to curl you must quote them.
832
833  An example that downloads two URLs (sequentially) would do:
834
835    curl '{curl,www}.haxx.se'
836
837  To be able to use those letters as actual parts of the URL (without using
838  them for the curl URL "globbing" system), use the -g/--globoff option:
839
840    curl -g 'www.site.com/weirdname[].html'
841
842  4.4 Why do I get downloaded data even though the web page doesn't exist?
843
844  Curl asks remote servers for the page you specify. If the page doesn't exist
845  at the server, the HTTP protocol defines how the server should respond and
846  that means that headers and a "page" will be returned. That's simply how
847  HTTP works.
848
849  By using the --fail option you can tell curl explicitly to not get any data
850  if the HTTP return code doesn't say success.
851
852  4.5 Why do I get return code XXX from a HTTP server?
853
854  RFC2616 clearly explains the return codes. This is a short transcript. Go
855  read the RFC for exact details:
856
857    4.5.1 "400 Bad Request"
858
859    The request could not be understood by the server due to malformed
860    syntax. The client SHOULD NOT repeat the request without modifications.
861
862    4.5.2 "401 Unauthorized"
863
864    The request requires user authentication.
865
866    4.5.3 "403 Forbidden"
867
868    The server understood the request, but is refusing to fulfil it.
869    Authorization will not help and the request SHOULD NOT be repeated.
870
871    4.5.4 "404 Not Found"
872
873    The server has not found anything matching the Request-URI. No indication
874    is given of whether the condition is temporary or permanent.
875
876    4.5.5 "405 Method Not Allowed"
877
878    The method specified in the Request-Line is not allowed for the resource
879    identified by the Request-URI. The response MUST include an Allow header
880    containing a list of valid methods for the requested resource.
881
882    4.5.6 "301 Moved Permanently"
883
884    If you get this return code and an HTML output similar to this:
885
886       <H1>Moved Permanently</H1> The document has moved <A
887       HREF="http://same_url_now_with_a_trailing_slash/">here</A>.
888
889    it might be because you request a directory URL but without the trailing
890    slash. Try the same operation again _with_ the trailing URL, or use the
891    -L/--location option to follow the redirection.
892
893  4.6 Can you tell me what error code 142 means?
894
895  All curl error codes are described at the end of the man page, in the
896  section called "EXIT CODES".
897
898  Error codes that are larger than the highest documented error code means
899  that curl has exited due to a crash. This is a serious error, and we
900  appreciate a detailed bug report from you that describes how we could go
901  ahead and repeat this!
902
903  4.7 How do I keep user names and passwords secret in Curl command lines?
904
905  This problem has two sides:
906
907  The first part is to avoid having clear-text passwords in the command line
908  so that they don't appear in 'ps' outputs and similar. That is easily
909  avoided by using the "-K" option to tell curl to read parameters from a file
910  or stdin to which you can pass the secret info. curl itself will also
911  attempt to "hide" the given password by blanking out the option - this
912  doesn't work on all platforms.
913
914  To keep the passwords in your account secret from the rest of the world is
915  not a task that curl addresses. You could of course encrypt them somehow to
916  at least hide them from being read by human eyes, but that is not what
917  anyone would call security.
918
919  Also note that regular HTTP (using Basic authentication) and FTP passwords
920  are sent in clear across the network. All it takes for anyone to fetch them
921  is to listen on the network.  Eavesdropping is very easy. Use more secure
922  authentication methods (like Digest, Negotiate or even NTLM) or consider the
923  SSL-based alternatives HTTPS and FTPS.
924
925  4.8 I found a bug!
926
927  It is not a bug if the behavior is documented. Read the docs first.
928  Especially check out the KNOWN_BUGS file, it may be a documented bug!
929
930  If it is a problem with a binary you've downloaded or a package for your
931  particular platform, try contacting the person who built the package/archive
932  you have.
933
934  If there is a bug, read the BUGS document first. Then report it as described
935  in there.
936
937  4.9 Curl can't authenticate to the server that requires NTLM?
938
939  NTLM support requires OpenSSL, GnuTLS, NSS, Secure Transport, or Microsoft
940  Windows libraries at build-time to provide this functionality.
941
942  NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You
943  should not use such ones.
944
945  4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
946
947  Many web servers allow or demand that the administrator configures the
948  server properly for these requests to work on the web server.
949
950  Some servers seem to support HEAD only on certain kinds of URLs.
951
952  To fully grasp this, try the documentation for the particular server
953  software you're trying to interact with. This is not anything curl can do
954  anything about.
955
956  4.11 Why does my HTTP range requests return the full document?
957
958  Because the range may not be supported by the server, or the server may
959  choose to ignore it and return the full document anyway.
960
961  4.12 Why do I get "certificate verify failed" ?
962
963  You invoke curl 7.10 or later to communicate on a https:// URL and get an
964  error back looking something similar to this:
965
966      curl: (35) SSL: error:14090086:SSL routines:
967      SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
968
969  Then it means that curl couldn't verify that the server's certificate was
970  good. Curl verifies the certificate using the CA cert bundle that comes with
971  the curl installation.
972
973  To disable the verification (which makes it act like curl did before 7.10),
974  use -k. This does however enable man-in-the-middle attacks.
975
976  If you get this failure but are having a CA cert bundle installed and used,
977  the server's certificate is not signed by one of the CA's in the bundle. It
978  might for example be self-signed. You then correct this problem by obtaining
979  a valid CA cert for the server. Or again, decrease the security by disabling
980  this check.
981
982  Details are also in the SSLCERTS file in the release archives, found online
983  here: http://curl.haxx.se/docs/sslcerts.html
984
985  4.13 Why is curl -R on Windows one hour off?
986
987  During daylight savings time, when -R is used, curl will set a time that
988  appears one hour off. This happens due to a flaw in how Windows stores and
989  uses file modification times and it is not easily worked around. For details
990  on this problem, read this: http://www.codeproject.com/datetime/dstbugs.asp
991
992  4.14 Redirects work in browser but not with curl!
993
994  curl supports HTTP redirects fine (see item 3.8). Browsers generally support
995  at least two other ways to perform redirects that curl does not:
996
997  Meta tags. You can write a HTML tag that will cause the browser to redirect
998  to another given URL after a certain time.
999
1000  Javascript. You can write a Javascript program embedded in a HTML page that
1001  redirects the browser to another given URL.
1002
1003  There is no way to make curl follow these redirects. You must either
1004  manually figure out what the page is set to do, or you write a script that
1005  parses the results and fetches the new URL.
1006
1007  4.15 FTPS doesn't work
1008
1009  curl supports FTPS (sometimes known as FTP-SSL) both implicit and explicit
1010  mode.
1011
1012  When a URL is used that starts with FTPS://, curl assumes implicit SSL on
1013  the control connection and will therefore immediately connect and try to
1014  speak SSL. FTPS:// connections default to port 990.
1015
1016  To use explicit FTPS, you use a FTP:// URL and the --ftp-ssl option (or one
1017  of its related flavours). This is the most common method, and the one
1018  mandated by RFC4217. This kind of connection then of course uses the
1019  standard FTP port 21 by default.
1020
1021  4.16 My HTTP POST or PUT requests are slow!
1022
1023  libcurl makes all POST and PUT requests (except for POST requests with a
1024  very tiny request body) use the "Expect: 100-continue" header. This header
1025  allows the server to deny the operation early so that libcurl can bail out
1026  already before having to send any data. This is useful in authentication
1027  cases and others.
1028
1029  However, many servers don't implement the Expect: stuff properly and if the
1030  server doesn't respond (positively) within 1 second libcurl will continue
1031  and send off the data anyway.
1032
1033  You can disable libcurl's use of the Expect: header the same way you disable
1034  any header, using -H / CURLOPT_HTTPHEADER, or by forcing it to use HTTP 1.0.
1035
1036  4.17 Non-functional connect timeouts
1037
1038  In most Windows setups having a timeout longer than 21 seconds make no
1039  difference, as it will only send 3 TCP SYN packets and no more. The second
1040  packet sent three seconds after the first and the third six seconds after
1041  the second.  No more than three packets are sent, no matter how long the
1042  timeout is set.
1043
1044  See option TcpMaxConnectRetransmissions on this page:
1045  https://support.microsoft.com/en-us/kb/175523/en-us
1046
1047  Also, even on non-Windows systems there may run a firewall or anti-virus
1048  software or similar that accepts the connection but does not actually do
1049  anything else. This will make (lib)curl to consider the connection connected
1050  and thus the connect timeout won't trigger.
1051
1052  4.18 file:// URLs containing drive letters (Windows, NetWare)
1053
1054  When using cURL to try to download a local file, one might use a URL
1055  in this format:
1056
1057  file://D:/blah.txt
1058
1059  You'll find that even if D:\blah.txt does exist, cURL returns a 'file
1060  not found' error.
1061
1062  According to RFC 1738 (https://www.ietf.org/rfc/rfc1738.txt),
1063  file:// URLs must contain a host component, but it is ignored by
1064  most implementations. In the above example, 'D:' is treated as the
1065  host component, and is taken away. Thus, cURL tries to open '/blah.txt'.
1066  If your system is installed to drive C:, that will resolve to 'C:\blah.txt',
1067  and if that doesn't exist you will get the not found error.
1068
1069  To fix this problem, use file:// URLs with *three* leading slashes:
1070
1071  file:///D:/blah.txt
1072
1073  Alternatively, if it makes more sense, specify 'localhost' as the host
1074  component:
1075
1076  file://localhost/D:/blah.txt
1077
1078  In either case, cURL should now be looking for the correct file.
1079
1080  4.19 Why doesn't cURL return an error when the network cable is unplugged?
1081
1082  Unplugging a cable is not an error situation. The TCP/IP protocol stack
1083  was designed to be fault tolerant, so even though there may be a physical
1084  break somewhere the connection shouldn't be affected, just possibly
1085  delayed.  Eventually, the physical break will be fixed or the data will be
1086  re-routed around the physical problem through another path.
1087
1088  In such cases, the TCP/IP stack is responsible for detecting when the
1089  network connection is irrevocably lost. Since with some protocols it is
1090  perfectly legal for the client wait indefinitely for data, the stack may
1091  never report a problem, and even when it does, it can take up to 20 minutes
1092  for it to detect an issue.  The curl option --keepalive-time enables
1093  keep-alive support in the TCP/IP stack which makes it periodically probe the
1094  connection to make sure it is still available to send data. That should
1095  reliably detect any TCP/IP network failure.
1096
1097  But even that won't detect the network going down before the TCP/IP
1098  connection is established (e.g. during a DNS lookup) or using protocols that
1099  don't use TCP.  To handle those situations, curl offers a number of timeouts
1100  on its own. --speed-limit/--speed-time will abort if the data transfer rate
1101  falls too low, and --connect-timeout and --max-time can be used to put an
1102  overall timeout on the connection phase or the entire transfer.
1103
1104  A libcurl-using application running in a known physical environment (e.g.
1105  an embedded device with only a single network connection) may want to act
1106  immediately if its lone network connection goes down.  That can be achieved
1107  by having the application monitor the network connection on its own using an
1108  OS-specific mechanism, then signalling libcurl to abort (see also item 5.13).
1109
1110  4.20 curl doesn't return error for HTTP non-200 responses!
1111
1112  Correct. Unless you use -f (--fail).
1113
1114  When doing HTTP transfers, curl will perform exactly what you're asking it
1115  to do and if successful it will not return an error. You can use curl to
1116  test your web server's "file not found" page (that gets 404 back), you can
1117  use it to check your authentication protected web pages (that get a 401
1118  back) and so on.
1119
1120  The specific HTTP response code does not constitute a problem or error for
1121  curl. It simply sends and delivers HTTP as you asked and if that worked,
1122  everything is fine and dandy. The response code is generally providing more
1123  higher level error information that curl doesn't care about. The error was
1124  not in the HTTP transfer.
1125
1126  If you want your command line to treat error codes in the 400 and up range
1127  as errors and thus return a non-zero value and possibly show an error
1128  message, curl has a dedicated option for that: -f (CURLOPT_FAILONERROR in
1129  libcurl speak).
1130
1131  You can also use the -w option and the variable %{response_code} to extract
1132  the exact response code that was return in the response.
1133
1134  4.21 Why is there a HTTP/1.1 in my HTTP/2 request?
1135
1136  If you use verbose to see the HTTP request when you send off a HTTP/2
1137  request, it will still say 1.1.
1138
1139  The reason for this is that we first generate the request to send using the
1140  old 1.1 style and show that request in the verbose output, and then we
1141  convert it over to the binary header-compressed HTTP/2 style. The actual
1142  "1.1" part from that request is then not actually used in the transfer. The
1143  binary HTTP/2 headers are not human readable.
1144
11455. libcurl Issues
1146
1147  5.1 Is libcurl thread-safe?
1148
1149  Yes.
1150
1151  We have written the libcurl code specifically adjusted for multi-threaded
1152  programs. libcurl will use thread-safe functions instead of non-safe ones if
1153  your system has such.  Note that you must never share the same handle in
1154  multiple threads.
1155
1156  libcurl's implementation of timeouts might use signals (depending on what it
1157  was built to use for name resolving), and signal handling is generally not
1158  thread-safe.  Multi-threaded Applicationss that call libcurl from different
1159  threads (on different handles) might want to use CURLOPT_NOSIGNAL, e.g.:
1160
1161    curl_easy_setopt(handle, CURLOPT_NOSIGNAL, true);
1162
1163  If you use a OpenSSL-powered libcurl in a multi-threaded environment, you
1164  need to provide one or two locking functions:
1165
1166    https://www.openssl.org/docs/crypto/threads.html
1167
1168  If you use a GnuTLS-powered libcurl in a multi-threaded environment, you
1169  need to provide locking function(s) for libgcrypt (which is used by GnuTLS
1170  for the crypto functions).
1171
1172    https://web.archive.org/web/20111103083330/http://www.gnu.org/software/gnutls/manual/html_node/Multi_002dthreaded-applications.html
1173
1174  No special locking is needed with a NSS-powered libcurl. NSS is thread-safe.
1175
1176  5.2 How can I receive all data into a large memory chunk?
1177
1178  [ See also the examples/getinmemory.c source ]
1179
1180  You are in full control of the callback function that gets called every time
1181  there is data received from the remote server. You can make that callback do
1182  whatever you want. You do not have to write the received data to a file.
1183
1184  One solution to this problem could be to have a pointer to a struct that you
1185  pass to the callback function. You set the pointer using the
1186  CURLOPT_WRITEDATA option. Then that pointer will be passed to the callback
1187  instead of a FILE * to a file:
1188
1189        /* imaginary struct */
1190        struct MemoryStruct {
1191          char *memory;
1192          size_t size;
1193        };
1194
1195        /* imaginary callback function */
1196        size_t
1197        WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
1198        {
1199          size_t realsize = size * nmemb;
1200          struct MemoryStruct *mem = (struct MemoryStruct *)data;
1201
1202          mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
1203          if (mem->memory) {
1204            memcpy(&(mem->memory[mem->size]), ptr, realsize);
1205            mem->size += realsize;
1206            mem->memory[mem->size] = 0;
1207          }
1208          return realsize;
1209        }
1210
1211  5.3 How do I fetch multiple files with libcurl?
1212
1213  libcurl has excellent support for transferring multiple files. You should
1214  just repeatedly set new URLs with curl_easy_setopt() and then transfer it
1215  with curl_easy_perform(). The handle you get from curl_easy_init() is not
1216  only reusable, but you're even encouraged to reuse it if you can, as that
1217  will enable libcurl to use persistent connections.
1218
1219  5.4 Does libcurl do Winsock initialization on win32 systems?
1220
1221  Yes, if told to in the curl_global_init() call.
1222
1223  5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
1224
1225  Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
1226  that DLL use the FILE * (as the DLL and the client application cannot access
1227  each others' variable memory areas). If you set CURLOPT_WRITEDATA you must
1228  also use CURLOPT_WRITEFUNCTION as well to set a function that writes the
1229  file, even if that simply writes the data to the specified FILE *.
1230  Similarly, if you use CURLOPT_READDATA you must also specify
1231  CURLOPT_READFUNCTION.
1232
1233  5.6 What about Keep-Alive or persistent connections?
1234
1235  curl and libcurl have excellent support for persistent connections when
1236  transferring several files from the same server.  Curl will attempt to reuse
1237  connections for all URLs specified on the same command line/config file, and
1238  libcurl will reuse connections for all transfers that are made using the
1239  same libcurl handle.
1240
1241  When you use the easy interface, the connection cache is kept within the
1242  easy handle. If you instead use the multi interface, the connection cache
1243  will be kept within the multi handle and will be shared among all the easy
1244  handles that are used within the same multi handle.
1245
1246  5.7 Link errors when building libcurl on Windows!
1247
1248  You need to make sure that your project, and all the libraries (both static
1249  and dynamic) that it links against, are compiled/linked against the same run
1250  time library.
1251
1252  This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
1253  options to the command line compiler. /MD (linking against MSVCRT dll) seems
1254  to be the most commonly used option.
1255
1256  When building an application that uses the static libcurl library, you must
1257  add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for
1258  dynamic import symbols. If you're using Visual Studio, you need to instead
1259  add CURL_STATICLIB in the "Preprocessor Definitions" section.
1260
1261  If you get linker error like "unknown symbol __imp__curl_easy_init ..." you
1262  have linked against the wrong (static) library.  If you want to use the
1263  libcurl.dll and import lib, you don't need any extra CFLAGS, but use one of
1264  the import libraries below. These are the libraries produced by the various
1265  lib/Makefile.* files:
1266
1267       Target:          static lib.   import lib for libcurl*.dll.
1268       -----------------------------------------------------------
1269       MingW:           libcurl.a     libcurldll.a
1270       MSVC (release):  libcurl.lib   libcurl_imp.lib
1271       MSVC (debug):    libcurld.lib  libcurld_imp.lib
1272       Borland:         libcurl.lib   libcurl_imp.lib
1273
1274  5.8 libcurl.so.X: open failed: No such file or directory
1275
1276  This is an error message you might get when you try to run a program linked
1277  with a shared version of libcurl and your run-time linker (ld.so) couldn't
1278  find the shared library named libcurl.so.X. (Where X is the number of the
1279  current libcurl ABI, typically 3 or 4).
1280
1281  You need to make sure that ld.so finds libcurl.so.X. You can do that
1282  multiple ways, and it differs somewhat between different operating systems,
1283  but they are usually:
1284
1285  * Add an option to the linker command line that specify the hard-coded path
1286    the run-time linker should check for the lib (usually -R)
1287
1288  * Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
1289    should check for libs
1290
1291  * Adjust the system's config to check for libs in the directory where you've
1292    put the dir (like Linux's /etc/ld.so.conf)
1293
1294  'man ld.so' and 'man ld' will tell you more details
1295
1296  5.9 How does libcurl resolve host names?
1297
1298  libcurl supports a large a number of different name resolve functions. One
1299  of them is picked at build-time and will be used unconditionally. Thus, if
1300  you want to change name resolver function you must rebuild libcurl and tell
1301  it to use a different function.
1302
1303  - The non-IPv6 resolver that can use one out of four host name resolve calls
1304    (depending on what your system supports):
1305
1306      A - gethostbyname()
1307      B - gethostbyname_r() with 3 arguments
1308      C - gethostbyname_r() with 5 arguments
1309      D - gethostbyname_r() with 6 arguments
1310
1311  - The IPv6-resolver that uses getaddrinfo()
1312
1313  - The c-ares based name resolver that uses the c-ares library for resolves.
1314    Using this offers asynchronous name resolves.
1315
1316  - The threaded resolver (default option on Windows). It uses:
1317
1318      A - gethostbyname() on plain IPv4 hosts
1319      B - getaddrinfo() on IPv6 enabled hosts
1320
1321  Also note that libcurl never resolves or reverse-lookups addresses given as
1322  pure numbers, such as 127.0.0.1 or ::1.
1323
1324  5.10 How do I prevent libcurl from writing the response to stdout?
1325
1326  libcurl provides a default built-in write function that writes received data
1327  to stdout. Set the CURLOPT_WRITEFUNCTION to receive the data, or possibly
1328  set CURLOPT_WRITEDATA to a different FILE * handle.
1329
1330  5.11 How do I make libcurl not receive the whole HTTP response?
1331
1332  You make the write callback (or progress callback) return an error and
1333  libcurl will then abort the transfer.
1334
1335  5.12 Can I make libcurl fake or hide my real IP address?
1336
1337  No. libcurl operates on a higher level. Besides, faking IP address would
1338  imply sending IP packet with a made-up source address, and then you normally
1339  get a problem with receiving the packet sent back as they would then not be
1340  routed to you!
1341
1342  If you use a proxy to access remote sites, the sites will not see your local
1343  IP address but instead the address of the proxy.
1344
1345  Also note that on many networks NATs or other IP-munging techniques are used
1346  that makes you see and use a different IP address locally than what the
1347  remote server will see you coming from. You may also consider using
1348  https://www.torproject.org/ .
1349
1350  5.13 How do I stop an ongoing transfer?
1351
1352  With the easy interface you make sure to return the correct error code from
1353  one of the callbacks, but none of them are instant. There is no function you
1354  can call from another thread or similar that will stop it immediately.
1355  Instead, you need to make sure that one of the callbacks you use returns an
1356  appropriate value that will stop the transfer.  Suitable callbacks that you
1357  can do this with include the progress callback, the read callback and the
1358  write callback.
1359
1360  If you're using the multi interface, you can also stop a transfer by
1361  removing the particular easy handle from the multi stack at any moment you
1362  think the transfer is done or when you wish to abort the transfer.
1363
1364  5.14 Using C++ non-static functions for callbacks?
1365
1366  libcurl is a C library, it doesn't know anything about C++ member functions.
1367
1368  You can overcome this "limitation" with a relative ease using a static
1369  member function that is passed a pointer to the class:
1370
1371     // f is the pointer to your object.
1372     static YourClass::func(void *buffer, size_t sz, size_t n, void *f)
1373     {
1374       // Call non-static member function.
1375       static_cast<YourClass*>(f)->nonStaticFunction();
1376     }
1377
1378     // This is how you pass pointer to the static function:
1379     curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass:func);
1380     curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);
1381
1382  5.15 How do I get an FTP directory listing?
1383
1384  If you end the FTP URL you request with a slash, libcurl will provide you
1385  with a directory listing of that given directory. You can also set
1386  CURLOPT_CUSTOMREQUEST to alter what exact listing command libcurl would use
1387  to list the files.
1388
1389  The follow-up question that tend to follow the previous one, is how a
1390  program is supposed to parse the directory listing. How does it know what's
1391  a file and what's a dir and what's a symlink etc. The harsh reality is that
1392  FTP provides no such fine and easy-to-parse output. The output format FTP
1393  servers respond to LIST commands are entirely at the server's own liking and
1394  the NLST output doesn't reveal any types and in many cases don't even
1395  include all the directory entries. Also, both LIST and NLST tend to hide
1396  unix-style hidden files (those that start with a dot) by default so you need
1397  to do "LIST -a" or similar to see them.
1398
1399  The application thus needs to parse the LIST output. One such existing
1400  list parser is available at http://cr.yp.to/ftpparse.html  Versions of
1401  libcurl since 7.21.0 also provide the ability to specify a wildcard to
1402  download multiple files from one FTP directory.
1403
1404  5.16 I want a different time-out!
1405
1406  Time and time again users realize that CURLOPT_TIMEOUT and
1407  CURLOPT_CONNECTIMEOUT are not sufficiently advanced or flexible to cover all
1408  the various use cases and scenarios applications end up with.
1409
1410  libcurl offers many more ways to time-out operations. A common alternative
1411  is to use the CURLOPT_LOW_SPEED_LIMIT and CURLOPT_LOW_SPEED_TIME options to
1412  specify the lowest possible speed to accept before to consider the transfer
1413  timed out.
1414
1415  The most flexible way is by writing your own time-out logic and using
1416  CURLOPT_PROGRESSFUNCTION (perhaps in combination with other callbacks) and
1417  use that to figure out exactly when the right condition is met when the
1418  transfer should get stopped.
1419
1420  5.17 Can I write a server with libcurl?
1421
1422  No. libcurl offers no functions or building blocks to build any kind of
1423  internet protocol server. libcurl is only a client-side library. For server
1424  libraries, you need to continue your search elsewhere but there exist many
1425  good open source ones out there for most protocols you could possibly want a
1426  server for. And there are really good stand-alone ones that have been tested
1427  and proven for many years. There's no need for you to reinvent them!
1428
1429  5.18 Does libcurl use threads?
1430
1431  Put simply: no, libcurl will execute in the same thread you call it in. All
1432  callbacks will be called in the same thread as the one you call libcurl in.
1433
1434  If you want to avoid your thread to be blocked by the libcurl call, you make
1435  sure you use the non-blocking API which will do transfers asynchronously -
1436  but still in the same single thread.
1437
1438  libcurl will potentially internally use threads for name resolving, if it
1439  was built to work like that, but in those cases it'll create the child
1440  threads by itself and they will only be used and then killed internally by
1441  libcurl and never exposed to the outside.
1442
14436. License Issues
1444
1445  Curl and libcurl are released under a MIT/X derivate license. The license is
1446  very liberal and should not impose a problem for your project. This section
1447  is just a brief summary for the cases we get the most questions. (Parts of
1448  this section was much enhanced by Bjorn Reese.)
1449
1450  We are not lawyers and this is not legal advice. You should probably consult
1451  one if you want true and accurate legal insights without our prejudice. Note
1452  especially that this section concerns the libcurl license only; compiling in
1453  features of libcurl that depend on other libraries (e.g. OpenSSL) may affect
1454  the licensing obligations of your application.
1455
1456  6.1 I have a GPL program, can I use the libcurl library?
1457
1458  Yes!
1459
1460  Since libcurl may be distributed under the MIT/X derivate license, it can be
1461  used together with GPL in any software.
1462
1463  6.2 I have a closed-source program, can I use the libcurl library?
1464
1465  Yes!
1466
1467  libcurl does not put any restrictions on the program that uses the library.
1468
1469  6.3 I have a BSD licensed program, can I use the libcurl library?
1470
1471  Yes!
1472
1473  libcurl does not put any restrictions on the program that uses the library.
1474
1475  6.4 I have a program that uses LGPL libraries, can I use libcurl?
1476
1477  Yes!
1478
1479  The LGPL license doesn't clash with other licenses.
1480
1481  6.5 Can I modify curl/libcurl for my program and keep the changes secret?
1482
1483  Yes!
1484
1485  The MIT/X derivate license practically allows you to do almost anything with
1486  the sources, on the condition that the copyright texts in the sources are
1487  left intact.
1488
1489  6.6 Can you please change the curl/libcurl license to XXXX?
1490
1491  No.
1492
1493  We have carefully picked this license after years of development and
1494  discussions and a large amount of people have contributed with source code
1495  knowing that this is the license we use. This license puts the restrictions
1496  we want on curl/libcurl and it does not spread to other programs or
1497  libraries that use it. It should be possible for everyone to use libcurl or
1498  curl in their projects, no matter what license they already have in use.
1499
1500  6.7 What are my obligations when using libcurl in my commercial apps?
1501
1502  Next to none. All you need to adhere to is the MIT-style license (stated in
1503  the COPYING file) which basically says you have to include the copyright
1504  notice in "all copies" and that you may not use the copyright holder's name
1505  when promoting your software.
1506
1507  You do not have to release any of your source code.
1508
1509  You do not have to reveal or make public any changes to the libcurl source
1510  code.
1511
1512  You do not have to broadcast to the world that you are using libcurl within
1513  your app.
1514
1515  All we ask is that you disclose "the copyright notice and this permission
1516  notice" somewhere. Most probably like in the documentation or in the section
1517  where other third party dependencies already are mentioned and acknowledged.
1518
1519  As can be seen here: http://curl.haxx.se/docs/companies.html and elsewhere,
1520  more and more companies are discovering the power of libcurl and take
1521  advantage of it even in commercial environments.
1522
1523
15247. PHP/CURL Issues
1525
1526  7.1 What is PHP/CURL?
1527
1528  The module for PHP that makes it possible for PHP programs to access curl-
1529  functions from within PHP.
1530
1531  In the cURL project we call this module PHP/CURL to differentiate it from
1532  curl the command line tool and libcurl the library. The PHP team however
1533  does not refer to it like this (for unknown reasons). They call it plain
1534  CURL (often using all caps) or sometimes ext/curl, but both cause much
1535  confusion to users which in turn gives us a higher question load.
1536
1537  7.2 Who wrote PHP/CURL?
1538
1539  PHP/CURL is a module that comes with the regular PHP package. It depends and
1540  uses libcurl, so you need to have libcurl installed properly first before
1541  PHP/CURL can be used. PHP/CURL was initially written by Sterling Hughes.
1542
1543  7.3 Can I perform multiple requests using the same handle?
1544
1545  Yes - at least in PHP version 4.3.8 and later (this has been known to not
1546  work in earlier versions, but the exact version when it started to work is
1547  unknown to me).
1548
1549  After a transfer, you just set new options in the handle and make another
1550  transfer. This will make libcurl to re-use the same connection if it can.
1551