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?
81 6.1 I have a GPL program, can I use the libcurl library?
82 6.2 I have a closed-source program, can I use the libcurl library?
83 6.3 I have a BSD licensed program, can I use the libcurl library?
84 6.4 I have a program that uses LGPL libraries, can I use libcurl?
85 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
86 6.6 Can you please change the curl/libcurl license to XXXX?
90 7.2 Who write PHP/CURL?
91 7.3 Can I perform multiple requests using the same handle?
93 ==============================================================================
99 cURL is the name of the project. The name is a play on 'Client for URLs',
100 originally with URL spelled in uppercase to make it obvious it deals with
101 URLs. The fact it can also be pronounced 'see URL' also helped, it works as
102 an abbrivation for "Client URL Request Library" or why not the recursive
103 version: "Curl URL Request Library".
105 The cURL project produces two products:
109 A free and easy-to-use client-side URL transfer library, supporting FTP,
110 FTPS, HTTP, HTTPS, GOPHER, TELNET, DICT, FILE and LDAP. libcurl supports
111 HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading, kerberos, HTTP
112 form based upload, proxies, cookies, user+password authentication, file
113 transfer resume, http proxy tunneling and more!
115 libcurl is highly portable, it builds and works identically on numerous
116 platforms, including Solaris, NetBSD, FreeBSD, OpenBSD, Darwin, HPUX,
117 IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, BeOs, Mac
118 OS X, Ultrix, QNX, OpenVMS, RISC OS, Novell NetWare, DOS and more...
120 libcurl is free, thread-safe, IPv6 compatible, feature rich, well
125 A command line tool for getting or sending files using URL syntax.
127 Since curl uses libcurl, it supports a range of common Internet protocols,
128 currently including HTTP, HTTPS, FTP, FTPS, GOPHER, LDAP, DICT, TELNET and
131 We pronounce curl and cURL with an initial k sound: [kurl].
133 NOTE: there are numerous sub-projects and related projects that also use the
134 word curl in the project names in various combinations, but you should take
135 notice that this FAQ is directed at the command-line tool named curl (and
136 libcurl the library), and may therefore not be valid for other curl-related
141 libcurl is a reliable and portable library which provides you with an easy
142 interface to a range of common Internet protocols.
144 You can use libcurl for free in your application, be it open source,
145 commercial or closed-source.
147 1.3 What is curl not?
149 Curl is *not* a wget clone. That is a common misconception. Never, during
150 curl's development, have we intended curl to replace wget or compete on its
151 market. Curl is targeted at single-shot file transfers.
153 Curl is not a web site mirroring program. If you want to use curl to mirror
154 something: fine, go ahead and write a script that wraps around curl to make
155 it reality (like curlmirror.pl does).
157 Curl is not an FTP site mirroring program. Sure, get and send FTP with curl
158 but if you want systematic and sequential behavior you should write a
159 script (or write a new program that interfaces libcurl) and do it.
161 Curl is not a PHP tool, even though it works perfectly well when used from
162 or with PHP (when using the PHP/CURL module).
164 Curl is not a single-OS program. Curl exists, compiles, builds and runs
165 under a wide range of operating systems, including all modern Unixes (and a
166 bunch of older ones too), Windows, Amiga, BeOS, OS/2, OS X, QNX etc.
168 1.4 When will you make curl do XXXX ?
170 We love suggestions of what to change in order to make curl and libcurl
171 better. We do however believe in a few rules when it comes to the future of
174 * Curl -- the command line tool -- is to remain a non-graphical command line
175 tool. If you want GUIs or fancy scripting capabilities, you should look
176 for another tool that uses libcurl.
178 * We do not add things to curl that other small and available tools already
179 do very fine at the side. Curl's output is fine to pipe into another
180 program or redirect to another file for the next program to interpret.
182 * We focus on protocol related issues and improvements. If you wanna do more
183 magic with the supported protocols than curl currently does, chances are
184 big we will agree. If you wanna add more protocols, we may very well
187 * If you want someone else to make all the work while you wait for us to
188 implement it for you, that is not a very friendly attitude. We spend a
189 considerable time already on maintaining and developing curl. In order to
190 get more out of us, you should consider trading in some of your time and
193 * If you write the code, chances are bigger that it will get into curl
198 curl and libcurl are not made by any single individual. Sure, Daniel
199 Stenberg writes the major parts, but other persons' submissions are
200 important and crucial. Anyone can contribute and post their changes and
201 improvements and have them inserted in the main sources (of course on the
202 condition that developers agree on that the fixes are good).
204 The list of contributors in the docs/THANKS file is only a small part of all
205 the people that every day provide us with bug reports, suggestions, ideas
208 curl is developed by a community, with Daniel at the wheel.
210 1.6 What do you get for making curl?
212 Project cURL is entirely free and open. No person gets paid for developing
213 (lib)curl. We do this voluntarily on our spare time.
215 We get some help from companies. Contactor Data hosts the curl web site,
216 Haxx owns the curl web site's domain and sourceforge.net hosts project
217 services we take advantage from, like the bug tracker. Also, some companies
218 have sponsored certain parts of the development in the past and I hope some
219 will continue to do so in the future.
221 If you want to support our project with a donation or similar, one way of
222 doing that would be to buy "gift certificates" at useful online shopping
223 sites, such as amazon.com or thinkgeek.com. Another way would be to sponsor
224 us through a banner-program or even better: by helping us coding,
225 documenting, testing etc. You're welcome to send us a buck using paypal, as
226 described here: http://curl.haxx.se/donation.html
228 1.7 What about CURL from curl.com?
230 During the summer 2001, curl.com was busy advertising their client-side
231 programming language for the web, named CURL.
233 We are in no way associated with curl.com or their CURL programming
236 Our project name curl has been in effective use since 1998. We were not the
237 first computer related project to use the name "curl" and do not claim any
238 first-hand rights to the name.
240 We recognize that we will be living in parallel with curl.com and wish them
243 1.8 I have a problem who do I mail?
245 Please do not mail any single individual unless you really need to. Keep
246 curl-related questions on a suitable mailing list. All available mailing
247 lists are listed in the MANUAL document and online at
248 http://curl.haxx.se/mail/
250 Keeping curl-related questions and discussions on mailing lists allows
251 others to join in and help, to share their ideas, contribute their
252 suggestions and spread their wisdom. Keeping discussions on public mailing
253 lists also allows for others to learn from this (both current and future
254 users thanks to the web based archives of the mailing lists), thus saving us
255 from having to repeat ourselves even more. Thanks for respecting this.
258 2. Install Related Problems
260 2.1 configure doesn't find OpenSSL even when it is installed
262 This may be because of several reasons.
264 2.1.1 native linker doesn't find openssl
267 Solaris (native cc compiler)
268 HPUX (native cc compiler)
269 SGI IRIX (native cc compiler)
270 SCO UNIX (native cc compiler)
272 When configuring curl, I specify --with-ssl. OpenSSL is installed in
273 /usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
274 CRYPTO_lock in -lcrypto
276 Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
277 -lcrypto, so ld can't find the library. This is due to a bug in the GNU
280 Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
281 ./configure places the -L/usr/local/ssl/lib early enough in the command
282 line to make things work
284 Solution submitted by: Bob Allison <allisonb@users.sourceforge.net>
286 2.1.2 only the libssl lib is missing
288 If all include files and the libcrypto lib is present, with only the
289 libssl being missing according to configure, this is mostly likely because
290 a few functions are left out from the libssl.
292 If the function names missing include RSA or RSAREF you can be certain
293 that this is because libssl requires the RSA and RSAREF libs to build.
295 See the INSTALL file section that explains how to add those libs to
296 configure. Make sure that you remove the config.cache file before you
297 rerun configure with the new flags.
299 2.2 Does curl work/build with other SSL libraries?
301 Curl has been written to use OpenSSL, although there should not be much
302 problems using a different library. If anyone does "port" curl to use a
303 different SSL library, we are of course very interested in getting the
306 2.3 Where can I find a copy of LIBEAY32.DLL?
308 That is an OpenSSL binary built for Windows.
310 Curl uses OpenSSL to do the SSL stuff. The LIBEAY32.DLL is what curl needs
311 on a windows machine to do https://. Check out the curl web site to find
312 accurate and up-to-date pointers to recent OpenSSL DLLs and other binary
315 2.4 Does curl support Socks (RFC 1928) ?
317 Yes, SOCKS5 is supported.
322 3.1 curl: (1) SSL is disabled, https: not supported
324 If you get this output when trying to get anything from a https:// server,
325 it means that the configure script couldn't find all libs and include files
326 it requires for SSL to work. If the configure script fails to find them,
327 curl is simply built without SSL support.
329 To get the https:// support into a curl that was previously built but that
330 reports that https:// is not supported, you should dig through the document
331 and logs and check out why the configure script doesn't find the SSL libs
332 and/or include files.
334 Also, check out the other paragraph in this FAQ labeled "configure doesn't
335 find OpenSSL even when it is installed".
337 3.2 How do I tell curl to resume a transfer?
339 Curl supports resumed transfers both ways on both FTP and HTTP.
343 3.3 Why doesn't my posting using -F work?
345 You can't simply use -F or -d at your choice. The web server that will
346 receive your post assumes one of the formats. If the form you're trying to
347 "fake" sets the type to 'multipart/form-data', then and only then you must
348 use the -F type. In all the most common cases, you should use -d which then
349 causes a posting with the type 'application/x-www-form-urlencoded'.
351 This is described in some detail in the MANUAL and TheArtOfHttpScripting
352 documents, and if you don't understand it the first time, read it again
353 before you post questions about this to the mailing list. Also, try reading
354 through the mailing list archives for old postings and questions regarding
357 3.4 How do I tell curl to run custom FTP commands?
359 You can tell curl to perform optional commands both before and/or after a
360 file transfer. Study the -Q/--quote option.
362 Since curl is used for file transfers, you don't use curl to just perform
363 FTP commands without transferring anything. Therefore you must always specify
364 a URL to transfer to/from even when doing custom FTP commands.
366 3.5 How can I disable the Pragma: nocache header?
368 You can change all internally generated headers by adding a replacement with
369 the -H/--header option. By adding a header with empty contents you safely
370 disable that one. Use -H "Pragma:" to disable that specific header.
372 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
374 To curl, all contents are alike. It doesn't matter how the page was
375 generated. It may be ASP, PHP, Perl, shell-script, SSI or plain
376 HTML-files. There's no difference to curl and it doesn't even know what kind
377 of language that generated the page.
379 See also item 3.14 regarding javascript.
381 3.7 Can I use curl to delete/rename a file through FTP?
383 Yes. You specify custom FTP commands with -Q/--quote.
385 One example would be to delete a file after you have downloaded it:
387 curl -O ftp://download.com/coolfile -Q '-DELE coolfile'
389 3.8 How do I tell curl to follow HTTP redirects?
391 Curl does not follow so-called redirects by default. The Location: header
392 that informs the client about this is only interpreted if you're using the
393 -L/--location option. As in:
395 curl -L http://redirector.com
397 3.9 How do I use curl in my favorite programming language?
399 There exist many language interfaces/bindings for curl that integrates it
400 better with various languages. If you are fluid in a script language, you
401 may very well opt to use such an interface instead of using the command line
404 Find out more about which languages that support curl directly, and how to
405 install and use them, in the libcurl section of the curl web site:
406 http://curl.haxx.se/libcurl/
408 In February 2003, there are interfaces available for the following
409 languages: Basic, C, C++, Cocoa, Dylan, Euphoria, Java, Lua, Object-Pascal,
410 Pascal, Perl, PHP, PostgreSQL, Python, Rexx, Ruby, Scheme and Tcl. By the
411 time you read this, additional ones may have appeared!
413 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
415 Curl adheres to the HTTP spec, which basically means you can play with *any*
416 protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and
417 XML-RPC are all such ones. You can use -X to set custom requests and -H to
418 set custom headers (or replace internally generated ones).
420 Using libcurl is of course just as fine and you'd just use the proper
421 library options to do the same.
423 3.11 How do I POST with a different Content-Type?
425 You can always replace the internally generated headers with -H/--header.
426 To make a simple HTTP POST with text/xml as content-type, do something like:
428 curl -d "datatopost" -H "Content-Type: text/xml" [URL]
430 3.12 Why do FTP specific features over HTTP proxy fail?
432 Because when you use a HTTP proxy, the protocol spoken on the network will
433 be HTTP, even if you specify a FTP URL. This effectively means that you
434 normally can't use FTP specific features such as FTP upload and FTP quote
437 There is one exception to this rule, and that is if you can "tunnel through"
438 the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
439 and is generally not available as proxy admins usually disable tunneling to
440 other ports than 443 (which is used for HTTPS access through proxies).
442 3.13 Why does my single/double quotes fail?
444 To specify a command line option that includes spaces, you might need to
445 put the entire option within quotes. Like in:
447 curl -d " with spaces " url.com
451 curl -d ' with spaces ' url.com
453 Exactly what kind of quotes and how to do this is entirely up to the shell
454 or command line interpreter that you are using. For most unix shells, you
455 can more or less pick either single (') or double (") quotes. For
456 Windows/DOS prompts I believe you're forced to use double (") quotes.
458 Please study the documentation for your particular environment. Examples in
459 the curl docs will use a mix of both these ones as shown above. You must
460 adjust them to work in your environment.
462 Remember that curl works and runs on more operating systems than most single
463 individuals have ever tried.
465 3.14 Does curl support javascript or pac (automated proxy config)?
467 Many web pages do magic stuff using embedded javascript. Curl and libcurl
468 have no built-in support for that, so it will be treated just like any other
471 .pac files are a netscape invention and are sometimes used by organizations
472 to allow them to differentiate which proxies to use. The .pac contents is
473 just a javascript program that gets invoked by the browser and that returns
474 the name of the proxy to connect to. Since curl doesn't support javascript,
475 it can't support .pac proxy configuration either.
477 Some work-arounds usually suggested to overcome this javascript dependency:
479 - Depending on the javascript complexity, write up a script that
480 translates it to another language and execute that.
482 - Read the javascript code and rewrite the same logic in another language.
484 - Implement a javascript interpreter, people have successfully used the
485 Mozilla javascript engine in the past.
487 - Ask your admins to stop this, for a static proxy setup or similar.
489 3.15 Can I do recursive fetches with curl?
491 No. curl itself has no code that performs recursive operations, such as
492 those performed by wget and similar tools.
494 There exist wrapper scripts with that functionality (for example the
495 curlmirror perl script), and you can write programs based on libcurl to do
496 it, but the command line tool curl itself cannot.
498 3.16 What certificates do I need when I use SSL?
500 There are three different kinds of "certificates" to keep track of when we
501 talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.
503 - Client certificate. The server you communicate may require that you can
504 provide this in order to prove that you actually are who you claim to be.
505 If the server doesn't require this, you don't need a client certificate.
507 - Server certificate. The server you communicate with has a server
508 certificate. You can and should verify this certficate to make sure that
509 you are truly talking to the real server and not a server impersonating
510 it. The server certificate verifaction process is made by using a
511 Certificate Authority certificate ("CA cert") that was used to sign the
512 server certificate. Server certificate verification is enabled by default
513 in curl and libcurl and is often the reason for problems as explained in
514 FAQ entry 4.12 and the SSLCERTS document
515 (http://curl.haxx.se/docs/sslcerts.html). Server certificates that are
516 "self-signed" or otherwise signed by a CA that you do not have a CA cert
517 for, cannot be verified. If the verification during a connect fails, you
518 are refused access. You then need to explicitly disable the verification
519 to connect to the server.
521 - Certificate Authority certificate ("CA cert"). You often have several CA
522 certs in a CA cert bundle that can be used to verify a server certificate
523 that was signed by one of the authorities in the bundle. curl comes with a
524 default CA cert bundle. You can override the default.
526 3.17 How do I list the root dir of an FTP server?
528 There are two ways. The way defined in the RFC is to use an encoded slash
529 in the first path part. List the "/tmp" dir like this:
531 curl ftp://ftp.sunet.se/%2ftmp/
533 or the not-quite-kosher-but-more-readable way, by simply starting the path
534 section of the URL with a slash:
536 curl ftp://ftp.sunet.se//tmp/
538 3.18 Can I use curl to send a POST/PUT and not wait for a response?
545 4.1 Problems connecting to SSL servers.
547 It took a very long time before we could sort out why curl had problems to
548 connect to certain SSL servers when using SSLeay or OpenSSL v0.9+. The
549 error sometimes showed up similar to:
551 16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:
553 It turned out to be because many older SSL servers don't deal with SSLv3
554 requests properly. To correct this problem, tell curl to select SSLv2 from
555 the command line (-2/--sslv2).
557 There have also been examples where the remote server didn't like the SSLv2
558 request and instead you had to force curl to use SSLv3 with -3/--sslv3.
560 4.2 Why do I get problems when I use & or % in the URL?
562 In general unix shells, the & letter is treated special and when used, it
563 runs the specified command in the background. To safely send the & as a part
564 of a URL, you should quote the entire URL by using single (') or double (")
567 An example that would invoke a remote CGI that uses &-letters could be:
569 curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
571 In Windows, the standard DOS shell treats the %-letter specially and you
572 need to use TWO %-letters for each single one you want to use in the URL.
574 Also note that if you want the literal %-letter to be part of the data you
575 pass in a POST using -d/--data you must encode it as '%25' (which then also
576 needs the %-letter doubled on Windows machines).
578 4.3 How can I use {, }, [ or ] to specify multiple URLs?
580 Because those letters have a special meaning to the shell, and to be used in
581 a URL specified to curl you must quote them.
583 An example that downloads two URLs (sequentially) would do:
585 curl '{curl,www}.haxx.se'
587 To be able to use those letters as actual parts of the URL (without using
588 them for the curl URL "globbing" system), use the -g/--globoff option:
590 curl -g 'www.site.com/weirdname[].html'
592 4.4 Why do I get downloaded data even though the web page doesn't exist?
594 Curl asks remote servers for the page you specify. If the page doesn't exist
595 at the server, the HTTP protocol defines how the server should respond and
596 that means that headers and a "page" will be returned. That's simply how
599 By using the --fail option you can tell curl explicitly to not get any data
600 if the HTTP return code doesn't say success.
602 4.5 Why do I get return code XXX from a HTTP server?
604 RFC2616 clearly explains the return codes. This is a short transcript. Go
605 read the RFC for exact details:
607 4.5.1 "400 Bad Request"
609 The request could not be understood by the server due to malformed
610 syntax. The client SHOULD NOT repeat the request without modifications.
612 4.5.2 "401 Unauthorized"
614 The request requires user authentication.
616 4.5.3 "403 Forbidden"
618 The server understood the request, but is refusing to fulfill it.
619 Authorization will not help and the request SHOULD NOT be repeated.
621 4.5.4 "404 Not Found"
623 The server has not found anything matching the Request-URI. No indication
624 is given of whether the condition is temporary or permanent.
626 4.5.5 "405 Method Not Allowed"
628 The method specified in the Request-Line is not allowed for the resource
629 identified by the Request-URI. The response MUST include an Allow header
630 containing a list of valid methods for the requested resource.
632 4.5.6 "301 Moved Permanently"
634 If you get this return code and an HTML output similar to this:
636 <H1>Moved Permanently</H1> The document has moved <A
637 HREF="http://same_url_now_with_a_trailing_slash/">here</A>.
639 it might be because you request a directory URL but without the trailing
640 slash. Try the same operation again _with_ the trailing URL, or use the
641 -L/--location option to follow the redirection.
643 4.6 Can you tell me what error code 142 means?
645 All curl error codes are described at the end of the man page, in the
646 section called "EXIT CODES".
648 Error codes that are larger than the highest documented error code means
649 that curl has exited due to a crash. This is a serious error, and we
650 appreciate a detailed bug report from you that describes how we could go
651 ahead and repeat this!
653 4.7 How do I keep user names and passwords secret in Curl command lines?
655 This problem has two sides:
657 The first part is to avoid having clear-text passwords in the command line
658 so that they don't appear in 'ps' outputs and similar. That is easily
659 avoided by using the "-K" option to tell curl to read parameters from a file
660 or stdin to which you can pass the secret info. curl itself will also
661 attempt to "hide" the given password by blanking out the option - this
662 doesn't work on all platforms.
664 To keep the passwords in your account secret from the rest of the world is
665 not a task that curl addresses. You could of course encrypt them somehow to
666 at least hide them from being read by human eyes, but that is not what
667 anyone would call security.
669 Also note that regular HTTP (using Basic authentication) and FTP passwords
670 are sent in clear across the network. All it takes for anyone to fetch them
671 is to listen on the network. Eavesdropping is very easy. Use more secure
672 authentication methods (like Digest, Negotiate or even NTLM) or consider the
673 SSL-based alternatives HTTPS and FTPS.
677 It is not a bug if the behavior is documented. Read the docs first.
678 Especially check out the KNOWN_BUGS file, it may be a documented bug!
680 If it is a problem with a binary you've downloaded or a package for your
681 particular platform, try contacting the person who built the package/archive
684 If there is a bug, read the BUGS document first. Then report it as described
687 4.9 Curl can't authenticate to the server that requires NTLM?
689 This is supported in curl 7.10.6 or later. No earlier curl version knows
692 NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You
693 should not use such ones.
695 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
697 Many web servers allow or demand that the administrator configures the
698 server properly for these requests to work on the web server.
700 Some servers seem to support HEAD only on certain kinds of URLs.
702 To fully grasp this, try the documentation for the particular server
703 software you're trying to interact with. This is not anything curl can do
706 4.11 Why does my HTTP range requests return the full document?
708 Because the range may not be supported by the server, or the server may
709 choose to ignore it and return the full document anyway.
711 4.12 Why do I get "certificate verify failed" ?
713 You invoke curl 7.10 or later to communicate on a https:// URL and get an
714 error back looking something similar to this:
716 curl: (35) SSL: error:14090086:SSL routines:
717 SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
719 Then it means that curl couldn't verify that the server's certificate was
720 good. Curl verifies the certificate using the CA cert bundle that comes with
721 the curl installation.
723 To disable the verification (which makes it act like curl did before 7.10),
724 use -k. This does however enable man-in-the-middle attacks.
726 If you get this failure but are having a CA cert bundle installed and used,
727 the server's certificate is not signed by one of the CA's in the bundle. It
728 might for example be self-signed. You then correct this problem by obtaining
729 a valid CA cert for the server. Or again, decrease the security by disabling
732 Details are also in the SSLCERTS file in the release archives, found online
733 here: http://curl.haxx.se/docs/sslcerts.html
738 5.1 Is libcurl thread-safe?
742 We have written the libcurl code specificly adjusted for multi-threaded
743 programs. libcurl will use thread-safe functions instead of non-safe ones if
744 your system has such.
746 We would appreciate some kind of report or README file from those who have
747 used libcurl in a threaded environment.
749 5.2 How can I receive all data into a large memory chunk?
751 [ See also the examples/getinmemory.c source ]
753 You are in full control of the callback function that gets called every time
754 there is data received from the remote server. You can make that callback do
755 whatever you want. You do not have to write the received data to a file.
757 One solution to this problem could be to have a pointer to a struct that you
758 pass to the callback function. You set the pointer using the
759 curl_easy_setopt(CURLOPT_FILE) function. Then that pointer will be passed to
760 the callback instead of a FILE * to a file:
762 /* imaginary struct */
763 struct MemoryStruct {
768 /* imaginary callback function */
770 WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
772 size_t realsize = size * nmemb;
773 struct MemoryStruct *mem = (struct MemoryStruct *)data;
775 mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
777 memcpy(&(mem->memory[mem->size]), ptr, realsize);
778 mem->size += realsize;
779 mem->memory[mem->size] = 0;
784 5.3 How do I fetch multiple files with libcurl?
786 libcurl has excellent support for transferring multiple files. You should
787 just repeatedly set new URLs with curl_easy_setopt() and then transfer it
788 with curl_easy_perform(). The handle you get from curl_easy_init() is not
789 only reusable, but you're even encouraged to reuse it if you can, as that
790 will enable libcurl to use persistent connections.
792 5.4 Does libcurl do Winsock initialization on win32 systems?
794 Yes, if told to in the curl_global_init() call.
796 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
798 Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
799 that DLL use the FILE * (as the DLL and the client application cannot access
800 each others' variable memory areas). If you set CURLOPT_WRITEDATA you must
801 also use CURLOPT_WRITEFUNCTION as well to set a function that writes the
802 file, even if that simply writes the data to the specified FILE *.
803 Similarly, if you use CURLOPT_READDATA you must also specify
804 CURLOPT_READFUNCTION.
806 (Provided by Joel DeYoung and Bob Schader)
808 5.6 What about Keep-Alive or persistent connections?
810 curl and libcurl have excellent support for persistent connections when
811 transferring several files from the same server. Curl will attempt to reuse
812 connections for all URLs specified on the same command line/config file, and
813 libcurl will reuse connections for all transfers that are made using the
816 5.7 Link errors when building libcurl on Windows!
818 You need to make sure that your project, and all the libraries (both static
819 and dynamic) that it links against, are compiled/linked against the same run
822 This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
823 options to the command line compiler. /MD (linking against MSVCRT dll) seems
824 to be the most commonly used option.
826 (Provided by Andrew Francis)
828 When building an application that uses the static libcurl library, you must
829 add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for
830 dynamic import symbols. If you get linker error like "unknown symbol
831 __imp__curl_easy_init ..." you have linked against the wrong (static)
832 library. If you want to use the libcurl.dll and import lib, you don't need
833 any extra CFLAGS, but use one of the import libraries below. These are the
834 libraries produced by the various lib/Makefile.* files:
836 Target: static lib. import lib for libcurl*.dll.
837 -----------------------------------------------------------
838 MingW: libcurl.a libcurldll.a
839 MSVC (release): libcurl.lib libcurl_imp.lib
840 MSVC (debug): libcurld.lib libcurld_imp.lib
841 Borland: libcurl.lib libcurl_imp.lib
844 5.8 libcurl.so.3: open failed: No such file or directory
846 This is an error message you might get when you try to run a program linked
847 with a shared version of libcurl and your run-time linker (ld.so) couldn't
848 find the shared library named libcurl.so.3.
850 You need to make sure that ld.so finds libcurl.so.3. You can do that
851 multiple ways, and it differs somewhat between different operating systems,
852 but they are usually:
854 * Add an option to the linker command line that specify the hard-coded path
855 the run-time linker should check for the lib (usually -R)
857 * Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
858 should check for libs
860 * Adjust the system's config to check for libs in the directory where you've
861 put the dir (like Linux's /etc/ld.so.conf)
863 'man ld.so' and 'man ld' will tell you more details
865 5.9 How does libcurl resolve host names?
867 libcurl includes a number of different name resolve functions:
869 - The non-ipv6 resolver that can use one out of four host name resolve calls
870 (depending on what your system supports):
873 B - gethostbyname_r() with 3 arguments
874 C - gethostbyname_r() with 5 arguments
875 D - gethostbyname_r() with 6 arguments
877 - The ipv6-resolver that uses getaddrinfo()
879 - The c-ares based name resolver that uses the c-ares library for resolves.
881 - The Windows threaded resolver. It use:
883 A - gethostbyname() on plain ipv4 windows hosts
884 B - getaddrinfo() on ipv6-enabled windows hosts
886 Also note that libcurl never resolves or reverse-lookups addresses given as
887 pure numbers, such as 127.0.0.1 or ::1.
889 5.10 How do I prevent libcurl from writing the response to stdout?
891 libcurl provides a default built-in write function that writes received data
892 to stdout. Set a WRITEFUNCTION to receive the data, or possibly set
893 WRITEDATA to a different FILE * handle.
898 Curl and libcurl are released under a MIT/X derivate license. The license is
899 very liberal and should not impose a problem for your project. This section
900 is just a brief summary for the cases we get the most questions. (Parts of
901 this section was much enhanced by Bjorn Reese.)
903 6.1 I have a GPL program, can I use the libcurl library?
907 Since libcurl may be distributed under the MIT/X derivate license, it can be
908 used together with GPL in any software.
910 6.2 I have a closed-source program, can I use the libcurl library?
914 libcurl does not put any restrictions on the program that uses the library.
916 6.3 I have a BSD licensed program, can I use the libcurl library?
920 libcurl does not put any restrictions on the program that uses the library.
922 6.4 I have a program that uses LGPL libraries, can I use libcurl?
926 The LGPL license doesn't clash with other licenses.
928 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
932 The MIT/X derivate license practically allows you to do almost anything with
933 the sources, on the condition that the copyright texts in the sources are
936 6.6 Can you please change the curl/libcurl license to XXXX?
940 We have carefully picked this license after years of development and
941 discussions and a large amount of people have contributed with source code
942 knowing that this is the license we use. This license puts the restrictions
943 we want on curl/libcurl and it does not spread to other programs or
944 libraries that use it. It should be possible for everyone to use libcurl or
945 curl in their projects, no matter what license they already have in use.
949 7.1 What is PHP/CURL?
951 The module for PHP that makes it possible for PHP programs to access curl-
952 functions from within PHP. We often call it PHP/CURL to differentiate from
953 curl the command line tool and libcurl the library.
955 7.2 Who write PHP/CURL?
957 PHP/CURL is a module that comes with the regular PHP package. It depends and
958 uses libcurl, so you need to have libcurl installed properly first before
959 PHP/CURL can be used. PHP/CURL is written by Sterling Hughes.
961 7.3 Can I perform multiple requests using the same handle?
963 Yes - at least in PHP version 4.3.8 and later (this has been known to not
964 work in earlier versions, but the exact version when it started to work is
967 After a transfer, you just set new options in the handle and make another
968 transfer. This will make libcurl to re-use the same connection if it can.
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