1 Updated: April 5, 2005 (http://curl.haxx.se/docs/faq.html)
6 \___|\___/|_| \_\_____|
14 1.4 When will you make curl do XXXX ?
16 1.6 What do you get for making curl?
17 1.7 What about CURL from curl.com?
18 1.8 I have a problem who do I mail?
20 2. Install Related Problems
21 2.1 configure doesn't find OpenSSL even when it is installed
22 2.1.1 native linker doesn't find OpenSSL
23 2.1.2 only the libssl lib is missing
24 2.2 Does curl work/build with other SSL libraries?
25 2.3 Where can I find a copy of LIBEAY32.DLL?
26 2.4 Does curl support Socks (RFC 1928) ?
29 3.1 curl: (1) SSL is disabled, https: not supported
30 3.2 How do I tell curl to resume a transfer?
31 3.3 Why doesn't my posting using -F work?
32 3.4 How do I tell curl to run custom FTP commands?
33 3.5 How can I disable the Pragma: nocache header?
34 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
35 3.7 Can I use curl to delete/rename a file through FTP?
36 3.8 How do I tell curl to follow HTTP redirects?
37 3.9 How do I use curl in my favorite programming language?
38 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
39 3.11 How do I POST with a different Content-Type?
40 3.12 Why do FTP specific features over HTTP proxy fail?
41 3.13 Why does my single/double quotes fail?
42 3.14 Does curl support javascript or pac (automated proxy config)?
43 3.15 Can I do recursive fetches with curl?
44 3.16 What certificates do I need when I use SSL?
45 3.17 How do I list the root dir of an FTP server?
46 3.18 Can I use curl to send a POST/PUT and not wait for a response?
49 4.1 Problems connecting to SSL servers.
50 4.2 Why do I get problems when I use & or % in the URL?
51 4.3 How can I use {, }, [ or ] to specify multiple URLs?
52 4.4 Why do I get downloaded data even though the web page doesn't exist?
53 4.5 Why do I get return code XXX from a HTTP server?
54 4.5.1 "400 Bad Request"
55 4.5.2 "401 Unauthorized"
58 4.5.5 "405 Method Not Allowed"
59 4.5.6 "301 Moved Permanently"
60 4.6 Can you tell me what error code 142 means?
61 4.7 How do I keep user names and passwords secret in Curl command lines?
63 4.9 Curl can't authenticate to the server that requires NTLM?
64 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
65 4.11 Why does my HTTP range requests return the full document?
66 4.12 Why do I get "certificate verify failed" ?
69 5.1 Is libcurl thread-safe?
70 5.2 How can I receive all data into a large memory chunk?
71 5.3 How do I fetch multiple files with libcurl?
72 5.4 Does libcurl do Winsock initing on win32 systems?
73 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
74 5.6 What about Keep-Alive or persistent connections?
75 5.7 Link errors when building libcurl on Windows!
76 5.8 libcurl.so.3: open failed: No such file or directory
77 5.9 How does libcurl resolve host names?
78 5.10 How do I prevent libcurl from writing the response to stdout?
79 5.11 How do I make libcurl not receive the whole HTTP response?
82 6.1 I have a GPL program, can I use the libcurl library?
83 6.2 I have a closed-source program, can I use the libcurl library?
84 6.3 I have a BSD licensed program, can I use the libcurl library?
85 6.4 I have a program that uses LGPL libraries, can I use libcurl?
86 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
87 6.6 Can you please change the curl/libcurl license to XXXX?
91 7.2 Who write PHP/CURL?
92 7.3 Can I perform multiple requests using the same handle?
94 ==============================================================================
100 cURL is the name of the project. The name is a play on 'Client for URLs',
101 originally with URL spelled in uppercase to make it obvious it deals with
102 URLs. The fact it can also be pronounced 'see URL' also helped, it works as
103 an abbrivation for "Client URL Request Library" or why not the recursive
104 version: "Curl URL Request Library".
106 The cURL project produces two products:
110 A free and easy-to-use client-side URL transfer library, supporting FTP,
111 FTPS, HTTP, HTTPS, GOPHER, TELNET, DICT, FILE and LDAP. libcurl supports
112 HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading, kerberos, HTTP
113 form based upload, proxies, cookies, user+password authentication, file
114 transfer resume, http proxy tunneling and more!
116 libcurl is highly portable, it builds and works identically on numerous
117 platforms, including Solaris, NetBSD, FreeBSD, OpenBSD, Darwin, HPUX,
118 IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, BeOs, Mac
119 OS X, Ultrix, QNX, OpenVMS, RISC OS, Novell NetWare, DOS and more...
121 libcurl is free, thread-safe, IPv6 compatible, feature rich, well
126 A command line tool for getting or sending files using URL syntax.
128 Since curl uses libcurl, it supports a range of common Internet protocols,
129 currently including HTTP, HTTPS, FTP, FTPS, GOPHER, LDAP, DICT, TELNET and
132 We pronounce curl and cURL with an initial k sound: [kurl].
134 NOTE: there are numerous sub-projects and related projects that also use the
135 word curl in the project names in various combinations, but you should take
136 notice that this FAQ is directed at the command-line tool named curl (and
137 libcurl the library), and may therefore not be valid for other curl-related
142 libcurl is a reliable and portable library which provides you with an easy
143 interface to a range of common Internet protocols.
145 You can use libcurl for free in your application, be it open source,
146 commercial or closed-source.
148 1.3 What is curl not?
150 Curl is *not* a wget clone. That is a common misconception. Never, during
151 curl's development, have we intended curl to replace wget or compete on its
152 market. Curl is targeted at single-shot file transfers.
154 Curl is not a web site mirroring program. If you want to use curl to mirror
155 something: fine, go ahead and write a script that wraps around curl to make
156 it reality (like curlmirror.pl does).
158 Curl is not an FTP site mirroring program. Sure, get and send FTP with curl
159 but if you want systematic and sequential behavior you should write a
160 script (or write a new program that interfaces libcurl) and do it.
162 Curl is not a PHP tool, even though it works perfectly well when used from
163 or with PHP (when using the PHP/CURL module).
165 Curl is not a single-OS program. Curl exists, compiles, builds and runs
166 under a wide range of operating systems, including all modern Unixes (and a
167 bunch of older ones too), Windows, Amiga, BeOS, OS/2, OS X, QNX etc.
169 1.4 When will you make curl do XXXX ?
171 We love suggestions of what to change in order to make curl and libcurl
172 better. We do however believe in a few rules when it comes to the future of
175 * Curl -- the command line tool -- is to remain a non-graphical command line
176 tool. If you want GUIs or fancy scripting capabilities, you should look
177 for another tool that uses libcurl.
179 * We do not add things to curl that other small and available tools already
180 do very fine at the side. Curl's output is fine to pipe into another
181 program or redirect to another file for the next program to interpret.
183 * We focus on protocol related issues and improvements. If you wanna do more
184 magic with the supported protocols than curl currently does, chances are
185 big we will agree. If you wanna add more protocols, we may very well
188 * If you want someone else to make all the work while you wait for us to
189 implement it for you, that is not a very friendly attitude. We spend a
190 considerable time already on maintaining and developing curl. In order to
191 get more out of us, you should consider trading in some of your time and
194 * If you write the code, chances are bigger that it will get into curl
199 curl and libcurl are not made by any single individual. Sure, Daniel
200 Stenberg writes the major parts, but other persons' submissions are
201 important and crucial. Anyone can contribute and post their changes and
202 improvements and have them inserted in the main sources (of course on the
203 condition that developers agree on that the fixes are good).
205 The list of contributors in the docs/THANKS file is only a small part of all
206 the people that every day provide us with bug reports, suggestions, ideas
209 curl is developed by a community, with Daniel at the wheel.
211 1.6 What do you get for making curl?
213 Project cURL is entirely free and open. No person gets paid for developing
214 (lib)curl. We do this voluntarily on our spare time.
216 We get some help from companies. Contactor Data hosts the curl web site,
217 Haxx owns the curl web site's domain and sourceforge.net hosts project
218 services we take advantage from, like the bug tracker. Also, some companies
219 have sponsored certain parts of the development in the past and I hope some
220 will continue to do so in the future.
222 If you want to support our project with a donation or similar, one way of
223 doing that would be to buy "gift certificates" at useful online shopping
224 sites, such as amazon.com or thinkgeek.com. Another way would be to sponsor
225 us through a banner-program or even better: by helping us coding,
226 documenting, testing etc. You're welcome to send us a buck using paypal, as
227 described here: http://curl.haxx.se/donation.html
229 1.7 What about CURL from curl.com?
231 During the summer 2001, curl.com was busy advertising their client-side
232 programming language for the web, named CURL.
234 We are in no way associated with curl.com or their CURL programming
237 Our project name curl has been in effective use since 1998. We were not the
238 first computer related project to use the name "curl" and do not claim any
239 first-hand rights to the name.
241 We recognize that we will be living in parallel with curl.com and wish them
244 1.8 I have a problem who do I mail?
246 Please do not mail any single individual unless you really need to. Keep
247 curl-related questions on a suitable mailing list. All available mailing
248 lists are listed in the MANUAL document and online at
249 http://curl.haxx.se/mail/
251 Keeping curl-related questions and discussions on mailing lists allows
252 others to join in and help, to share their ideas, contribute their
253 suggestions and spread their wisdom. Keeping discussions on public mailing
254 lists also allows for others to learn from this (both current and future
255 users thanks to the web based archives of the mailing lists), thus saving us
256 from having to repeat ourselves even more. Thanks for respecting this.
259 2. Install Related Problems
261 2.1 configure doesn't find OpenSSL even when it is installed
263 This may be because of several reasons.
265 2.1.1 native linker doesn't find openssl
268 Solaris (native cc compiler)
269 HPUX (native cc compiler)
270 SGI IRIX (native cc compiler)
271 SCO UNIX (native cc compiler)
273 When configuring curl, I specify --with-ssl. OpenSSL is installed in
274 /usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
275 CRYPTO_lock in -lcrypto
277 Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
278 -lcrypto, so ld can't find the library. This is due to a bug in the GNU
281 Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
282 ./configure places the -L/usr/local/ssl/lib early enough in the command
283 line to make things work
285 Solution submitted by: Bob Allison <allisonb@users.sourceforge.net>
287 2.1.2 only the libssl lib is missing
289 If all include files and the libcrypto lib is present, with only the
290 libssl being missing according to configure, this is mostly likely because
291 a few functions are left out from the libssl.
293 If the function names missing include RSA or RSAREF you can be certain
294 that this is because libssl requires the RSA and RSAREF libs to build.
296 See the INSTALL file section that explains how to add those libs to
297 configure. Make sure that you remove the config.cache file before you
298 rerun configure with the new flags.
300 2.2 Does curl work/build with other SSL libraries?
302 Curl has been written to use OpenSSL, although there should not be much
303 problems using a different library. If anyone does "port" curl to use a
304 different SSL library, we are of course very interested in getting the
307 2.3 Where can I find a copy of LIBEAY32.DLL?
309 That is an OpenSSL binary built for Windows.
311 Curl uses OpenSSL to do the SSL stuff. The LIBEAY32.DLL is what curl needs
312 on a windows machine to do https://. Check out the curl web site to find
313 accurate and up-to-date pointers to recent OpenSSL DLLs and other binary
316 2.4 Does curl support Socks (RFC 1928) ?
318 Yes, SOCKS5 is supported.
323 3.1 curl: (1) SSL is disabled, https: not supported
325 If you get this output when trying to get anything from a https:// server,
326 it means that the configure script couldn't find all libs and include files
327 it requires for SSL to work. If the configure script fails to find them,
328 curl is simply built without SSL support.
330 To get the https:// support into a curl that was previously built but that
331 reports that https:// is not supported, you should dig through the document
332 and logs and check out why the configure script doesn't find the SSL libs
333 and/or include files.
335 Also, check out the other paragraph in this FAQ labeled "configure doesn't
336 find OpenSSL even when it is installed".
338 3.2 How do I tell curl to resume a transfer?
340 Curl supports resumed transfers both ways on both FTP and HTTP.
344 3.3 Why doesn't my posting using -F work?
346 You can't simply use -F or -d at your choice. The web server that will
347 receive your post assumes one of the formats. If the form you're trying to
348 "fake" sets the type to 'multipart/form-data', then and only then you must
349 use the -F type. In all the most common cases, you should use -d which then
350 causes a posting with the type 'application/x-www-form-urlencoded'.
352 This is described in some detail in the MANUAL and TheArtOfHttpScripting
353 documents, and if you don't understand it the first time, read it again
354 before you post questions about this to the mailing list. Also, try reading
355 through the mailing list archives for old postings and questions regarding
358 3.4 How do I tell curl to run custom FTP commands?
360 You can tell curl to perform optional commands both before and/or after a
361 file transfer. Study the -Q/--quote option.
363 Since curl is used for file transfers, you don't use curl to just perform
364 FTP commands without transferring anything. Therefore you must always specify
365 a URL to transfer to/from even when doing custom FTP commands.
367 3.5 How can I disable the Pragma: nocache header?
369 You can change all internally generated headers by adding a replacement with
370 the -H/--header option. By adding a header with empty contents you safely
371 disable that one. Use -H "Pragma:" to disable that specific header.
373 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
375 To curl, all contents are alike. It doesn't matter how the page was
376 generated. It may be ASP, PHP, Perl, shell-script, SSI or plain
377 HTML-files. There's no difference to curl and it doesn't even know what kind
378 of language that generated the page.
380 See also item 3.14 regarding javascript.
382 3.7 Can I use curl to delete/rename a file through FTP?
384 Yes. You specify custom FTP commands with -Q/--quote.
386 One example would be to delete a file after you have downloaded it:
388 curl -O ftp://download.com/coolfile -Q '-DELE coolfile'
390 3.8 How do I tell curl to follow HTTP redirects?
392 Curl does not follow so-called redirects by default. The Location: header
393 that informs the client about this is only interpreted if you're using the
394 -L/--location option. As in:
396 curl -L http://redirector.com
398 3.9 How do I use curl in my favorite programming language?
400 There exist many language interfaces/bindings for curl that integrates it
401 better with various languages. If you are fluid in a script language, you
402 may very well opt to use such an interface instead of using the command line
405 Find out more about which languages that support curl directly, and how to
406 install and use them, in the libcurl section of the curl web site:
407 http://curl.haxx.se/libcurl/
409 In February 2003, there are interfaces available for the following
410 languages: Basic, C, C++, Cocoa, Dylan, Euphoria, Java, Lua, Object-Pascal,
411 Pascal, Perl, PHP, PostgreSQL, Python, Rexx, Ruby, Scheme and Tcl. By the
412 time you read this, additional ones may have appeared!
414 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
416 Curl adheres to the HTTP spec, which basically means you can play with *any*
417 protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and
418 XML-RPC are all such ones. You can use -X to set custom requests and -H to
419 set custom headers (or replace internally generated ones).
421 Using libcurl is of course just as fine and you'd just use the proper
422 library options to do the same.
424 3.11 How do I POST with a different Content-Type?
426 You can always replace the internally generated headers with -H/--header.
427 To make a simple HTTP POST with text/xml as content-type, do something like:
429 curl -d "datatopost" -H "Content-Type: text/xml" [URL]
431 3.12 Why do FTP specific features over HTTP proxy fail?
433 Because when you use a HTTP proxy, the protocol spoken on the network will
434 be HTTP, even if you specify a FTP URL. This effectively means that you
435 normally can't use FTP specific features such as FTP upload and FTP quote
438 There is one exception to this rule, and that is if you can "tunnel through"
439 the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
440 and is generally not available as proxy admins usually disable tunneling to
441 other ports than 443 (which is used for HTTPS access through proxies).
443 3.13 Why does my single/double quotes fail?
445 To specify a command line option that includes spaces, you might need to
446 put the entire option within quotes. Like in:
448 curl -d " with spaces " url.com
452 curl -d ' with spaces ' url.com
454 Exactly what kind of quotes and how to do this is entirely up to the shell
455 or command line interpreter that you are using. For most unix shells, you
456 can more or less pick either single (') or double (") quotes. For
457 Windows/DOS prompts I believe you're forced to use double (") quotes.
459 Please study the documentation for your particular environment. Examples in
460 the curl docs will use a mix of both these ones as shown above. You must
461 adjust them to work in your environment.
463 Remember that curl works and runs on more operating systems than most single
464 individuals have ever tried.
466 3.14 Does curl support javascript or pac (automated proxy config)?
468 Many web pages do magic stuff using embedded javascript. Curl and libcurl
469 have no built-in support for that, so it will be treated just like any other
472 .pac files are a netscape invention and are sometimes used by organizations
473 to allow them to differentiate which proxies to use. The .pac contents is
474 just a javascript program that gets invoked by the browser and that returns
475 the name of the proxy to connect to. Since curl doesn't support javascript,
476 it can't support .pac proxy configuration either.
478 Some work-arounds usually suggested to overcome this javascript dependency:
480 - Depending on the javascript complexity, write up a script that
481 translates it to another language and execute that.
483 - Read the javascript code and rewrite the same logic in another language.
485 - Implement a javascript interpreter, people have successfully used the
486 Mozilla javascript engine in the past.
488 - Ask your admins to stop this, for a static proxy setup or similar.
490 3.15 Can I do recursive fetches with curl?
492 No. curl itself has no code that performs recursive operations, such as
493 those performed by wget and similar tools.
495 There exist wrapper scripts with that functionality (for example the
496 curlmirror perl script), and you can write programs based on libcurl to do
497 it, but the command line tool curl itself cannot.
499 3.16 What certificates do I need when I use SSL?
501 There are three different kinds of "certificates" to keep track of when we
502 talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.
504 - Client certificate. The server you communicate may require that you can
505 provide this in order to prove that you actually are who you claim to be.
506 If the server doesn't require this, you don't need a client certificate.
508 - Server certificate. The server you communicate with has a server
509 certificate. You can and should verify this certficate to make sure that
510 you are truly talking to the real server and not a server impersonating
511 it. The server certificate verifaction process is made by using a
512 Certificate Authority certificate ("CA cert") that was used to sign the
513 server certificate. Server certificate verification is enabled by default
514 in curl and libcurl and is often the reason for problems as explained in
515 FAQ entry 4.12 and the SSLCERTS document
516 (http://curl.haxx.se/docs/sslcerts.html). Server certificates that are
517 "self-signed" or otherwise signed by a CA that you do not have a CA cert
518 for, cannot be verified. If the verification during a connect fails, you
519 are refused access. You then need to explicitly disable the verification
520 to connect to the server.
522 - Certificate Authority certificate ("CA cert"). You often have several CA
523 certs in a CA cert bundle that can be used to verify a server certificate
524 that was signed by one of the authorities in the bundle. curl comes with a
525 default CA cert bundle. You can override the default.
527 3.17 How do I list the root dir of an FTP server?
529 There are two ways. The way defined in the RFC is to use an encoded slash
530 in the first path part. List the "/tmp" dir like this:
532 curl ftp://ftp.sunet.se/%2ftmp/
534 or the not-quite-kosher-but-more-readable way, by simply starting the path
535 section of the URL with a slash:
537 curl ftp://ftp.sunet.se//tmp/
539 3.18 Can I use curl to send a POST/PUT and not wait for a response?
546 4.1 Problems connecting to SSL servers.
548 It took a very long time before we could sort out why curl had problems to
549 connect to certain SSL servers when using SSLeay or OpenSSL v0.9+. The
550 error sometimes showed up similar to:
552 16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:
554 It turned out to be because many older SSL servers don't deal with SSLv3
555 requests properly. To correct this problem, tell curl to select SSLv2 from
556 the command line (-2/--sslv2).
558 There have also been examples where the remote server didn't like the SSLv2
559 request and instead you had to force curl to use SSLv3 with -3/--sslv3.
561 4.2 Why do I get problems when I use & or % in the URL?
563 In general unix shells, the & letter is treated special and when used, it
564 runs the specified command in the background. To safely send the & as a part
565 of a URL, you should quote the entire URL by using single (') or double (")
568 An example that would invoke a remote CGI that uses &-letters could be:
570 curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
572 In Windows, the standard DOS shell treats the %-letter specially and you
573 need to use TWO %-letters for each single one you want to use in the URL.
575 Also note that if you want the literal %-letter to be part of the data you
576 pass in a POST using -d/--data you must encode it as '%25' (which then also
577 needs the %-letter doubled on Windows machines).
579 4.3 How can I use {, }, [ or ] to specify multiple URLs?
581 Because those letters have a special meaning to the shell, and to be used in
582 a URL specified to curl you must quote them.
584 An example that downloads two URLs (sequentially) would do:
586 curl '{curl,www}.haxx.se'
588 To be able to use those letters as actual parts of the URL (without using
589 them for the curl URL "globbing" system), use the -g/--globoff option:
591 curl -g 'www.site.com/weirdname[].html'
593 4.4 Why do I get downloaded data even though the web page doesn't exist?
595 Curl asks remote servers for the page you specify. If the page doesn't exist
596 at the server, the HTTP protocol defines how the server should respond and
597 that means that headers and a "page" will be returned. That's simply how
600 By using the --fail option you can tell curl explicitly to not get any data
601 if the HTTP return code doesn't say success.
603 4.5 Why do I get return code XXX from a HTTP server?
605 RFC2616 clearly explains the return codes. This is a short transcript. Go
606 read the RFC for exact details:
608 4.5.1 "400 Bad Request"
610 The request could not be understood by the server due to malformed
611 syntax. The client SHOULD NOT repeat the request without modifications.
613 4.5.2 "401 Unauthorized"
615 The request requires user authentication.
617 4.5.3 "403 Forbidden"
619 The server understood the request, but is refusing to fulfill it.
620 Authorization will not help and the request SHOULD NOT be repeated.
622 4.5.4 "404 Not Found"
624 The server has not found anything matching the Request-URI. No indication
625 is given of whether the condition is temporary or permanent.
627 4.5.5 "405 Method Not Allowed"
629 The method specified in the Request-Line is not allowed for the resource
630 identified by the Request-URI. The response MUST include an Allow header
631 containing a list of valid methods for the requested resource.
633 4.5.6 "301 Moved Permanently"
635 If you get this return code and an HTML output similar to this:
637 <H1>Moved Permanently</H1> The document has moved <A
638 HREF="http://same_url_now_with_a_trailing_slash/">here</A>.
640 it might be because you request a directory URL but without the trailing
641 slash. Try the same operation again _with_ the trailing URL, or use the
642 -L/--location option to follow the redirection.
644 4.6 Can you tell me what error code 142 means?
646 All curl error codes are described at the end of the man page, in the
647 section called "EXIT CODES".
649 Error codes that are larger than the highest documented error code means
650 that curl has exited due to a crash. This is a serious error, and we
651 appreciate a detailed bug report from you that describes how we could go
652 ahead and repeat this!
654 4.7 How do I keep user names and passwords secret in Curl command lines?
656 This problem has two sides:
658 The first part is to avoid having clear-text passwords in the command line
659 so that they don't appear in 'ps' outputs and similar. That is easily
660 avoided by using the "-K" option to tell curl to read parameters from a file
661 or stdin to which you can pass the secret info. curl itself will also
662 attempt to "hide" the given password by blanking out the option - this
663 doesn't work on all platforms.
665 To keep the passwords in your account secret from the rest of the world is
666 not a task that curl addresses. You could of course encrypt them somehow to
667 at least hide them from being read by human eyes, but that is not what
668 anyone would call security.
670 Also note that regular HTTP (using Basic authentication) and FTP passwords
671 are sent in clear across the network. All it takes for anyone to fetch them
672 is to listen on the network. Eavesdropping is very easy. Use more secure
673 authentication methods (like Digest, Negotiate or even NTLM) or consider the
674 SSL-based alternatives HTTPS and FTPS.
678 It is not a bug if the behavior is documented. Read the docs first.
679 Especially check out the KNOWN_BUGS file, it may be a documented bug!
681 If it is a problem with a binary you've downloaded or a package for your
682 particular platform, try contacting the person who built the package/archive
685 If there is a bug, read the BUGS document first. Then report it as described
688 4.9 Curl can't authenticate to the server that requires NTLM?
690 This is supported in curl 7.10.6 or later. No earlier curl version knows
693 NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You
694 should not use such ones.
696 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
698 Many web servers allow or demand that the administrator configures the
699 server properly for these requests to work on the web server.
701 Some servers seem to support HEAD only on certain kinds of URLs.
703 To fully grasp this, try the documentation for the particular server
704 software you're trying to interact with. This is not anything curl can do
707 4.11 Why does my HTTP range requests return the full document?
709 Because the range may not be supported by the server, or the server may
710 choose to ignore it and return the full document anyway.
712 4.12 Why do I get "certificate verify failed" ?
714 You invoke curl 7.10 or later to communicate on a https:// URL and get an
715 error back looking something similar to this:
717 curl: (35) SSL: error:14090086:SSL routines:
718 SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
720 Then it means that curl couldn't verify that the server's certificate was
721 good. Curl verifies the certificate using the CA cert bundle that comes with
722 the curl installation.
724 To disable the verification (which makes it act like curl did before 7.10),
725 use -k. This does however enable man-in-the-middle attacks.
727 If you get this failure but are having a CA cert bundle installed and used,
728 the server's certificate is not signed by one of the CA's in the bundle. It
729 might for example be self-signed. You then correct this problem by obtaining
730 a valid CA cert for the server. Or again, decrease the security by disabling
733 Details are also in the SSLCERTS file in the release archives, found online
734 here: http://curl.haxx.se/docs/sslcerts.html
739 5.1 Is libcurl thread-safe?
743 We have written the libcurl code specificly adjusted for multi-threaded
744 programs. libcurl will use thread-safe functions instead of non-safe ones if
745 your system has such.
747 We would appreciate some kind of report or README file from those who have
748 used libcurl in a threaded environment.
750 5.2 How can I receive all data into a large memory chunk?
752 [ See also the examples/getinmemory.c source ]
754 You are in full control of the callback function that gets called every time
755 there is data received from the remote server. You can make that callback do
756 whatever you want. You do not have to write the received data to a file.
758 One solution to this problem could be to have a pointer to a struct that you
759 pass to the callback function. You set the pointer using the
760 curl_easy_setopt(CURLOPT_FILE) function. Then that pointer will be passed to
761 the callback instead of a FILE * to a file:
763 /* imaginary struct */
764 struct MemoryStruct {
769 /* imaginary callback function */
771 WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
773 size_t realsize = size * nmemb;
774 struct MemoryStruct *mem = (struct MemoryStruct *)data;
776 mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
778 memcpy(&(mem->memory[mem->size]), ptr, realsize);
779 mem->size += realsize;
780 mem->memory[mem->size] = 0;
785 5.3 How do I fetch multiple files with libcurl?
787 libcurl has excellent support for transferring multiple files. You should
788 just repeatedly set new URLs with curl_easy_setopt() and then transfer it
789 with curl_easy_perform(). The handle you get from curl_easy_init() is not
790 only reusable, but you're even encouraged to reuse it if you can, as that
791 will enable libcurl to use persistent connections.
793 5.4 Does libcurl do Winsock initialization on win32 systems?
795 Yes, if told to in the curl_global_init() call.
797 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
799 Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
800 that DLL use the FILE * (as the DLL and the client application cannot access
801 each others' variable memory areas). If you set CURLOPT_WRITEDATA you must
802 also use CURLOPT_WRITEFUNCTION as well to set a function that writes the
803 file, even if that simply writes the data to the specified FILE *.
804 Similarly, if you use CURLOPT_READDATA you must also specify
805 CURLOPT_READFUNCTION.
807 (Provided by Joel DeYoung and Bob Schader)
809 5.6 What about Keep-Alive or persistent connections?
811 curl and libcurl have excellent support for persistent connections when
812 transferring several files from the same server. Curl will attempt to reuse
813 connections for all URLs specified on the same command line/config file, and
814 libcurl will reuse connections for all transfers that are made using the
817 5.7 Link errors when building libcurl on Windows!
819 You need to make sure that your project, and all the libraries (both static
820 and dynamic) that it links against, are compiled/linked against the same run
823 This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
824 options to the command line compiler. /MD (linking against MSVCRT dll) seems
825 to be the most commonly used option.
827 (Provided by Andrew Francis)
829 When building an application that uses the static libcurl library, you must
830 add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for
831 dynamic import symbols. If you get linker error like "unknown symbol
832 __imp__curl_easy_init ..." you have linked against the wrong (static)
833 library. If you want to use the libcurl.dll and import lib, you don't need
834 any extra CFLAGS, but use one of the import libraries below. These are the
835 libraries produced by the various lib/Makefile.* files:
837 Target: static lib. import lib for libcurl*.dll.
838 -----------------------------------------------------------
839 MingW: libcurl.a libcurldll.a
840 MSVC (release): libcurl.lib libcurl_imp.lib
841 MSVC (debug): libcurld.lib libcurld_imp.lib
842 Borland: libcurl.lib libcurl_imp.lib
845 5.8 libcurl.so.3: open failed: No such file or directory
847 This is an error message you might get when you try to run a program linked
848 with a shared version of libcurl and your run-time linker (ld.so) couldn't
849 find the shared library named libcurl.so.3.
851 You need to make sure that ld.so finds libcurl.so.3. You can do that
852 multiple ways, and it differs somewhat between different operating systems,
853 but they are usually:
855 * Add an option to the linker command line that specify the hard-coded path
856 the run-time linker should check for the lib (usually -R)
858 * Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
859 should check for libs
861 * Adjust the system's config to check for libs in the directory where you've
862 put the dir (like Linux's /etc/ld.so.conf)
864 'man ld.so' and 'man ld' will tell you more details
866 5.9 How does libcurl resolve host names?
868 libcurl includes a number of different name resolve functions:
870 - The non-ipv6 resolver that can use one out of four host name resolve calls
871 (depending on what your system supports):
874 B - gethostbyname_r() with 3 arguments
875 C - gethostbyname_r() with 5 arguments
876 D - gethostbyname_r() with 6 arguments
878 - The ipv6-resolver that uses getaddrinfo()
880 - The c-ares based name resolver that uses the c-ares library for resolves.
882 - The Windows threaded resolver. It use:
884 A - gethostbyname() on plain ipv4 windows hosts
885 B - getaddrinfo() on ipv6-enabled windows hosts
887 Also note that libcurl never resolves or reverse-lookups addresses given as
888 pure numbers, such as 127.0.0.1 or ::1.
890 5.10 How do I prevent libcurl from writing the response to stdout?
892 libcurl provides a default built-in write function that writes received data
893 to stdout. Set a WRITEFUNCTION to receive the data, or possibly set
894 WRITEDATA to a different FILE * handle.
896 5.11 How do I make libcurl not receive the whole HTTP response?
898 You make the write callback (or progress callback) return an error and
899 libcurl will then abort the transfer.
904 Curl and libcurl are released under a MIT/X derivate license. The license is
905 very liberal and should not impose a problem for your project. This section
906 is just a brief summary for the cases we get the most questions. (Parts of
907 this section was much enhanced by Bjorn Reese.)
909 6.1 I have a GPL program, can I use the libcurl library?
913 Since libcurl may be distributed under the MIT/X derivate license, it can be
914 used together with GPL in any software.
916 6.2 I have a closed-source program, can I use the libcurl library?
920 libcurl does not put any restrictions on the program that uses the library.
922 6.3 I have a BSD licensed program, can I use the libcurl library?
926 libcurl does not put any restrictions on the program that uses the library.
928 6.4 I have a program that uses LGPL libraries, can I use libcurl?
932 The LGPL license doesn't clash with other licenses.
934 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
938 The MIT/X derivate license practically allows you to do almost anything with
939 the sources, on the condition that the copyright texts in the sources are
942 6.6 Can you please change the curl/libcurl license to XXXX?
946 We have carefully picked this license after years of development and
947 discussions and a large amount of people have contributed with source code
948 knowing that this is the license we use. This license puts the restrictions
949 we want on curl/libcurl and it does not spread to other programs or
950 libraries that use it. It should be possible for everyone to use libcurl or
951 curl in their projects, no matter what license they already have in use.
955 7.1 What is PHP/CURL?
957 The module for PHP that makes it possible for PHP programs to access curl-
958 functions from within PHP. We often call it PHP/CURL to differentiate from
959 curl the command line tool and libcurl the library.
961 7.2 Who write PHP/CURL?
963 PHP/CURL is a module that comes with the regular PHP package. It depends and
964 uses libcurl, so you need to have libcurl installed properly first before
965 PHP/CURL can be used. PHP/CURL is written by Sterling Hughes.
967 7.3 Can I perform multiple requests using the same handle?
969 Yes - at least in PHP version 4.3.8 and later (this has been known to not
970 work in earlier versions, but the exact version when it started to work is
973 After a transfer, you just set new options in the handle and make another
974 transfer. This will make libcurl to re-use the same connection if it can.
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