5 \___|\___/|_| \_\_____|
13 1.4 When will you make curl do XXXX ?
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?
25 2. Install Related Problems
26 2.1 configure doesn't find OpenSSL even when it is installed
27 2.1.1 native linker doesn't find OpenSSL
28 2.1.2 only the libssl lib is missing
29 2.2 Does curl work/build with other SSL libraries?
30 2.3 Where can I find a copy of LIBEAY32.DLL?
31 2.4 Does curl support SOCKS (RFC 1928) ?
34 3.1 curl: (1) SSL is disabled, https: not supported
35 3.2 How do I tell curl to resume a transfer?
36 3.3 Why doesn't my posting using -F work?
37 3.4 How do I tell curl to run custom FTP commands?
38 3.5 How can I disable the Accept: */* header?
39 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
40 3.7 Can I use curl to delete/rename a file through FTP?
41 3.8 How do I tell curl to follow HTTP redirects?
42 3.9 How do I use curl in my favorite programming language?
43 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
44 3.11 How do I POST with a different Content-Type?
45 3.12 Why do FTP specific features over HTTP proxy fail?
46 3.13 Why does my single/double quotes fail?
47 3.14 Does curl support Javascript or PAC (automated proxy config)?
48 3.15 Can I do recursive fetches with curl?
49 3.16 What certificates do I need when I use SSL?
50 3.17 How do I list the root dir of an FTP server?
51 3.18 Can I use curl to send a POST/PUT and not wait for a response?
52 3.19 How do I get HTTP from a host using a specific IP address?
53 3.20 How to SFTP from my user's home directory?
54 3.21 Protocol xxx not supported or disabled in libcurl
55 3.22 curl -X gives me HTTP problems
58 4.1 Problems connecting to SSL servers.
59 4.2 Why do I get problems when I use & or % in the URL?
60 4.3 How can I use {, }, [ or ] to specify multiple URLs?
61 4.4 Why do I get downloaded data even though the web page doesn't exist?
62 4.5 Why do I get return code XXX from a HTTP server?
63 4.5.1 "400 Bad Request"
64 4.5.2 "401 Unauthorized"
67 4.5.5 "405 Method Not Allowed"
68 4.5.6 "301 Moved Permanently"
69 4.6 Can you tell me what error code 142 means?
70 4.7 How do I keep user names and passwords secret in Curl command lines?
72 4.9 Curl can't authenticate to the server that requires NTLM?
73 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
74 4.11 Why does my HTTP range requests return the full document?
75 4.12 Why do I get "certificate verify failed" ?
76 4.13 Why is curl -R on Windows one hour off?
77 4.14 Redirects work in browser but not with curl!
78 4.15 FTPS doesn't work
79 4.16 My HTTP POST or PUT requests are slow!
80 4.17 Non-functional connect timeouts on Windows
81 4.18 file:// URLs containing drive letters (Windows, NetWare)
82 4.19 Why doesn't cURL return an error when the network cable is unplugged?
83 4.20 curl doesn't return error for HTTP non-200 responses!
86 5.1 Is libcurl thread-safe?
87 5.2 How can I receive all data into a large memory chunk?
88 5.3 How do I fetch multiple files with libcurl?
89 5.4 Does libcurl do Winsock initing on win32 systems?
90 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
91 5.6 What about Keep-Alive or persistent connections?
92 5.7 Link errors when building libcurl on Windows!
93 5.8 libcurl.so.X: open failed: No such file or directory
94 5.9 How does libcurl resolve host names?
95 5.10 How do I prevent libcurl from writing the response to stdout?
96 5.11 How do I make libcurl not receive the whole HTTP response?
97 5.12 Can I make libcurl fake or hide my real IP address?
98 5.13 How do I stop an ongoing transfer?
99 5.14 Using C++ non-static functions for callbacks?
100 5.15 How do I get an FTP directory listing?
101 5.16 I want a different time-out!
102 5.17 Can I write a server with libcurl?
103 5.18 Does libcurl use threads?
106 6.1 I have a GPL program, can I use the libcurl library?
107 6.2 I have a closed-source program, can I use the libcurl library?
108 6.3 I have a BSD licensed program, can I use the libcurl library?
109 6.4 I have a program that uses LGPL libraries, can I use libcurl?
110 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
111 6.6 Can you please change the curl/libcurl license to XXXX?
112 6.7 What are my obligations when using libcurl in my commercial apps?
115 7.1 What is PHP/CURL?
116 7.2 Who wrote PHP/CURL?
117 7.3 Can I perform multiple requests using the same handle?
119 ==============================================================================
125 cURL is the name of the project. The name is a play on 'Client for URLs',
126 originally with URL spelled in uppercase to make it obvious it deals with
127 URLs. The fact it can also be pronounced 'see URL' also helped, it works as
128 an abbreviation for "Client URL Request Library" or why not the recursive
129 version: "Curl URL Request Library".
131 The cURL project produces two products:
135 A free and easy-to-use client-side URL transfer library, supporting DICT,
136 FILE, FTP, FTPS, GOPHER, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, POP3,
137 POP3S, RTMP, RTSP, SCP, SFTP, SMB, SMBS, SMTP, SMTPS, TELNET and TFTP.
139 libcurl supports HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading,
140 Kerberos, SPNEGO, HTTP form based upload, proxies, cookies, user+password
141 authentication, file transfer resume, http proxy tunneling and more!
143 libcurl is highly portable, it builds and works identically on numerous
144 platforms, including Solaris, NetBSD, FreeBSD, OpenBSD, Darwin, HP-UX,
145 IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, BeOS, Mac
146 OS X, Ultrix, QNX, OpenVMS, RISC OS, Novell NetWare, DOS, Symbian, OSF,
147 Android, Minix, IBM TPF and more...
149 libcurl is free, thread-safe, IPv6 compatible, feature rich, well
154 A command line tool for getting or sending files using URL syntax.
156 Since curl uses libcurl, curl supports the same wide range of common
157 Internet protocols that libcurl does.
159 We pronounce curl with an initial k sound. It rhymes with words like girl
160 and earl. This is a short WAV file to help you:
162 http://media.merriam-webster.com/soundc11/c/curl0001.wav
164 There are numerous sub-projects and related projects that also use the word
165 curl in the project names in various combinations, but you should take
166 notice that this FAQ is directed at the command-line tool named curl (and
167 libcurl the library), and may therefore not be valid for other curl-related
168 projects. (There is however a small section for the PHP/CURL in this FAQ.)
172 libcurl is a reliable and portable library which provides you with an easy
173 interface to a range of common Internet protocols.
175 You can use libcurl for free in your application, be it open source,
176 commercial or closed-source.
178 libcurl is most probably the most portable, most powerful and most often
179 used C-based multi-platform file transfer library on this planet - be it
180 open source or commercial.
182 1.3 What is curl not?
184 Curl is not a wget clone. That is a common misconception. Never, during
185 curl's development, have we intended curl to replace wget or compete on its
186 market. Curl is targeted at single-shot file transfers.
188 Curl is not a web site mirroring program. If you want to use curl to mirror
189 something: fine, go ahead and write a script that wraps around curl to make
190 it reality (like curlmirror.pl does).
192 Curl is not an FTP site mirroring program. Sure, get and send FTP with curl
193 but if you want systematic and sequential behavior you should write a
194 script (or write a new program that interfaces libcurl) and do it.
196 Curl is not a PHP tool, even though it works perfectly well when used from
197 or with PHP (when using the PHP/CURL module).
199 Curl is not a program for a single operating system. Curl exists, compiles,
200 builds and runs under a wide range of operating systems, including all
201 modern Unixes (and a bunch of older ones too), Windows, Amiga, BeOS, OS/2,
204 1.4 When will you make curl do XXXX ?
206 We love suggestions of what to change in order to make curl and libcurl
207 better. We do however believe in a few rules when it comes to the future of
210 Curl -- the command line tool -- is to remain a non-graphical command line
211 tool. If you want GUIs or fancy scripting capabilities, you should look for
212 another tool that uses libcurl.
214 We do not add things to curl that other small and available tools already do
215 very fine at the side. Curl's output is fine to pipe into another program or
216 redirect to another file for the next program to interpret.
218 We focus on protocol related issues and improvements. If you wanna do more
219 magic with the supported protocols than curl currently does, chances are big
220 we will agree. If you wanna add more protocols, we may very well agree.
222 If you want someone else to make all the work while you wait for us to
223 implement it for you, that is not a very friendly attitude. We spend a
224 considerable time already on maintaining and developing curl. In order to
225 get more out of us, you should consider trading in some of your time and
228 If you write the code, chances are bigger that it will get into curl faster.
232 curl and libcurl are not made by any single individual. Daniel Stenberg is
233 project leader and main developer, but other persons' submissions are
234 important and crucial. Anyone can contribute and post their changes and
235 improvements and have them inserted in the main sources (of course on the
236 condition that developers agree on that the fixes are good).
238 The full list of all contributors is found in the docs/THANKS file.
240 curl is developed by a community, with Daniel at the wheel.
242 1.6 What do you get for making curl?
244 Project cURL is entirely free and open. No person gets paid for developing
245 curl on full time. We do this voluntarily, mostly on spare time.
246 Occasionally companies pay individual developers to work on curl, but that's
247 up to each company and developer. It is not controlled by nor supervised in
248 any way by the project.
250 We still get help from companies. Haxx provides web site, bandwidth, mailing
251 lists etc, sourceforge.net hosts project services we take advantage from,
252 like the bug tracker and github hosts the primary git repository. Also
253 again, some companies have sponsored certain parts of the development in the
254 past and I hope some will continue to do so in the future.
256 If you want to support our project, consider a donation or a banner-program
257 or even better: by helping us coding, documenting, testing etc.
259 1.7 What about CURL from curl.com?
261 During the summer 2001, curl.com was busy advertising their client-side
262 programming language for the web, named CURL.
264 We are in no way associated with curl.com or their CURL programming
267 Our project name curl has been in effective use since 1998. We were not the
268 first computer related project to use the name "curl" and do not claim any
271 We recognize that we will be living in parallel with curl.com and wish them
274 1.8 I have a problem who do I mail?
276 Please do not mail any single individual unless you really need to. Keep
277 curl-related questions on a suitable mailing list. All available mailing
278 lists are listed in the MANUAL document and online at
279 http://curl.haxx.se/mail/
281 Keeping curl-related questions and discussions on mailing lists allows
282 others to join in and help, to share their ideas, contribute their
283 suggestions and spread their wisdom. Keeping discussions on public mailing
284 lists also allows for others to learn from this (both current and future
285 users thanks to the web based archives of the mailing lists), thus saving us
286 from having to repeat ourselves even more. Thanks for respecting this.
288 If you have found or simply suspect a security problem in curl or libcurl,
289 mail curl-security at haxx.se (closed list of receivers, mails are not
290 disclosed) and tell. Then we can produce a fix in a timely manner before the
291 flaw is announced to the world, thus lessen the impact the problem will have
294 1.9 Where do I buy commercial support for curl?
296 curl is fully open source. It means you can hire any skilled engineer to fix
297 your curl-related problems.
299 We list available alternatives on the curl web site:
300 http://curl.haxx.se/support.html
302 1.10 How many are using curl?
304 It is impossible to tell.
306 We don't know how many users that knowingly have installed and use curl.
308 We don't know how many users that use curl without knowing that they are in
311 We don't know how many users that downloaded or installed curl and then
314 In May 2012 Daniel did a counting game and came up with a number that may
315 be completely wrong or somewhat accurate. Over 500 million!
317 See http://daniel.haxx.se/blog/2012/05/16/300m-users/
319 1.11 Why don't you update ca-bundle.crt
321 The ca cert bundle that used to shipped with curl was very outdated and must
322 be replaced with an up-to-date version by anyone who wants to verify
323 peers. It is no longer provided by curl. The last curl release ever that
324 shipped a ca cert bundle was curl 7.18.0.
326 In the cURL project we've decided not to attempt to keep this file updated
327 (or even present anymore) since deciding what to add to a ca cert bundle is
328 an undertaking we've not been ready to accept, and the one we can get from
329 Mozilla is perfectly fine so there's no need to duplicate that work.
331 Today, with many services performed over HTTPS, every operating system
332 should come with a default ca cert bundle that can be deemed somewhat
333 trustworthy and that collection (if reasonably updated) should be deemed to
334 be a lot better than a private curl version.
336 If you want the most recent collection of ca certs that Mozilla Firefox
337 uses, we recommend that you extract the collection yourself from Mozilla
338 Firefox (by running 'make ca-bundle), or by using our online service setup
339 for this purpose: http://curl.haxx.se/docs/caextract.html
341 1.12 I have a problem who can I chat with?
343 There's a bunch of friendly people hanging out in the #curl channel on the
344 IRC network irc.freenode.net. If you're polite and nice, chances are big
345 that you can get -- or provide -- help instantly.
347 1.13 curl's ECCN number?
349 The US government restricts exports of software that contains or uses
350 cryptography. When doing so, the Export Control Classification Number (ECCN)
351 is used to identify the level of export control etc.
353 ASF gives a good explanation at http://www.apache.org/dev/crypto.html
355 We believe curl's number might be ECCN 5D002, another possibility is
356 5D992. It seems necessary to write them, asking to confirm.
358 Comprehensible explanations of the meaning of such numbers and how to
359 obtain them (resp.) are here
361 http://www.bis.doc.gov/licensing/exportingbasics.htm
362 http://www.bis.doc.gov/licensing/do_i_needaneccn.html
364 An incomprehensible description of the two numbers above is here
365 http://www.access.gpo.gov/bis/ear/pdf/ccl5-pt2.pdf
367 1.14 How do I submit my patch?
369 When you have made a patch or a change of whatever sort, and want to submit
370 that to the project, there are a few different ways we prefer:
372 o send a patch to the curl-library mailing list. We're many subscribers
373 there and there are lots of people who can review patches, comment on them
374 and "receive" them properly.
376 o if your patch changes or fixes a bug, you can also opt to submit a bug
377 report in the bug tracker and attach your patch there. There are less
378 people involved there.
380 Lots of more details are found in the CONTRIBUTE and INTERNALS docs.
383 2. Install Related Problems
385 2.1 configure doesn't find OpenSSL even when it is installed
387 This may be because of several reasons.
389 2.1.1 native linker doesn't find openssl
392 Solaris (native cc compiler)
393 HPUX (native cc compiler)
394 SGI IRIX (native cc compiler)
395 SCO UNIX (native cc compiler)
397 When configuring curl, I specify --with-ssl. OpenSSL is installed in
398 /usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
399 CRYPTO_lock in -lcrypto
401 Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
402 -lcrypto, so ld can't find the library. This is due to a bug in the GNU
405 Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
406 ./configure places the -L/usr/local/ssl/lib early enough in the command
407 line to make things work
409 2.1.2 only the libssl lib is missing
411 If all include files and the libcrypto lib is present, with only the
412 libssl being missing according to configure, this is mostly likely because
413 a few functions are left out from the libssl.
415 If the function names missing include RSA or RSAREF you can be certain
416 that this is because libssl requires the RSA and RSAREF libs to build.
418 See the INSTALL file section that explains how to add those libs to
419 configure. Make sure that you remove the config.cache file before you
420 rerun configure with the new flags.
422 2.2 Does curl work/build with other SSL libraries?
424 Curl has been written to use a generic SSL function layer internally, and
425 that SSL functionality can then be provided by one out of many different SSL
428 curl can be built to use one of the following SSL alternatives: OpenSSL,
429 GnuTLS, yassl, NSS, PolarSSL, axTLS, Secure Transport (native iOS/OS X),
430 WinSSL (native Windows) or GSKit (native IBM i). They all have their pros
431 and cons, and we try to maintain a comparison of them here:
432 http://curl.haxx.se/docs/ssl-compared.html
434 2.3 Where can I find a copy of LIBEAY32.DLL?
436 That is an OpenSSL binary built for Windows.
438 Curl can be built with OpenSSL to do the SSL stuff. The LIBEAY32.DLL is then
439 what curl needs on a windows machine to do https:// etc. Check out the curl
440 web site to find accurate and up-to-date pointers to recent OpenSSL DLLs and
441 other binary packages.
443 2.4 Does curl support SOCKS (RFC 1928) ?
445 Yes, SOCKS 4 and 5 are supported.
450 3.1 curl: (1) SSL is disabled, https: not supported
452 If you get this output when trying to get anything from a https:// server,
453 it means that the instance of curl/libcurl that you're using was built
454 without support for this protocol.
456 This could've happened if the configure script that was run at build time
457 couldn't find all libs and include files curl requires for SSL to work. If
458 the configure script fails to find them, curl is simply built without SSL
461 To get the https:// support into a curl that was previously built but that
462 reports that https:// is not supported, you should dig through the document
463 and logs and check out why the configure script doesn't find the SSL libs
464 and/or include files.
466 Also, check out the other paragraph in this FAQ labelled "configure doesn't
467 find OpenSSL even when it is installed".
469 3.2 How do I tell curl to resume a transfer?
471 Curl supports resumed transfers both ways on both FTP and HTTP.
474 3.3 Why doesn't my posting using -F work?
476 You can't simply use -F or -d at your choice. The web server that will
477 receive your post expects one of the formats. If the form you're trying to
478 submit uses the type 'multipart/form-data', then and only then you must use
479 the -F type. In all the most common cases, you should use -d which then
480 causes a posting with the type 'application/x-www-form-urlencoded'.
482 This is described in some detail in the MANUAL and TheArtOfHttpScripting
483 documents, and if you don't understand it the first time, read it again
484 before you post questions about this to the mailing list. Also, try reading
485 through the mailing list archives for old postings and questions regarding
488 3.4 How do I tell curl to run custom FTP commands?
490 You can tell curl to perform optional commands both before and/or after a
491 file transfer. Study the -Q/--quote option.
493 Since curl is used for file transfers, you don't normally use curl to
494 perform FTP commands without transferring anything. Therefore you must
495 always specify a URL to transfer to/from even when doing custom FTP
496 commands, or use -I which implies the "no body" option sent to libcurl.
498 3.5 How can I disable the Accept: */* header?
500 You can change all internally generated headers by adding a replacement with
501 the -H/--header option. By adding a header with empty contents you safely
502 disable that one. Use -H "Accept:" to disable that specific header.
504 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
506 To curl, all contents are alike. It doesn't matter how the page was
507 generated. It may be ASP, PHP, Perl, shell-script, SSI or plain HTML
508 files. There's no difference to curl and it doesn't even know what kind of
509 language that generated the page.
511 See also item 3.14 regarding javascript.
513 3.7 Can I use curl to delete/rename a file through FTP?
515 Yes. You specify custom FTP commands with -Q/--quote.
517 One example would be to delete a file after you have downloaded it:
519 curl -O ftp://download.com/coolfile -Q '-DELE coolfile'
521 or rename a file after upload:
523 curl -T infile ftp://upload.com/dir/ -Q "-RNFR infile" -Q "-RNTO newname"
525 3.8 How do I tell curl to follow HTTP redirects?
527 Curl does not follow so-called redirects by default. The Location: header
528 that informs the client about this is only interpreted if you're using the
529 -L/--location option. As in:
531 curl -L http://redirector.com
533 Not all redirects are HTTP ones, see 4.14
535 3.9 How do I use curl in my favorite programming language?
537 There exist many language interfaces/bindings for curl that integrates it
538 better with various languages. If you are fluid in a script language, you
539 may very well opt to use such an interface instead of using the command line
542 Find out more about which languages that support curl directly, and how to
543 install and use them, in the libcurl section of the curl web site:
544 http://curl.haxx.se/libcurl/
546 All the various bindings to libcurl are made by other projects and people,
547 outside of the cURL project. The cURL project itself only produces libcurl
548 with its plain C API. If you don't find anywhere else to ask you can ask
549 about bindings on the curl-library list too, but be prepared that people on
550 that list may not know anything about bindings.
552 In October 2009, there were interfaces available for the following
553 languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Dylan, Eiffel, Euphoria,
554 Ferite, Gambas, glib/GTK+, Haskell, ILE/RPG, Java, Lisp, Lua, Mono, .NET,
555 Object-Pascal, O'Caml, Pascal, Perl, PHP, PostgreSQL, Python, R, Rexx, Ruby,
556 Scheme, S-Lang, Smalltalk, SP-Forth, SPL, Tcl, Visual Basic, Visual FoxPro,
557 Q, wxwidgets and XBLite. By the time you read this, additional ones may have
560 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
562 Curl adheres to the HTTP spec, which basically means you can play with *any*
563 protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and
564 XML-RPC are all such ones. You can use -X to set custom requests and -H to
565 set custom headers (or replace internally generated ones).
567 Using libcurl is of course just as fine and you'd just use the proper
568 library options to do the same.
570 3.11 How do I POST with a different Content-Type?
572 You can always replace the internally generated headers with -H/--header.
573 To make a simple HTTP POST with text/xml as content-type, do something like:
575 curl -d "datatopost" -H "Content-Type: text/xml" [URL]
577 3.12 Why do FTP specific features over HTTP proxy fail?
579 Because when you use a HTTP proxy, the protocol spoken on the network will
580 be HTTP, even if you specify a FTP URL. This effectively means that you
581 normally can't use FTP specific features such as FTP upload and FTP quote
584 There is one exception to this rule, and that is if you can "tunnel through"
585 the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
586 and is generally not available as proxy admins usually disable tunneling to
587 other ports than 443 (which is used for HTTPS access through proxies).
589 3.13 Why does my single/double quotes fail?
591 To specify a command line option that includes spaces, you might need to
592 put the entire option within quotes. Like in:
594 curl -d " with spaces " url.com
598 curl -d ' with spaces ' url.com
600 Exactly what kind of quotes and how to do this is entirely up to the shell
601 or command line interpreter that you are using. For most unix shells, you
602 can more or less pick either single (') or double (") quotes. For
603 Windows/DOS prompts I believe you're forced to use double (") quotes.
605 Please study the documentation for your particular environment. Examples in
606 the curl docs will use a mix of both these ones as shown above. You must
607 adjust them to work in your environment.
609 Remember that curl works and runs on more operating systems than most single
610 individuals have ever tried.
612 3.14 Does curl support Javascript or PAC (automated proxy config)?
614 Many web pages do magic stuff using embedded Javascript. Curl and libcurl
615 have no built-in support for that, so it will be treated just like any other
618 .pac files are a netscape invention and are sometimes used by organizations
619 to allow them to differentiate which proxies to use. The .pac contents is
620 just a Javascript program that gets invoked by the browser and that returns
621 the name of the proxy to connect to. Since curl doesn't support Javascript,
622 it can't support .pac proxy configuration either.
624 Some workarounds usually suggested to overcome this Javascript dependency:
626 Depending on the Javascript complexity, write up a script that translates it
627 to another language and execute that.
629 Read the Javascript code and rewrite the same logic in another language.
631 Implement a Javascript interpreter, people have successfully used the
632 Mozilla Javascript engine in the past.
634 Ask your admins to stop this, for a static proxy setup or similar.
636 3.15 Can I do recursive fetches with curl?
638 No. curl itself has no code that performs recursive operations, such as
639 those performed by wget and similar tools.
641 There exist wrapper scripts with that functionality (for example the
642 curlmirror perl script), and you can write programs based on libcurl to do
643 it, but the command line tool curl itself cannot.
645 3.16 What certificates do I need when I use SSL?
647 There are three different kinds of "certificates" to keep track of when we
648 talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.
652 The server you communicate may require that you can provide this in order to
653 prove that you actually are who you claim to be. If the server doesn't
654 require this, you don't need a client certificate.
656 A client certificate is always used together with a private key, and the
657 private key has a pass phrase that protects it.
661 The server you communicate with has a server certificate. You can and should
662 verify this certificate to make sure that you are truly talking to the real
663 server and not a server impersonating it.
665 CERTIFICATE AUTHORITY CERTIFICATE ("CA cert")
667 You often have several CA certs in a CA cert bundle that can be used to
668 verify a server certificate that was signed by one of the authorities in the
669 bundle. curl does not come with a CA cert bundle but most curl installs
670 provide one. You can also override the default.
672 The server certificate verification process is made by using a Certificate
673 Authority certificate ("CA cert") that was used to sign the server
674 certificate. Server certificate verification is enabled by default in curl
675 and libcurl and is often the reason for problems as explained in FAQ entry
676 4.12 and the SSLCERTS document
677 (http://curl.haxx.se/docs/sslcerts.html). Server certificates that are
678 "self-signed" or otherwise signed by a CA that you do not have a CA cert
679 for, cannot be verified. If the verification during a connect fails, you are
680 refused access. You then need to explicitly disable the verification to
681 connect to the server.
683 3.17 How do I list the root dir of an FTP server?
685 There are two ways. The way defined in the RFC is to use an encoded slash
686 in the first path part. List the "/tmp" dir like this:
688 curl ftp://ftp.sunet.se/%2ftmp/
690 or the not-quite-kosher-but-more-readable way, by simply starting the path
691 section of the URL with a slash:
693 curl ftp://ftp.sunet.se//tmp/
695 3.18 Can I use curl to send a POST/PUT and not wait for a response?
699 But you could easily write your own program using libcurl to do such stunts.
701 3.19 How do I get HTTP from a host using a specific IP address?
703 For example, you may be trying out a web site installation that isn't yet in
704 the DNS. Or you have a site using multiple IP addresses for a given host
705 name and you want to address a specific one out of the set.
707 Set a custom Host: header that identifies the server name you want to reach
708 but use the target IP address in the URL:
710 curl --header "Host: www.example.com" http://127.0.0.1/
712 You can also opt to add faked host name entries to curl with the --resolve
713 option. That has the added benefit that things like redirects will also work
714 properly. The above operation would instead be done as:
716 curl --resolve www.example.com:80:127.0.0.1 http://www.example.com/
718 3.20 How to SFTP from my user's home directory?
720 Contrary to how FTP works, SFTP and SCP URLs specify the exact directory to
721 work with. It means that if you don't specify that you want the user's home
722 directory, you get the actual root directory.
724 To specify a file in your user's home directory, you need to use the correct
725 URL syntax which for sftp might look similar to:
727 curl -O -u user:password sftp://example.com/~/file.txt
729 and for SCP it is just a different protocol prefix:
731 curl -O -u user:password scp://example.com/~/file.txt
733 3.21 Protocol xxx not supported or disabled in libcurl
735 When passing on a URL to curl to use, it may respond that the particular
736 protocol is not supported or disabled. The particular way this error message
737 is phrased is because curl doesn't make a distinction internally of whether
738 a particular protocol is not supported (i.e. never got any code added that
739 knows how to speak that protocol) or if it was explicitly disabled. curl can
740 be built to only support a given set of protocols, and the rest would then
741 be disabled or not supported.
743 Note that this error will also occur if you pass a wrongly spelled protocol
744 part as in "htpt://example.com" or as in the less evident case if you prefix
745 the protocol part with a space as in " http://example.com/".
747 3.22 curl -X gives me HTTP problems
749 In normal circumstances, -X should hardly ever be used.
751 By default you use curl without explicitly saying which request method to
752 use when the URL identifies a HTTP transfer. If you just pass in a URL like
753 "curl http://example.com" it will use GET. If you use -d or -F curl will use
754 POST, -I will cause a HEAD and -T will make it a PUT.
756 If for whatever reason you're not happy with these default choices that curl
757 does for you, you can override those request methods by specifying -X
758 [WHATEVER]. This way you can for example send a DELETE by doing "curl -X
761 It is thus pointless to do "curl -XGET [URL]" as GET would be used
762 anyway. In the same vein it is pointless to do "curl -X POST -d data
763 [URL]"... But you can make a fun and somewhat rare request that sends a
764 request-body in a GET request with something like "curl -X GET -d data
767 Note that -X doesn't change curl's behavior. It only modifies the actual
768 string sent in the request.
770 Accordingly, by using -XPOST on a command line that for example would follow
771 a 303 redirect, you will effectively prevent curl from behaving
777 4.1 Problems connecting to SSL servers.
779 It took a very long time before we could sort out why curl had problems to
780 connect to certain SSL servers when using SSLeay or OpenSSL v0.9+. The
781 error sometimes showed up similar to:
783 16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:
785 It turned out to be because many older SSL servers don't deal with SSLv3
786 requests properly. To correct this problem, tell curl to select SSLv2 from
787 the command line (-2/--sslv2).
789 There have also been examples where the remote server didn't like the SSLv2
790 request and instead you had to force curl to use SSLv3 with -3/--sslv3.
792 4.2 Why do I get problems when I use & or % in the URL?
794 In general unix shells, the & symbol is treated specially and when used, it
795 runs the specified command in the background. To safely send the & as a part
796 of a URL, you should quote the entire URL by using single (') or double (")
797 quotes around it. Similar problems can also occur on some shells with other
798 characters, including ?*!$~(){}<>\|;`. When in doubt, quote the URL.
800 An example that would invoke a remote CGI that uses &-symbols could be:
802 curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
804 In Windows, the standard DOS shell treats the percent sign specially and you
805 need to use TWO percent signs for each single one you want to use in the
808 If you want a literal percent sign to be part of the data you pass in a POST
809 using -d/--data you must encode it as '%25' (which then also needs the
810 percent sign doubled on Windows machines).
812 4.3 How can I use {, }, [ or ] to specify multiple URLs?
814 Because those letters have a special meaning to the shell, and to be used in
815 a URL specified to curl you must quote them.
817 An example that downloads two URLs (sequentially) would do:
819 curl '{curl,www}.haxx.se'
821 To be able to use those letters as actual parts of the URL (without using
822 them for the curl URL "globbing" system), use the -g/--globoff option:
824 curl -g 'www.site.com/weirdname[].html'
826 4.4 Why do I get downloaded data even though the web page doesn't exist?
828 Curl asks remote servers for the page you specify. If the page doesn't exist
829 at the server, the HTTP protocol defines how the server should respond and
830 that means that headers and a "page" will be returned. That's simply how
833 By using the --fail option you can tell curl explicitly to not get any data
834 if the HTTP return code doesn't say success.
836 4.5 Why do I get return code XXX from a HTTP server?
838 RFC2616 clearly explains the return codes. This is a short transcript. Go
839 read the RFC for exact details:
841 4.5.1 "400 Bad Request"
843 The request could not be understood by the server due to malformed
844 syntax. The client SHOULD NOT repeat the request without modifications.
846 4.5.2 "401 Unauthorized"
848 The request requires user authentication.
850 4.5.3 "403 Forbidden"
852 The server understood the request, but is refusing to fulfil it.
853 Authorization will not help and the request SHOULD NOT be repeated.
855 4.5.4 "404 Not Found"
857 The server has not found anything matching the Request-URI. No indication
858 is given of whether the condition is temporary or permanent.
860 4.5.5 "405 Method Not Allowed"
862 The method specified in the Request-Line is not allowed for the resource
863 identified by the Request-URI. The response MUST include an Allow header
864 containing a list of valid methods for the requested resource.
866 4.5.6 "301 Moved Permanently"
868 If you get this return code and an HTML output similar to this:
870 <H1>Moved Permanently</H1> The document has moved <A
871 HREF="http://same_url_now_with_a_trailing_slash/">here</A>.
873 it might be because you request a directory URL but without the trailing
874 slash. Try the same operation again _with_ the trailing URL, or use the
875 -L/--location option to follow the redirection.
877 4.6 Can you tell me what error code 142 means?
879 All curl error codes are described at the end of the man page, in the
880 section called "EXIT CODES".
882 Error codes that are larger than the highest documented error code means
883 that curl has exited due to a crash. This is a serious error, and we
884 appreciate a detailed bug report from you that describes how we could go
885 ahead and repeat this!
887 4.7 How do I keep user names and passwords secret in Curl command lines?
889 This problem has two sides:
891 The first part is to avoid having clear-text passwords in the command line
892 so that they don't appear in 'ps' outputs and similar. That is easily
893 avoided by using the "-K" option to tell curl to read parameters from a file
894 or stdin to which you can pass the secret info. curl itself will also
895 attempt to "hide" the given password by blanking out the option - this
896 doesn't work on all platforms.
898 To keep the passwords in your account secret from the rest of the world is
899 not a task that curl addresses. You could of course encrypt them somehow to
900 at least hide them from being read by human eyes, but that is not what
901 anyone would call security.
903 Also note that regular HTTP (using Basic authentication) and FTP passwords
904 are sent in clear across the network. All it takes for anyone to fetch them
905 is to listen on the network. Eavesdropping is very easy. Use more secure
906 authentication methods (like Digest, Negotiate or even NTLM) or consider the
907 SSL-based alternatives HTTPS and FTPS.
911 It is not a bug if the behavior is documented. Read the docs first.
912 Especially check out the KNOWN_BUGS file, it may be a documented bug!
914 If it is a problem with a binary you've downloaded or a package for your
915 particular platform, try contacting the person who built the package/archive
918 If there is a bug, read the BUGS document first. Then report it as described
921 4.9 Curl can't authenticate to the server that requires NTLM?
923 NTLM support requires OpenSSL, GnuTLS, NSS, Secure Transport, or Microsoft
924 Windows libraries at build-time to provide this functionality.
926 NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You
927 should not use such ones.
929 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
931 Many web servers allow or demand that the administrator configures the
932 server properly for these requests to work on the web server.
934 Some servers seem to support HEAD only on certain kinds of URLs.
936 To fully grasp this, try the documentation for the particular server
937 software you're trying to interact with. This is not anything curl can do
940 4.11 Why does my HTTP range requests return the full document?
942 Because the range may not be supported by the server, or the server may
943 choose to ignore it and return the full document anyway.
945 4.12 Why do I get "certificate verify failed" ?
947 You invoke curl 7.10 or later to communicate on a https:// URL and get an
948 error back looking something similar to this:
950 curl: (35) SSL: error:14090086:SSL routines:
951 SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
953 Then it means that curl couldn't verify that the server's certificate was
954 good. Curl verifies the certificate using the CA cert bundle that comes with
955 the curl installation.
957 To disable the verification (which makes it act like curl did before 7.10),
958 use -k. This does however enable man-in-the-middle attacks.
960 If you get this failure but are having a CA cert bundle installed and used,
961 the server's certificate is not signed by one of the CA's in the bundle. It
962 might for example be self-signed. You then correct this problem by obtaining
963 a valid CA cert for the server. Or again, decrease the security by disabling
966 Details are also in the SSLCERTS file in the release archives, found online
967 here: http://curl.haxx.se/docs/sslcerts.html
969 4.13 Why is curl -R on Windows one hour off?
971 During daylight savings time, when -R is used, curl will set a time that
972 appears one hour off. This happens due to a flaw in how Windows stores and
973 uses file modification times and it is not easily worked around. For details
974 on this problem, read this: http://www.codeproject.com/datetime/dstbugs.asp
976 4.14 Redirects work in browser but not with curl!
978 curl supports HTTP redirects fine (see item 3.8). Browsers generally support
979 at least two other ways to perform redirects that curl does not:
981 Meta tags. You can write a HTML tag that will cause the browser to redirect
982 to another given URL after a certain time.
984 Javascript. You can write a Javascript program embedded in a HTML page that
985 redirects the browser to another given URL.
987 There is no way to make curl follow these redirects. You must either
988 manually figure out what the page is set to do, or you write a script that
989 parses the results and fetches the new URL.
991 4.15 FTPS doesn't work
993 curl supports FTPS (sometimes known as FTP-SSL) both implicit and explicit
996 When a URL is used that starts with FTPS://, curl assumes implicit SSL on
997 the control connection and will therefore immediately connect and try to
998 speak SSL. FTPS:// connections default to port 990.
1000 To use explicit FTPS, you use a FTP:// URL and the --ftp-ssl option (or one
1001 of its related flavours). This is the most common method, and the one
1002 mandated by RFC4217. This kind of connection then of course uses the
1003 standard FTP port 21 by default.
1005 4.16 My HTTP POST or PUT requests are slow!
1007 libcurl makes all POST and PUT requests (except for POST requests with a
1008 very tiny request body) use the "Expect: 100-continue" header. This header
1009 allows the server to deny the operation early so that libcurl can bail out
1010 already before having to send any data. This is useful in authentication
1013 However, many servers don't implement the Expect: stuff properly and if the
1014 server doesn't respond (positively) within 1 second libcurl will continue
1015 and send off the data anyway.
1017 You can disable libcurl's use of the Expect: header the same way you disable
1018 any header, using -H / CURLOPT_HTTPHEADER, or by forcing it to use HTTP 1.0.
1020 4.17 Non-functional connect timeouts
1022 In most Windows setups having a timeout longer than 21 seconds make no
1023 difference, as it will only send 3 TCP SYN packets and no more. The second
1024 packet sent three seconds after the first and the third six seconds after
1025 the second. No more than three packets are sent, no matter how long the
1028 See option TcpMaxConnectRetransmissions on this page:
1029 http://support.microsoft.com/?scid=kb%3Ben-us%3B175523&x=6&y=7
1031 Also, even on non-Windows systems there may run a firewall or anti-virus
1032 software or similar that accepts the connection but does not actually do
1033 anything else. This will make (lib)curl to consider the connection connected
1034 and thus the connect timeout won't trigger.
1036 4.18 file:// URLs containing drive letters (Windows, NetWare)
1038 When using cURL to try to download a local file, one might use a URL
1043 You'll find that even if D:\blah.txt does exist, cURL returns a 'file
1046 According to RFC 1738 (http://www.faqs.org/rfcs/rfc1738.html),
1047 file:// URLs must contain a host component, but it is ignored by
1048 most implementations. In the above example, 'D:' is treated as the
1049 host component, and is taken away. Thus, cURL tries to open '/blah.txt'.
1050 If your system is installed to drive C:, that will resolve to 'C:\blah.txt',
1051 and if that doesn't exist you will get the not found error.
1053 To fix this problem, use file:// URLs with *three* leading slashes:
1057 Alternatively, if it makes more sense, specify 'localhost' as the host
1060 file://localhost/D:/blah.txt
1062 In either case, cURL should now be looking for the correct file.
1064 4.19 Why doesn't cURL return an error when the network cable is unplugged?
1066 Unplugging a cable is not an error situation. The TCP/IP protocol stack
1067 was designed to be fault tolerant, so even though there may be a physical
1068 break somewhere the connection shouldn't be affected, just possibly
1069 delayed. Eventually, the physical break will be fixed or the data will be
1070 re-routed around the physical problem through another path.
1072 In such cases, the TCP/IP stack is responsible for detecting when the
1073 network connection is irrevocably lost. Since with some protocols it is
1074 perfectly legal for the client wait indefinitely for data, the stack may
1075 never report a problem, and even when it does, it can take up to 20 minutes
1076 for it to detect an issue. The curl option --keepalive-time enables
1077 keep-alive support in the TCP/IP stack which makes it periodically probe the
1078 connection to make sure it is still available to send data. That should
1079 reliably detect any TCP/IP network failure.
1081 But even that won't detect the network going down before the TCP/IP
1082 connection is established (e.g. during a DNS lookup) or using protocols that
1083 don't use TCP. To handle those situations, curl offers a number of timeouts
1084 on its own. --speed-limit/--speed-time will abort if the data transfer rate
1085 falls too low, and --connect-timeout and --max-time can be used to put an
1086 overall timeout on the connection phase or the entire transfer.
1088 A libcurl-using application running in a known physical environment (e.g.
1089 an embedded device with only a single network connection) may want to act
1090 immediately if its lone network connection goes down. That can be achieved
1091 by having the application monitor the network connection on its own using an
1092 OS-specific mechanism, then signalling libcurl to abort (see also item 5.13).
1094 4.20 curl doesn't return error for HTTP non-200 responses!
1096 Correct. Unless you use -f (--fail).
1098 When doing HTTP transfers, curl will perform exactly what you're asking it
1099 to do and if successful it will not return an error. You can use curl to
1100 test your web server's "file not found" page (that gets 404 back), you can
1101 use it to check your authentication protected web pages (that get a 401
1104 The specific HTTP response code does not constitute a problem or error for
1105 curl. It simply sends and delivers HTTP as you asked and if that worked,
1106 everything is fine and dandy. The response code is generally providing more
1107 higher level error information that curl doesn't care about. The error was
1108 not in the HTTP transfer.
1110 If you want your command line to treat error codes in the 400 and up range
1111 as errors and thus return a non-zero value and possibly show an error
1112 message, curl has a dedicated option for that: -f (CURLOPT_FAILONERROR in
1115 You can also use the -w option and the variable %{response_code} to extract
1116 the exact response code that was return in the response.
1121 5.1 Is libcurl thread-safe?
1125 We have written the libcurl code specifically adjusted for multi-threaded
1126 programs. libcurl will use thread-safe functions instead of non-safe ones if
1127 your system has such. Note that you must never share the same handle in
1130 libcurl's implementation of timeouts might use signals (depending on what it
1131 was built to use for name resolving), and signal handling is generally not
1132 thread-safe. Multi-threaded Applicationss that call libcurl from different
1133 threads (on different handles) might want to use CURLOPT_NOSIGNAL, e.g.:
1135 curl_easy_setopt(handle, CURLOPT_NOSIGNAL, true);
1137 If you use a OpenSSL-powered libcurl in a multi-threaded environment, you
1138 need to provide one or two locking functions:
1140 http://www.openssl.org/docs/crypto/threads.html
1142 If you use a GnuTLS-powered libcurl in a multi-threaded environment, you
1143 need to provide locking function(s) for libgcrypt (which is used by GnuTLS
1144 for the crypto functions).
1146 http://www.gnu.org/software/gnutls/manual/html_node/Multi_002dthreaded-applications.html
1148 No special locking is needed with a NSS-powered libcurl. NSS is thread-safe.
1150 5.2 How can I receive all data into a large memory chunk?
1152 [ See also the examples/getinmemory.c source ]
1154 You are in full control of the callback function that gets called every time
1155 there is data received from the remote server. You can make that callback do
1156 whatever you want. You do not have to write the received data to a file.
1158 One solution to this problem could be to have a pointer to a struct that you
1159 pass to the callback function. You set the pointer using the
1160 CURLOPT_WRITEDATA option. Then that pointer will be passed to the callback
1161 instead of a FILE * to a file:
1163 /* imaginary struct */
1164 struct MemoryStruct {
1169 /* imaginary callback function */
1171 WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
1173 size_t realsize = size * nmemb;
1174 struct MemoryStruct *mem = (struct MemoryStruct *)data;
1176 mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
1178 memcpy(&(mem->memory[mem->size]), ptr, realsize);
1179 mem->size += realsize;
1180 mem->memory[mem->size] = 0;
1185 5.3 How do I fetch multiple files with libcurl?
1187 libcurl has excellent support for transferring multiple files. You should
1188 just repeatedly set new URLs with curl_easy_setopt() and then transfer it
1189 with curl_easy_perform(). The handle you get from curl_easy_init() is not
1190 only reusable, but you're even encouraged to reuse it if you can, as that
1191 will enable libcurl to use persistent connections.
1193 5.4 Does libcurl do Winsock initialization on win32 systems?
1195 Yes, if told to in the curl_global_init() call.
1197 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
1199 Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
1200 that DLL use the FILE * (as the DLL and the client application cannot access
1201 each others' variable memory areas). If you set CURLOPT_WRITEDATA you must
1202 also use CURLOPT_WRITEFUNCTION as well to set a function that writes the
1203 file, even if that simply writes the data to the specified FILE *.
1204 Similarly, if you use CURLOPT_READDATA you must also specify
1205 CURLOPT_READFUNCTION.
1207 5.6 What about Keep-Alive or persistent connections?
1209 curl and libcurl have excellent support for persistent connections when
1210 transferring several files from the same server. Curl will attempt to reuse
1211 connections for all URLs specified on the same command line/config file, and
1212 libcurl will reuse connections for all transfers that are made using the
1213 same libcurl handle.
1215 When you use the easy interface, the connection cache is kept within the
1216 easy handle. If you instead use the multi interface, the connection cache
1217 will be kept within the multi handle and will be shared among all the easy
1218 handles that are used within the same multi handle.
1220 5.7 Link errors when building libcurl on Windows!
1222 You need to make sure that your project, and all the libraries (both static
1223 and dynamic) that it links against, are compiled/linked against the same run
1226 This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
1227 options to the command line compiler. /MD (linking against MSVCRT dll) seems
1228 to be the most commonly used option.
1230 When building an application that uses the static libcurl library, you must
1231 add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for
1232 dynamic import symbols. If you're using Visual Studio, you need to instead
1233 add CURL_STATICLIB in the "Preprocessor Definitions" section.
1235 If you get linker error like "unknown symbol __imp__curl_easy_init ..." you
1236 have linked against the wrong (static) library. If you want to use the
1237 libcurl.dll and import lib, you don't need any extra CFLAGS, but use one of
1238 the import libraries below. These are the libraries produced by the various
1239 lib/Makefile.* files:
1241 Target: static lib. import lib for libcurl*.dll.
1242 -----------------------------------------------------------
1243 MingW: libcurl.a libcurldll.a
1244 MSVC (release): libcurl.lib libcurl_imp.lib
1245 MSVC (debug): libcurld.lib libcurld_imp.lib
1246 Borland: libcurl.lib libcurl_imp.lib
1248 5.8 libcurl.so.X: open failed: No such file or directory
1250 This is an error message you might get when you try to run a program linked
1251 with a shared version of libcurl and your run-time linker (ld.so) couldn't
1252 find the shared library named libcurl.so.X. (Where X is the number of the
1253 current libcurl ABI, typically 3 or 4).
1255 You need to make sure that ld.so finds libcurl.so.X. You can do that
1256 multiple ways, and it differs somewhat between different operating systems,
1257 but they are usually:
1259 * Add an option to the linker command line that specify the hard-coded path
1260 the run-time linker should check for the lib (usually -R)
1262 * Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
1263 should check for libs
1265 * Adjust the system's config to check for libs in the directory where you've
1266 put the dir (like Linux's /etc/ld.so.conf)
1268 'man ld.so' and 'man ld' will tell you more details
1270 5.9 How does libcurl resolve host names?
1272 libcurl supports a large a number of different name resolve functions. One
1273 of them is picked at build-time and will be used unconditionally. Thus, if
1274 you want to change name resolver function you must rebuild libcurl and tell
1275 it to use a different function.
1277 - The non-IPv6 resolver that can use one out of four host name resolve calls
1278 (depending on what your system supports):
1281 B - gethostbyname_r() with 3 arguments
1282 C - gethostbyname_r() with 5 arguments
1283 D - gethostbyname_r() with 6 arguments
1285 - The IPv6-resolver that uses getaddrinfo()
1287 - The c-ares based name resolver that uses the c-ares library for resolves.
1288 Using this offers asynchronous name resolves.
1290 - The threaded resolver (default option on Windows). It uses:
1292 A - gethostbyname() on plain IPv4 hosts
1293 B - getaddrinfo() on IPv6 enabled hosts
1295 Also note that libcurl never resolves or reverse-lookups addresses given as
1296 pure numbers, such as 127.0.0.1 or ::1.
1298 5.10 How do I prevent libcurl from writing the response to stdout?
1300 libcurl provides a default built-in write function that writes received data
1301 to stdout. Set the CURLOPT_WRITEFUNCTION to receive the data, or possibly
1302 set CURLOPT_WRITEDATA to a different FILE * handle.
1304 5.11 How do I make libcurl not receive the whole HTTP response?
1306 You make the write callback (or progress callback) return an error and
1307 libcurl will then abort the transfer.
1309 5.12 Can I make libcurl fake or hide my real IP address?
1311 No. libcurl operates on a higher level. Besides, faking IP address would
1312 imply sending IP packet with a made-up source address, and then you normally
1313 get a problem with receiving the packet sent back as they would then not be
1316 If you use a proxy to access remote sites, the sites will not see your local
1317 IP address but instead the address of the proxy.
1319 Also note that on many networks NATs or other IP-munging techniques are used
1320 that makes you see and use a different IP address locally than what the
1321 remote server will see you coming from. You may also consider using
1322 http://www.torproject.org .
1324 5.13 How do I stop an ongoing transfer?
1326 With the easy interface you make sure to return the correct error code from
1327 one of the callbacks, but none of them are instant. There is no function you
1328 can call from another thread or similar that will stop it immediately.
1329 Instead, you need to make sure that one of the callbacks you use returns an
1330 appropriate value that will stop the transfer. Suitable callbacks that you
1331 can do this with include the progress callback, the read callback and the
1334 If you're using the multi interface, you can also stop a transfer by
1335 removing the particular easy handle from the multi stack at any moment you
1336 think the transfer is done or when you wish to abort the transfer.
1338 5.14 Using C++ non-static functions for callbacks?
1340 libcurl is a C library, it doesn't know anything about C++ member functions.
1342 You can overcome this "limitation" with a relative ease using a static
1343 member function that is passed a pointer to the class:
1345 // f is the pointer to your object.
1346 static YourClass::func(void *buffer, size_t sz, size_t n, void *f)
1348 // Call non-static member function.
1349 static_cast<YourClass*>(f)->nonStaticFunction();
1352 // This is how you pass pointer to the static function:
1353 curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass:func);
1354 curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);
1356 5.15 How do I get an FTP directory listing?
1358 If you end the FTP URL you request with a slash, libcurl will provide you
1359 with a directory listing of that given directory. You can also set
1360 CURLOPT_CUSTOMREQUEST to alter what exact listing command libcurl would use
1363 The follow-up question that tend to follow the previous one, is how a
1364 program is supposed to parse the directory listing. How does it know what's
1365 a file and what's a dir and what's a symlink etc. The harsh reality is that
1366 FTP provides no such fine and easy-to-parse output. The output format FTP
1367 servers respond to LIST commands are entirely at the server's own liking and
1368 the NLST output doesn't reveal any types and in many cases don't even
1369 include all the directory entries. Also, both LIST and NLST tend to hide
1370 unix-style hidden files (those that start with a dot) by default so you need
1371 to do "LIST -a" or similar to see them.
1373 The application thus needs to parse the LIST output. One such existing
1374 list parser is available at http://cr.yp.to/ftpparse.html Versions of
1375 libcurl since 7.21.0 also provide the ability to specify a wildcard to
1376 download multiple files from one FTP directory.
1378 5.16 I want a different time-out!
1380 Time and time again users realize that CURLOPT_TIMEOUT and
1381 CURLOPT_CONNECTIMEOUT are not sufficiently advanced or flexible to cover all
1382 the various use cases and scenarios applications end up with.
1384 libcurl offers many more ways to time-out operations. A common alternative
1385 is to use the CURLOPT_LOW_SPEED_LIMIT and CURLOPT_LOW_SPEED_TIME options to
1386 specify the lowest possible speed to accept before to consider the transfer
1389 The most flexible way is by writing your own time-out logic and using
1390 CURLOPT_PROGRESSFUNCTION (perhaps in combination with other callbacks) and
1391 use that to figure out exactly when the right condition is met when the
1392 transfer should get stopped.
1394 5.17 Can I write a server with libcurl?
1396 No. libcurl offers no functions or building blocks to build any kind of
1397 internet protocol server. libcurl is only a client-side library. For server
1398 libraries, you need to continue your search elsewhere but there exist many
1399 good open source ones out there for most protocols you could possibly want a
1400 server for. And there are really good stand-alone ones that have been tested
1401 and proven for many years. There's no need for you to reinvent them!
1403 5.18 Does libcurl use threads?
1405 Put simply: no, libcurl will execute in the same thread you call it in. All
1406 callbacks will be called in the same thread as the one you call libcurl in.
1408 If you want to avoid your thread to be blocked by the libcurl call, you make
1409 sure you use the non-blocking API which will do transfers asynchronously -
1410 but still in the same single thread.
1412 libcurl will potentially internally use threads for name resolving, if it
1413 was built to work like that, but in those cases it'll create the child
1414 threads by itself and they will only be used and then killed internally by
1415 libcurl and never exposed to the outside.
1419 Curl and libcurl are released under a MIT/X derivate license. The license is
1420 very liberal and should not impose a problem for your project. This section
1421 is just a brief summary for the cases we get the most questions. (Parts of
1422 this section was much enhanced by Bjorn Reese.)
1424 We are not lawyers and this is not legal advice. You should probably consult
1425 one if you want true and accurate legal insights without our prejudice. Note
1426 especially that this section concerns the libcurl license only; compiling in
1427 features of libcurl that depend on other libraries (e.g. OpenSSL) may affect
1428 the licensing obligations of your application.
1430 6.1 I have a GPL program, can I use the libcurl library?
1434 Since libcurl may be distributed under the MIT/X derivate license, it can be
1435 used together with GPL in any software.
1437 6.2 I have a closed-source program, can I use the libcurl library?
1441 libcurl does not put any restrictions on the program that uses the library.
1443 6.3 I have a BSD licensed program, can I use the libcurl library?
1447 libcurl does not put any restrictions on the program that uses the library.
1449 6.4 I have a program that uses LGPL libraries, can I use libcurl?
1453 The LGPL license doesn't clash with other licenses.
1455 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
1459 The MIT/X derivate license practically allows you to do almost anything with
1460 the sources, on the condition that the copyright texts in the sources are
1463 6.6 Can you please change the curl/libcurl license to XXXX?
1467 We have carefully picked this license after years of development and
1468 discussions and a large amount of people have contributed with source code
1469 knowing that this is the license we use. This license puts the restrictions
1470 we want on curl/libcurl and it does not spread to other programs or
1471 libraries that use it. It should be possible for everyone to use libcurl or
1472 curl in their projects, no matter what license they already have in use.
1474 6.7 What are my obligations when using libcurl in my commercial apps?
1476 Next to none. All you need to adhere to is the MIT-style license (stated in
1477 the COPYING file) which basically says you have to include the copyright
1478 notice in "all copies" and that you may not use the copyright holder's name
1479 when promoting your software.
1481 You do not have to release any of your source code.
1483 You do not have to reveal or make public any changes to the libcurl source
1486 You do not have to broadcast to the world that you are using libcurl within
1489 All we ask is that you disclose "the copyright notice and this permission
1490 notice" somewhere. Most probably like in the documentation or in the section
1491 where other third party dependencies already are mentioned and acknowledged.
1493 As can be seen here: http://curl.haxx.se/docs/companies.html and elsewhere,
1494 more and more companies are discovering the power of libcurl and take
1495 advantage of it even in commercial environments.
1500 7.1 What is PHP/CURL?
1502 The module for PHP that makes it possible for PHP programs to access curl-
1503 functions from within PHP.
1505 In the cURL project we call this module PHP/CURL to differentiate it from
1506 curl the command line tool and libcurl the library. The PHP team however
1507 does not refer to it like this (for unknown reasons). They call it plain
1508 CURL (often using all caps) or sometimes ext/curl, but both cause much
1509 confusion to users which in turn gives us a higher question load.
1511 7.2 Who wrote PHP/CURL?
1513 PHP/CURL is a module that comes with the regular PHP package. It depends and
1514 uses libcurl, so you need to have libcurl installed properly first before
1515 PHP/CURL can be used. PHP/CURL was initially written by Sterling Hughes.
1517 7.3 Can I perform multiple requests using the same handle?
1519 Yes - at least in PHP version 4.3.8 and later (this has been known to not
1520 work in earlier versions, but the exact version when it started to work is
1523 After a transfer, you just set new options in the handle and make another
1524 transfer. This will make libcurl to re-use the same connection if it can.