1 Updated: August 18, 2004 (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?
47 4.1 Problems connecting to SSL servers.
48 4.2 Why do I get problems when I use & or % in the URL?
49 4.3 How can I use {, }, [ or ] to specify multiple URLs?
50 4.4 Why do I get downloaded data even though the web page doesn't exist?
51 4.5 Why do I get return code XXX from a HTTP server?
52 4.5.1 "400 Bad Request"
53 4.5.2 "401 Unauthorized"
56 4.5.5 "405 Method Not Allowed"
57 4.5.6 "301 Moved Permanently"
58 4.6 Can you tell me what error code 142 means?
59 4.7 How do I keep user names and passwords secret in Curl command lines?
61 4.9 Curl can't authenticate to the server that requires NTLM?
62 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
63 4.11 Why does my HTTP range requests return the full document?
64 4.12 Why do I get "certificate verify failed" ?
67 5.1 Is libcurl thread-safe?
68 5.2 How can I receive all data into a large memory chunk?
69 5.3 How do I fetch multiple files with libcurl?
70 5.4 Does libcurl do Winsock initing on win32 systems?
71 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
72 5.6 What about Keep-Alive or persistent connections?
73 5.7 Link errors when building libcurl on Windows!
74 5.8 libcurl.so.3: open failed: No such file or directory
75 5.9 How does libcurl resolve host names?
78 6.1 I have a GPL program, can I use the libcurl library?
79 6.2 I have a closed-source program, can I use the libcurl library?
80 6.3 I have a BSD licensed program, can I use the libcurl library?
81 6.4 I have a program that uses LGPL libraries, can I use libcurl?
82 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
83 6.6 Can you please change the curl/libcurl license to XXXX?
87 7.2 Who write PHP/CURL?
88 7.3 Can I perform multiple requests using the same handle?
90 ==============================================================================
96 cURL (or simply just 'curl') is a command line tool for getting or sending
97 files using URL syntax. The name is a play on 'Client for URLs', originally
98 with URL spelled in uppercase to make it obvious it deals with URLs. The
99 fact it can also be pronounced 'see URL' also helped, it works as an
100 abbrivation for "Client URL Request Library" or why not the recursive
101 version: "Curl URL Request Library".
103 Curl supports a range of common Internet protocols, currently including
104 HTTP, HTTPS, FTP, FTPS, GOPHER, LDAP, DICT, TELNET and FILE.
106 We spell it cURL or just curl. We pronounce it with an initial k sound:
109 NOTE: there are numerous sub-projects and related projects that also use the
110 word curl in the project names in various combinations, but you should take
111 notice that this FAQ is directed at the command-line tool named curl (and
112 libcurl the library), and may therefore not be valid for other curl-related
117 libcurl is a reliable and portable library which provides you with an easy
118 interface to a range of common Internet protocols.
120 You can use libcurl for free in your application, be it open source,
121 commercial or closed-source.
123 1.3 What is cURL not?
125 Curl is *not* a wget clone. That is a common misconception. Never, during
126 curl's development, have we intended curl to replace wget or compete on its
127 market. Curl is targeted at single-shot file transfers.
129 Curl is not a web site mirroring program. If you want to use curl to mirror
130 something: fine, go ahead and write a script that wraps around curl to make
131 it reality (like curlmirror.pl does).
133 Curl is not an FTP site mirroring program. Sure, get and send FTP with curl
134 but if you want systematic and sequential behavior you should write a
135 script (or write a new program that interfaces libcurl) and do it.
137 Curl is not a PHP tool, even though it works perfectly well when used from
140 Curl is not a single-OS program. Curl exists, compiles, builds and runs
141 under a wide range of operating systems, including all modern Unixes (and a
142 bunch of older ones too), Windows, Amiga, BeOS, OS/2, OS X, QNX etc.
144 1.4 When will you make curl do XXXX ?
146 We love suggestions of what to change in order to make curl and libcurl
147 better. We do however believe in a few rules when it comes to the future of
150 * Curl -- the command line tool -- is to remain a non-graphical command line
151 tool. If you want GUIs or fancy scripting capabilities, you should look
152 for another tool that uses libcurl.
154 * We do not add things to curl that other small and available tools already
155 do very fine at the side. Curl's output is fine to pipe into another
156 program or redirect to another file for the next program to interpret.
158 * We focus on protocol related issues and improvements. If you wanna do more
159 magic with the supported protocols than curl currently does, chances are
160 big we will agree. If you wanna add more protocols, we may very well
163 * If you want someone else to make all the work while you wait for us to
164 implement it for you, that is not a very friendly attitude. We spend a
165 considerable time already on maintaining and developing curl. In order to
166 get more out of us, you should consider trading in some of your time and
169 * If you write the code, chances are bigger that it will get into curl
174 cURL and libcurl are not made by any single individual. Sure, Daniel
175 Stenberg writes the major parts, but other persons' submissions are
176 important and crucial. Anyone can contribute and post their changes and
177 improvements and have them inserted in the main sources (of course on the
178 condition that developers agree on that the fixes are good).
180 The list of contributors in the docs/THANKS file is only a small part of all
181 the people that every day provide us with bug reports, suggestions, ideas
184 curl is developed by a community, with Daniel at the wheel.
186 1.6 What do you get for making cURL?
188 Project cURL is entirely free and open. No person gets paid for developing
189 curl. We do this voluntarily on our spare time.
191 We get some help from companies. Contactor Data hosts the curl web site,
192 Haxx owns the curl web site's domain and sourceforge.net hosts project
193 services we take advantage from, like the bug tracker.
195 If you want to support our project with a donation or similar, one way of
196 doing that would be to buy "gift certificates" at useful online shopping
197 sites, such as amazon.com or thinkgeek.com. Another way would be to sponsor
198 us through a banner-program or even better: by helping us coding,
199 documenting, testing etc. You're welcome to send us a buck using paypal, as
200 described here: http://curl.haxx.se/donation.html
202 1.7 What about CURL from curl.com?
204 During the summer 2001, curl.com was busy advertising their client-side
205 programming language for the web, named CURL.
207 We are in no way associated with curl.com or their CURL programming
210 Our project name curl has been in effective use since 1998. We were not the
211 first computer related project to use the name "curl" and do not claim any
212 first-hand rights to the name.
214 We recognize that we will be living in parallel with curl.com and wish them
217 1.8 I have a problem who do I mail?
219 Please do not mail any single individual unless you really need to. Keep
220 curl-related questions on a suitable mailing list. All available mailing
221 lists are listed in the MANUAL document and online at
222 http://curl.haxx.se/mail/
224 Keeping curl-related questions and discussions on mailing lists allows
225 others to join in and help, to share their ideas, contribute their
226 suggestions and spread their wisdom. Keeping discussions on public mailing
227 lists also allows for others to learn from this (both current and future
228 users thanks to the web based archives of the mailing lists), thus saving us
229 from having to repeat ourselves even more. Thanks for respecting this.
232 2. Install Related Problems
234 2.1 configure doesn't find OpenSSL even when it is installed
236 This may be because of several reasons.
238 2.1.1 native linker doesn't find openssl
241 Solaris (native cc compiler)
242 HPUX (native cc compiler)
243 SGI IRIX (native cc compiler)
244 SCO UNIX (native cc compiler)
246 When configuring curl, I specify --with-ssl. OpenSSL is installed in
247 /usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
248 CRYPTO_lock in -lcrypto
250 Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
251 -lcrypto, so ld can't find the library. This is due to a bug in the GNU
254 Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
255 ./configure places the -L/usr/local/ssl/lib early enough in the command
256 line to make things work
258 Solution submitted by: Bob Allison <allisonb@users.sourceforge.net>
260 2.1.2 only the libssl lib is missing
262 If all include files and the libcrypto lib is present, with only the
263 libssl being missing according to configure, this is mostly likely because
264 a few functions are left out from the libssl.
266 If the function names missing include RSA or RSAREF you can be certain
267 that this is because libssl requires the RSA and RSAREF libs to build.
269 See the INSTALL file section that explains how to add those libs to
270 configure. Make sure that you remove the config.cache file before you
271 rerun configure with the new flags.
273 2.2 Does curl work/build with other SSL libraries?
275 Curl has been written to use OpenSSL, although there should not be much
276 problems using a different library. If anyone does "port" curl to use a
277 different SSL library, we are of course very interested in getting the
280 2.3 Where can I find a copy of LIBEAY32.DLL?
282 That is an OpenSSL binary built for Windows.
284 Curl uses OpenSSL to do the SSL stuff. The LIBEAY32.DLL is what curl needs
285 on a windows machine to do https://. Check out the curl web site to find
286 accurate and up-to-date pointers to recent OpenSSL DLLs and other binary
289 2.4 Does cURL support Socks (RFC 1928) ?
291 Yes, SOCKS5 is supported.
296 3.1 curl: (1) SSL is disabled, https: not supported
298 If you get this output when trying to get anything from a https:// server,
299 it means that the configure script couldn't find all libs and include files
300 it requires for SSL to work. If the configure script fails to find them,
301 curl is simply built without SSL support.
303 To get the https:// support into a curl that was previously built but that
304 reports that https:// is not supported, you should dig through the document
305 and logs and check out why the configure script doesn't find the SSL libs
306 and/or include files.
308 Also, check out the other paragraph in this FAQ labeled "configure doesn't
309 find OpenSSL even when it is installed".
311 3.2 How do I tell curl to resume a transfer?
313 Curl supports resumed transfers both ways on both FTP and HTTP.
317 3.3 Why doesn't my posting using -F work?
319 You can't simply use -F or -d at your choice. The web server that will
320 receive your post assumes one of the formats. If the form you're trying to
321 "fake" sets the type to 'multipart/form-data', then and only then you must
322 use the -F type. In all the most common cases, you should use -d which then
323 causes a posting with the type 'application/x-www-form-urlencoded'.
325 This is described in some detail in the MANUAL and TheArtOfHttpScripting
326 documents, and if you don't understand it the first time, read it again
327 before you post questions about this to the mailing list. Also, try reading
328 through the mailing list archives for old postings and questions regarding
331 3.4 How do I tell curl to run custom FTP commands?
333 You can tell curl to perform optional commands both before and/or after a
334 file transfer. Study the -Q/--quote option.
336 Since curl is used for file transfers, you don't use curl to just perform
337 FTP commands without transferring anything. Therefore you must always specify
338 a URL to transfer to/from even when doing custom FTP commands.
340 3.5 How can I disable the Pragma: nocache header?
342 You can change all internally generated headers by adding a replacement with
343 the -H/--header option. By adding a header with empty contents you safely
344 disable that one. Use -H "Pragma:" to disable that specific header.
346 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
348 To curl, all contents are alike. It doesn't matter how the page was
349 generated. It may be ASP, PHP, Perl, shell-script, SSI or plain
350 HTML-files. There's no difference to curl and it doesn't even know what kind
351 of language that generated the page.
353 See also item 3.14 regarding javascript.
355 3.7 Can I use curl to delete/rename a file through FTP?
357 Yes. You specify custom FTP commands with -Q/--quote.
359 One example would be to delete a file after you have downloaded it:
361 curl -O ftp://download.com/coolfile -Q '-DELE coolfile'
363 3.8 How do I tell curl to follow HTTP redirects?
365 Curl does not follow so-called redirects by default. The Location: header
366 that informs the client about this is only interpreted if you're using the
367 -L/--location option. As in:
369 curl -L http://redirector.com
371 3.9 How do I use curl in my favorite programming language?
373 There exist many language interfaces/bindings for curl that integrates it
374 better with various languages. If you are fluid in a script language, you
375 may very well opt to use such an interface instead of using the command line
378 Find out more about which languages that support curl directly, and how to
379 install and use them, in the libcurl section of the curl web site:
380 http://curl.haxx.se/libcurl/
382 In February 2003, there are interfaces available for the following
383 languages: Basic, C, C++, Cocoa, Dylan, Euphoria, Java, Lua, Object-Pascal,
384 Pascal, Perl, PHP, PostgreSQL, Python, Rexx, Ruby, Scheme and Tcl. By the
385 time you read this, additional ones may have appeared!
387 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
389 Curl adheres to the HTTP spec, which basically means you can play with *any*
390 protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and
391 XML-RPC are all such ones. You can use -X to set custom requests and -H to
392 set custom headers (or replace internally generated ones).
394 Using libcurl is of course just as fine and you'd just use the proper
395 library options to do the same.
397 3.11 How do I POST with a different Content-Type?
399 You can always replace the internally generated headers with -H/--header.
400 To make a simple HTTP POST with text/xml as content-type, do something like:
402 curl -d "datatopost" -H "Content-Type: text/xml" [URL]
404 3.12 Why do FTP specific features over HTTP proxy fail?
406 Because when you use a HTTP proxy, the protocol spoken on the network will
407 be HTTP, even if you specify a FTP URL. This effectively means that you
408 normally can't use FTP specific features such as FTP upload and FTP quote
411 There is one exception to this rule, and that is if you can "tunnel through"
412 the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
413 and is generally not available as proxy admins usually disable tunneling to
414 other ports than 443 (which is used for HTTPS access through proxies).
416 3.13 Why does my single/double quotes fail?
418 To specify a command line option that includes spaces, you might need to
419 put the entire option within quotes. Like in:
421 curl -d " with spaces " url.com
425 curl -d ' with spaces ' url.com
427 Exactly what kind of quotes and how to do this is entirely up to the shell
428 or command line interpreter that you are using. For most unix shells, you
429 can more or less pick either single (') or double (") quotes. For
430 Windows/DOS prompts I believe you're forced to use double (") quotes.
432 Please study the documentation for your particular environment. Examples in
433 the curl docs will use a mix of both these ones as shown above. You must
434 adjust them to work in your environment.
436 Remember that curl works and runs on more operating systems than most single
437 individuals have ever tried.
439 3.14 Does curl support javascript or pac (automated proxy config)?
441 Many web pages do magic stuff using embedded javascript. Curl and libcurl
442 have no built-in support for that, so it will be treated just like any other
445 .pac files are a netscape invention and are sometimes used by organizations
446 to allow them to differentiate which proxies to use. The .pac contents is
447 just a javascript program that gets invoked by the browser and that returns
448 the name of the proxy to connect to. Since curl doesn't support javascript,
449 it can't support .pac proxy configuration either.
451 Some work-arounds usually suggested to overcome this javascript dependency:
453 - Depending on the javascript complexity, write up a script that
454 translates it to another language and execute that.
456 - Read the javascript code and rewrite the same logic in another language.
458 - Implement a javascript interpreter, people have successfully used the
459 Mozilla javascript engine in the past.
461 - Ask your admins to stop this, for a static proxy setup or similar.
463 3.15 Can I do recursive fetches with curl?
465 No. curl itself has no code that performs recursive operations, such as
466 those performed by wget and similar tools.
468 There exist wrapper scripts with that functionality (for example the
469 curlmirror perl script), and you can write programs based on libcurl to do
470 it, but the command line tool curl itself cannot.
472 3.16 What certificates do I need when I use SSL?
474 There are three different kinds of "certificates" to keep track of when we
475 talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.
477 - Client certificate. The server you communicate may require that you can
478 provide this in order to prove that you actually are who you claim to be.
479 If the server doesn't require this, you don't need a client certificate.
481 - Server certificate. The server you communicate with has a server
482 certificate. You can and should verify this certficate to make sure that
483 you are truly talking to the real server and not a server impersonating
484 it. The server certificate verifaction process is made by using a
485 Certificate Authority certificate ("CA cert") that was used to sign the
486 server certificate. Server certificate verification is enabled by default
487 in curl and libcurl and is often the reason for problems as explained in
488 FAQ entry 4.12 and the SSLCERTS document
489 (http://curl.haxx.se/docs/sslcerts.html). Server certificates that are
490 "self-signed" or otherwise signed by a CA that you do not have a CA cert
491 for, cannot be verified. If the verification during a connect fails, you
492 are refused access. You then need to explicitly disable the verification
493 to connect to the server.
495 - Certificate Authority certificate ("CA cert"). You often have several CA
496 certs in a CA cert bundle that can be used to verify a server certificate
497 that was signed by one of the authorities in the bundle. curl comes with a
498 default CA cert bundle. You can override the default.
503 4.1 Problems connecting to SSL servers.
505 It took a very long time before we could sort out why curl had problems to
506 connect to certain SSL servers when using SSLeay or OpenSSL v0.9+. The
507 error sometimes showed up similar to:
509 16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:
511 It turned out to be because many older SSL servers don't deal with SSLv3
512 requests properly. To correct this problem, tell curl to select SSLv2 from
513 the command line (-2/--sslv2).
515 There have also been examples where the remote server didn't like the SSLv2
516 request and instead you had to force curl to use SSLv3 with -3/--sslv3.
518 4.2 Why do I get problems when I use & or % in the URL?
520 In general unix shells, the & letter is treated special and when used, it
521 runs the specified command in the background. To safely send the & as a part
522 of a URL, you should quote the entire URL by using single (') or double (")
525 An example that would invoke a remote CGI that uses &-letters could be:
527 curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
529 In Windows, the standard DOS shell treats the %-letter specially and you
530 need to use TWO %-letters for each single one you want to use in the URL.
532 Also note that if you want the literal %-letter to be part of the data you
533 pass in a POST using -d/--data you must encode it as '%25' (which then also
534 needs the %-letter doubled on Windows machines).
536 4.3 How can I use {, }, [ or ] to specify multiple URLs?
538 Because those letters have a special meaning to the shell, and to be used in
539 a URL specified to curl you must quote them.
541 An example that downloads two URLs (sequentially) would do:
543 curl '{curl,www}.haxx.se'
545 To be able to use those letters as actual parts of the URL (without using
546 them for the curl URL "globbing" system), use the -g/--globoff option:
548 curl -g 'www.site.com/weirdname[].html'
550 4.4 Why do I get downloaded data even though the web page doesn't exist?
552 Curl asks remote servers for the page you specify. If the page doesn't exist
553 at the server, the HTTP protocol defines how the server should respond and
554 that means that headers and a "page" will be returned. That's simply how
557 By using the --fail option you can tell curl explicitly to not get any data
558 if the HTTP return code doesn't say success.
560 4.5 Why do I get return code XXX from a HTTP server?
562 RFC2616 clearly explains the return codes. This is a short transcript. Go
563 read the RFC for exact details:
565 4.5.1 "400 Bad Request"
567 The request could not be understood by the server due to malformed
568 syntax. The client SHOULD NOT repeat the request without modifications.
570 4.5.2 "401 Unauthorized"
572 The request requires user authentication.
574 4.5.3 "403 Forbidden"
576 The server understood the request, but is refusing to fulfill it.
577 Authorization will not help and the request SHOULD NOT be repeated.
579 4.5.4 "404 Not Found"
581 The server has not found anything matching the Request-URI. No indication
582 is given of whether the condition is temporary or permanent.
584 4.5.5 "405 Method Not Allowed"
586 The method specified in the Request-Line is not allowed for the resource
587 identified by the Request-URI. The response MUST include an Allow header
588 containing a list of valid methods for the requested resource.
590 4.5.6 "301 Moved Permanently"
592 If you get this return code and an HTML output similar to this:
594 <H1>Moved Permanently</H1> The document has moved <A
595 HREF="http://same_url_now_with_a_trailing_slash/">here</A>.
597 it might be because you request a directory URL but without the trailing
598 slash. Try the same operation again _with_ the trailing URL, or use the
599 -L/--location option to follow the redirection.
601 4.6 Can you tell me what error code 142 means?
603 All error codes that are larger than the highest documented error code means
604 that curl has exited due to a crash. This is a serious error, and we
605 appreciate a detailed bug report from you that describes how we could go
606 ahead and repeat this!
608 4.7 How do I keep user names and passwords secret in Curl command lines?
610 This problem has two sides:
612 The first part is to avoid having clear-text passwords in the command line
613 so that they don't appear in 'ps' outputs and similar. That is easily
614 avoided by using the "-K" option to tell curl to read parameters from a file
615 or stdin to which you can pass the secret info. curl itself will also
616 attempt to "hide" the given password by blanking out the option - this
617 doesn't work on all platforms.
619 To keep the passwords in your account secret from the rest of the world is
620 not a task that curl addresses. You could of course encrypt them somehow to
621 at least hide them from being read by human eyes, but that is not what
622 anyone would call security.
624 Also note that regular HTTP (using Basic authentication) and FTP passwords
625 are sent in clear across the network. All it takes for anyone to fetch them
626 is to listen on the network. Eavesdropping is very easy. Use more secure
627 authentication methods (like Digest, Negotiate or even NTLM) or consider the
628 SSL-based alternatives HTTPS and FTPS.
632 It is not a bug if the behavior is documented. Read the docs first.
633 Especially check out the KNOWN_BUGS file, it may be a documented bug!
635 If it is a problem with a binary you've downloaded or a package for your
636 particular platform, try contacting the person who built the package/archive
639 If there is a bug, read the BUGS document first. Then report it as described
642 4.9 Curl can't authenticate to the server that requires NTLM?
644 This is supported in curl 7.10.6 or later. No earlier curl version knows
647 NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You
648 should not use such ones.
650 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
652 Many web servers allow or demand that the administrator configures the
653 server properly for these requests to work on the web server.
655 Some servers seem to support HEAD only on certain kinds of URLs.
657 To fully grasp this, try the documentation for the particular server
658 software you're trying to interact with. This is not anything curl can do
661 4.11 Why does my HTTP range requests return the full document?
663 Because the range may not be supported by the server, or the server may
664 choose to ignore it and return the full document anyway.
666 4.12 Why do I get "certificate verify failed" ?
668 You invoke curl 7.10 or later to communicate on a https:// URL and get an
669 error back looking something similar to this:
671 curl: (35) SSL: error:14090086:SSL routines:
672 SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
674 Then it means that curl couldn't verify that the server's certificate was
675 good. Curl verifies the certificate using the CA cert bundle that comes with
676 the curl installation.
678 To disable the verification (which makes it act like curl did before 7.10),
679 use -k. This does however enable man-in-the-middle attacks.
681 If you get this failure but are having a CA cert bundle installed and used,
682 the server's certificate is not signed by one of the CA's in the bundle. It
683 might for example be self-signed. You then correct this problem by obtaining
684 a valid CA cert for the server. Or again, decrease the security by disabling
687 Details are also in the SSLCERTS file in the release archives, found online
688 here: http://curl.haxx.se/docs/sslcerts.html
693 5.1 Is libcurl thread-safe?
697 We have written the libcurl code specificly adjusted for multi-threaded
698 programs. libcurl will use thread-safe functions instead of non-safe ones if
699 your system has such.
701 We would appreciate some kind of report or README file from those who have
702 used libcurl in a threaded environment.
704 5.2 How can I receive all data into a large memory chunk?
706 [ See also the examples/getinmemory.c source ]
708 You are in full control of the callback function that gets called every time
709 there is data received from the remote server. You can make that callback do
710 whatever you want. You do not have to write the received data to a file.
712 One solution to this problem could be to have a pointer to a struct that you
713 pass to the callback function. You set the pointer using the
714 curl_easy_setopt(CURLOPT_FILE) function. Then that pointer will be passed to
715 the callback instead of a FILE * to a file:
717 /* imaginary struct */
718 struct MemoryStruct {
723 /* imaginary callback function */
725 WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
727 size_t realsize = size * nmemb;
728 struct MemoryStruct *mem = (struct MemoryStruct *)data;
730 mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
732 memcpy(&(mem->memory[mem->size]), ptr, realsize);
733 mem->size += realsize;
734 mem->memory[mem->size] = 0;
739 5.3 How do I fetch multiple files with libcurl?
741 libcurl has excellent support for transferring multiple files. You should
742 just repeatedly set new URLs with curl_easy_setopt() and then transfer it
743 with curl_easy_perform(). The handle you get from curl_easy_init() is not
744 only reusable, but you're even encouraged to reuse it if you can, as that
745 will enable libcurl to use persistent connections.
747 5.4 Does libcurl do Winsock initialization on win32 systems?
749 Yes, if told to in the curl_global_init() call.
751 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
753 Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
754 that DLL use the FILE * (as the DLL and the client application cannot access
755 each others' variable memory areas). If you set CURLOPT_WRITEDATA you must
756 also use CURLOPT_WRITEFUNCTION as well to set a function that writes the
757 file, even if that simply writes the data to the specified FILE *.
758 Similarly, if you use CURLOPT_READDATA you must also specify
759 CURLOPT_READFUNCTION.
761 (Provided by Joel DeYoung and Bob Schader)
763 5.6 What about Keep-Alive or persistent connections?
765 curl and libcurl have excellent support for persistent connections when
766 transferring several files from the same server. Curl will attempt to reuse
767 connections for all URLs specified on the same command line/config file, and
768 libcurl will reuse connections for all transfers that are made using the
771 5.7 Link errors when building libcurl on Windows!
773 You need to make sure that your project, and all the libraries (both static
774 and dynamic) that it links against, are compiled/linked against the same run
777 This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
778 options to the command line compiler. /MD (linking against MSVCRT dll) seems
779 to be the most commonly used option.
781 (Provided by Andrew Francis)
783 5.8 libcurl.so.3: open failed: No such file or directory
785 This is an error message you might get when you try to run a program linked
786 with a shared version of libcurl and your run-time linker (ld.so) couldn't
787 find the shared library named libcurl.so.3.
789 You need to make sure that ld.so finds libcurl.so.3. You can do that
790 multiple ways, and it differs somewhat between different operating systems,
791 but they are usually:
793 * Add an option to the linker command line that specify the hard-coded path
794 the run-time linker should check for the lib (usually -R)
796 * Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
797 should check for libs
799 * Adjust the system's config to check for libs in the directory where you've
800 put the dir (like Linux's /etc/ld.so.conf)
802 'man ld.so' and 'man ld' will tell you more details
804 5.9 How does libcurl resolve host names?
806 libcurl includes a number of different name resolve functions:
808 - The non-ipv6 resolver that can use one out of four host name resolve calls
809 (depending on what your system supports):
812 B - gethostbyname_r() with 3 arguments
813 C - gethostbyname_r() with 5 arguments
814 D - gethostbyname_r() with 6 arguments
816 - The ipv6-resolver that uses getaddrinfo()
818 - The c-ares based name resolver that uses the c-ares library for resolves.
820 - The Windows threaded resolver. It use:
822 A - gethostbyname() on plain ipv4 windows hosts
823 B - getaddrinfo() on ipv6-enabled windows hosts
827 Curl and libcurl are released under a MIT/X derivate license. The license is
828 very liberal and should not impose a problem for your project. This section
829 is just a brief summary for the cases we get the most questions. (Parts of
830 this section was much enhanced by Bjorn Reese.)
832 6.1 I have a GPL program, can I use the libcurl library?
836 Since libcurl may be distributed under the MIT/X derivate license, it can be
837 used together with GPL in any software.
839 6.2 I have a closed-source program, can I use the libcurl library?
843 libcurl does not put any restrictions on the program that uses the library.
845 6.3 I have a BSD licensed program, can I use the libcurl library?
849 libcurl does not put any restrictions on the program that uses the library.
851 6.4 I have a program that uses LGPL libraries, can I use libcurl?
855 The LGPL license doesn't clash with other licenses.
857 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
861 The MIT/X derivate license practically allows you to do almost anything with
862 the sources, on the condition that the copyright texts in the sources are
865 6.6 Can you please change the curl/libcurl license to XXXX?
869 We have carefully picked this license after years of development and
870 discussions and a large amount of people have contributed with source code
871 knowing that this is the license we use. This license puts the restrictions
872 we want on curl/libcurl and it does not spread to other programs or
873 libraries that use it. It should be possible for everyone to use libcurl or
874 curl in their projects, no matter what license they already have in use.
878 7.1 What is PHP/CURL?
880 The module for PHP that makes it possible for PHP programs to access curl-
881 functions from within PHP. We often call it PHP/CURL to differentiate from
882 curl the command line tool and libcurl the library.
884 7.2 Who write PHP/CURL?
886 PHP/CURL is a module that comes with the regular PHP package. It depends and
887 uses libcurl, so you need to have libcurl installed properly first before
888 PHP/CURL can be used. PHP/CURL is written by Sterling Hughes.
890 7.3 Can I perform multiple requests using the same handle?
892 With libcurl you can. With PHP/CURL you cannot. This is a limitation in the
893 PHP/CURL code. Therefore, you need to close each handle after each request
894 and create a new one for the following one.
896 I've heard rumours that this situation has been fixed in PHP5, but that is