5 \___|\___/|_| \_\_____|
13 1.4 When will you make curl do XXXX ?
15 1.6 What do you get for making curl?
16 1.7 What about CURL from curl.com?
17 1.8 I have a problem who do I mail?
18 1.9 Where do I buy commercial support for curl?
19 1.10 How many are using curl?
20 1.11 Why don't you update ca-bundle.crt
21 1.12 I have a problem who can I chat with?
22 1.13 curl's ECCN number?
23 1.14 How do I submit my patch?
24 1.15 How do I port libcurl to my OS?
26 2. Install Related Problems
27 2.1 configure doesn't find OpenSSL even when it is installed
28 2.1.1 native linker doesn't find OpenSSL
29 2.1.2 only the libssl lib is missing
30 2.2 Does curl work/build with other SSL libraries?
31 2.3 Where can I find a copy of LIBEAY32.DLL?
32 2.4 Does curl support SOCKS (RFC 1928) ?
33 2.5 Install libcurl for both 32bit and 64bit?
36 3.1 curl: (1) SSL is disabled, https: not supported
37 3.2 How do I tell curl to resume a transfer?
38 3.3 Why doesn't my posting using -F work?
39 3.4 How do I tell curl to run custom FTP commands?
40 3.5 How can I disable the Accept: */* header?
41 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
42 3.7 Can I use curl to delete/rename a file through FTP?
43 3.8 How do I tell curl to follow HTTP redirects?
44 3.9 How do I use curl in my favorite programming language?
45 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
46 3.11 How do I POST with a different Content-Type?
47 3.12 Why do FTP specific features over HTTP proxy fail?
48 3.13 Why does my single/double quotes fail?
49 3.14 Does curl support Javascript or PAC (automated proxy config)?
50 3.15 Can I do recursive fetches with curl?
51 3.16 What certificates do I need when I use SSL?
52 3.17 How do I list the root dir of an FTP server?
53 3.18 Can I use curl to send a POST/PUT and not wait for a response?
54 3.19 How do I get HTTP from a host using a specific IP address?
55 3.20 How to SFTP from my user's home directory?
56 3.21 Protocol xxx not supported or disabled in libcurl
57 3.22 curl -X gives me HTTP problems
60 4.1 Problems connecting to SSL servers.
61 4.2 Why do I get problems when I use & or % in the URL?
62 4.3 How can I use {, }, [ or ] to specify multiple URLs?
63 4.4 Why do I get downloaded data even though the web page doesn't exist?
64 4.5 Why do I get return code XXX from a HTTP server?
65 4.5.1 "400 Bad Request"
66 4.5.2 "401 Unauthorized"
69 4.5.5 "405 Method Not Allowed"
70 4.5.6 "301 Moved Permanently"
71 4.6 Can you tell me what error code 142 means?
72 4.7 How do I keep user names and passwords secret in Curl command lines?
74 4.9 Curl can't authenticate to the server that requires NTLM?
75 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
76 4.11 Why does my HTTP range requests return the full document?
77 4.12 Why do I get "certificate verify failed" ?
78 4.13 Why is curl -R on Windows one hour off?
79 4.14 Redirects work in browser but not with curl!
80 4.15 FTPS doesn't work
81 4.16 My HTTP POST or PUT requests are slow!
82 4.17 Non-functional connect timeouts on Windows
83 4.18 file:// URLs containing drive letters (Windows, NetWare)
84 4.19 Why doesn't cURL return an error when the network cable is unplugged?
85 4.20 curl doesn't return error for HTTP non-200 responses!
86 4.21 Why is there a HTTP/1.1 in my HTTP/2 request?
89 5.1 Is libcurl thread-safe?
90 5.2 How can I receive all data into a large memory chunk?
91 5.3 How do I fetch multiple files with libcurl?
92 5.4 Does libcurl do Winsock initing on win32 systems?
93 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
94 5.6 What about Keep-Alive or persistent connections?
95 5.7 Link errors when building libcurl on Windows!
96 5.8 libcurl.so.X: open failed: No such file or directory
97 5.9 How does libcurl resolve host names?
98 5.10 How do I prevent libcurl from writing the response to stdout?
99 5.11 How do I make libcurl not receive the whole HTTP response?
100 5.12 Can I make libcurl fake or hide my real IP address?
101 5.13 How do I stop an ongoing transfer?
102 5.14 Using C++ non-static functions for callbacks?
103 5.15 How do I get an FTP directory listing?
104 5.16 I want a different time-out!
105 5.17 Can I write a server with libcurl?
106 5.18 Does libcurl use threads?
109 6.1 I have a GPL program, can I use the libcurl library?
110 6.2 I have a closed-source program, can I use the libcurl library?
111 6.3 I have a BSD licensed program, can I use the libcurl library?
112 6.4 I have a program that uses LGPL libraries, can I use libcurl?
113 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
114 6.6 Can you please change the curl/libcurl license to XXXX?
115 6.7 What are my obligations when using libcurl in my commercial apps?
118 7.1 What is PHP/CURL?
119 7.2 Who wrote PHP/CURL?
120 7.3 Can I perform multiple requests using the same handle?
122 ==============================================================================
128 cURL is the name of the project. The name is a play on 'Client for URLs',
129 originally with URL spelled in uppercase to make it obvious it deals with
130 URLs. The fact it can also be pronounced 'see URL' also helped, it works as
131 an abbreviation for "Client URL Request Library" or why not the recursive
132 version: "Curl URL Request Library".
134 The cURL project produces two products:
138 A free and easy-to-use client-side URL transfer library, supporting DICT,
139 FILE, FTP, FTPS, GOPHER, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, POP3,
140 POP3S, RTMP, RTSP, SCP, SFTP, SMB, SMBS, SMTP, SMTPS, TELNET and TFTP.
142 libcurl supports HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading,
143 Kerberos, SPNEGO, HTTP form based upload, proxies, cookies, user+password
144 authentication, file transfer resume, http proxy tunneling and more!
146 libcurl is highly portable, it builds and works identically on numerous
147 platforms, including Solaris, NetBSD, FreeBSD, OpenBSD, Darwin, HP-UX,
148 IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, BeOS, Mac
149 OS X, Ultrix, QNX, OpenVMS, RISC OS, Novell NetWare, DOS, Symbian, OSF,
150 Android, Minix, IBM TPF and more...
152 libcurl is free, thread-safe, IPv6 compatible, feature rich, well
157 A command line tool for getting or sending files using URL syntax.
159 Since curl uses libcurl, curl supports the same wide range of common
160 Internet protocols that libcurl does.
162 We pronounce curl with an initial k sound. It rhymes with words like girl
163 and earl. This is a short WAV file to help you:
165 http://media.merriam-webster.com/soundc11/c/curl0001.wav
167 There are numerous sub-projects and related projects that also use the word
168 curl in the project names in various combinations, but you should take
169 notice that this FAQ is directed at the command-line tool named curl (and
170 libcurl the library), and may therefore not be valid for other curl-related
171 projects. (There is however a small section for the PHP/CURL in this FAQ.)
175 libcurl is a reliable and portable library which provides you with an easy
176 interface to a range of common Internet protocols.
178 You can use libcurl for free in your application, be it open source,
179 commercial or closed-source.
181 libcurl is most probably the most portable, most powerful and most often
182 used C-based multi-platform file transfer library on this planet - be it
183 open source or commercial.
185 1.3 What is curl not?
187 Curl is not a wget clone. That is a common misconception. Never, during
188 curl's development, have we intended curl to replace wget or compete on its
189 market. Curl is targeted at single-shot file transfers.
191 Curl is not a web site mirroring program. If you want to use curl to mirror
192 something: fine, go ahead and write a script that wraps around curl to make
193 it reality (like curlmirror.pl does).
195 Curl is not an FTP site mirroring program. Sure, get and send FTP with curl
196 but if you want systematic and sequential behavior you should write a
197 script (or write a new program that interfaces libcurl) and do it.
199 Curl is not a PHP tool, even though it works perfectly well when used from
200 or with PHP (when using the PHP/CURL module).
202 Curl is not a program for a single operating system. Curl exists, compiles,
203 builds and runs under a wide range of operating systems, including all
204 modern Unixes (and a bunch of older ones too), Windows, Amiga, BeOS, OS/2,
207 1.4 When will you make curl do XXXX ?
209 We love suggestions of what to change in order to make curl and libcurl
210 better. We do however believe in a few rules when it comes to the future of
213 Curl -- the command line tool -- is to remain a non-graphical command line
214 tool. If you want GUIs or fancy scripting capabilities, you should look for
215 another tool that uses libcurl.
217 We do not add things to curl that other small and available tools already do
218 very fine at the side. Curl's output is fine to pipe into another program or
219 redirect to another file for the next program to interpret.
221 We focus on protocol related issues and improvements. If you wanna do more
222 magic with the supported protocols than curl currently does, chances are big
223 we will agree. If you wanna add more protocols, we may very well agree.
225 If you want someone else to make all the work while you wait for us to
226 implement it for you, that is not a very friendly attitude. We spend a
227 considerable time already on maintaining and developing curl. In order to
228 get more out of us, you should consider trading in some of your time and
229 efforts in return. Simply go to the GitHub repo which resides at
230 https://github.com/curl/curl, fork the project, and create pull requests
231 with your proposed changes.
233 If you write the code, chances are bigger that it will get into curl faster.
237 curl and libcurl are not made by any single individual. Daniel Stenberg is
238 project leader and main developer, but other persons' submissions are
239 important and crucial. Anyone can contribute and post their changes and
240 improvements and have them inserted in the main sources (of course on the
241 condition that developers agree on that the fixes are good).
243 The full list of all contributors is found in the docs/THANKS file.
245 curl is developed by a community, with Daniel at the wheel.
247 1.6 What do you get for making curl?
249 Project cURL is entirely free and open. No person gets paid for developing
250 curl on full time. We do this voluntarily, mostly on spare time.
251 Occasionally companies pay individual developers to work on curl, but that's
252 up to each company and developer. It is not controlled by nor supervised in
253 any way by the project.
255 We still get help from companies. Haxx provides web site, bandwidth, mailing
256 lists etc, sourceforge.net hosts project services we take advantage from,
257 like the bug tracker, and GitHub hosts the primary git repository at
258 https://github.com/curl/curl. Also again, some companies have sponsored
259 certain parts of the development in the past and I hope some will continue to
262 If you want to support our project, consider a donation or a banner-program
263 or even better: by helping us coding, documenting, testing etc.
265 1.7 What about CURL from curl.com?
267 During the summer 2001, curl.com was busy advertising their client-side
268 programming language for the web, named CURL.
270 We are in no way associated with curl.com or their CURL programming
273 Our project name curl has been in effective use since 1998. We were not the
274 first computer related project to use the name "curl" and do not claim any
277 We recognize that we will be living in parallel with curl.com and wish them
280 1.8 I have a problem who do I mail?
282 Please do not mail any single individual unless you really need to. Keep
283 curl-related questions on a suitable mailing list. All available mailing
284 lists are listed in the MANUAL document and online at
285 https://curl.haxx.se/mail/
287 Keeping curl-related questions and discussions on mailing lists allows
288 others to join in and help, to share their ideas, contribute their
289 suggestions and spread their wisdom. Keeping discussions on public mailing
290 lists also allows for others to learn from this (both current and future
291 users thanks to the web based archives of the mailing lists), thus saving us
292 from having to repeat ourselves even more. Thanks for respecting this.
294 If you have found or simply suspect a security problem in curl or libcurl,
295 mail curl-security at haxx.se (closed list of receivers, mails are not
296 disclosed) and tell. Then we can produce a fix in a timely manner before the
297 flaw is announced to the world, thus lessen the impact the problem will have
300 1.9 Where do I buy commercial support for curl?
302 curl is fully open source. It means you can hire any skilled engineer to fix
303 your curl-related problems.
305 We list available alternatives on the curl web site:
306 https://curl.haxx.se/support.html
308 1.10 How many are using curl?
310 It is impossible to tell.
312 We don't know how many users that knowingly have installed and use curl.
314 We don't know how many users that use curl without knowing that they are in
317 We don't know how many users that downloaded or installed curl and then
320 In May 2012 Daniel did a counting game and came up with a number that may
321 be completely wrong or somewhat accurate. Over 500 million!
323 See https://daniel.haxx.se/blog/2012/05/16/300m-users/
325 1.11 Why don't you update ca-bundle.crt
327 The ca cert bundle that used to shipped with curl was very outdated and must
328 be replaced with an up-to-date version by anyone who wants to verify
329 peers. It is no longer provided by curl. The last curl release ever that
330 shipped a ca cert bundle was curl 7.18.0.
332 In the cURL project we've decided not to attempt to keep this file updated
333 (or even present anymore) since deciding what to add to a ca cert bundle is
334 an undertaking we've not been ready to accept, and the one we can get from
335 Mozilla is perfectly fine so there's no need to duplicate that work.
337 Today, with many services performed over HTTPS, every operating system
338 should come with a default ca cert bundle that can be deemed somewhat
339 trustworthy and that collection (if reasonably updated) should be deemed to
340 be a lot better than a private curl version.
342 If you want the most recent collection of ca certs that Mozilla Firefox
343 uses, we recommend that you extract the collection yourself from Mozilla
344 Firefox (by running 'make ca-bundle), or by using our online service setup
345 for this purpose: https://curl.haxx.se/docs/caextract.html
347 1.12 I have a problem who can I chat with?
349 There's a bunch of friendly people hanging out in the #curl channel on the
350 IRC network irc.freenode.net. If you're polite and nice, chances are big
351 that you can get -- or provide -- help instantly.
353 1.13 curl's ECCN number?
355 The US government restricts exports of software that contains or uses
356 cryptography. When doing so, the Export Control Classification Number (ECCN)
357 is used to identify the level of export control etc.
359 Apache Software Foundation gives a good explanation of ECCNs at
360 https://www.apache.org/dev/crypto.html
362 We believe curl's number might be ECCN 5D002, another possibility is
363 5D992. It seems necessary to write them (the authority that administers ECCN
364 numbers), asking to confirm.
366 Comprehensible explanations of the meaning of such numbers and how to obtain
367 them (resp.) are here
369 http://www.bis.doc.gov/licensing/exportingbasics.htm
370 http://www.bis.doc.gov/licensing/do_i_needaneccn.html
372 An incomprehensible description of the two numbers above is here
373 http://www.access.gpo.gov/bis/ear/pdf/ccl5-pt2.pdf
375 1.14 How do I submit my patch?
377 When you have made a patch or a change of whatever sort, and want to submit
378 that to the project, there are a few different ways we prefer:
380 o send a patch to the curl-library mailing list. We're many subscribers
381 there and there are lots of people who can review patches, comment on them
382 and "receive" them properly.
384 o if your patch changes or fixes a bug, you can also opt to submit a bug
385 report in the bug tracker and attach your patch there. There are less
386 people involved there.
388 Lots of more details are found in the CONTRIBUTE and INTERNALS docs.
390 1.15 How do I port libcurl to my OS?
392 Here's a rough step-by-step:
394 1. copy a suitable lib/config-*.h file as a start to lib/config-[youros].h
396 2. edit lib/config-[youros].h to match your OS and setup
398 3. edit lib/curl_setup.h to include config-[youros].h when your OS is
399 detected by the preprocessor, in the style others already exist
401 4. compile lib/*.c and make them into a library
404 2. Install Related Problems
406 2.1 configure doesn't find OpenSSL even when it is installed
408 This may be because of several reasons.
410 2.1.1 native linker doesn't find openssl
413 Solaris (native cc compiler)
414 HPUX (native cc compiler)
415 SGI IRIX (native cc compiler)
416 SCO UNIX (native cc compiler)
418 When configuring curl, I specify --with-ssl. OpenSSL is installed in
419 /usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
420 CRYPTO_lock in -lcrypto
422 Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
423 -lcrypto, so ld can't find the library. This is due to a bug in the GNU
426 Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
427 ./configure places the -L/usr/local/ssl/lib early enough in the command
428 line to make things work
430 2.1.2 only the libssl lib is missing
432 If all include files and the libcrypto lib is present, with only the
433 libssl being missing according to configure, this is mostly likely because
434 a few functions are left out from the libssl.
436 If the function names missing include RSA or RSAREF you can be certain
437 that this is because libssl requires the RSA and RSAREF libs to build.
439 See the INSTALL file section that explains how to add those libs to
440 configure. Make sure that you remove the config.cache file before you
441 rerun configure with the new flags.
443 2.2 Does curl work/build with other SSL libraries?
445 Curl has been written to use a generic SSL function layer internally, and
446 that SSL functionality can then be provided by one out of many different SSL
449 curl can be built to use one of the following SSL alternatives: OpenSSL,
450 GnuTLS, yassl, NSS, PolarSSL, axTLS, Secure Transport (native iOS/OS X),
451 WinSSL (native Windows) or GSKit (native IBM i). They all have their pros
452 and cons, and we try to maintain a comparison of them here:
453 https://curl.haxx.se/docs/ssl-compared.html
455 2.3 Where can I find a copy of LIBEAY32.DLL?
457 That is an OpenSSL binary built for Windows.
459 Curl can be built with OpenSSL to do the SSL stuff. The LIBEAY32.DLL is then
460 what curl needs on a windows machine to do https:// etc. Check out the curl
461 web site to find accurate and up-to-date pointers to recent OpenSSL DLLs and
462 other binary packages.
464 2.4 Does curl support SOCKS (RFC 1928) ?
466 Yes, SOCKS 4 and 5 are supported.
468 2.5 Install libcurl for both 32bit and 64bit?
470 In curl's configure procedure one of the regular include files get created
471 with platform specific information. The file 'curl/curlbuild.h' in the
472 installed libcurl file tree is therefore somewhat tied to that particular
475 To allow applications to get built for either 32bit or 64bit you need to
476 install libcurl headers for both setups and unfortunately curl doesn't do
479 A commonly used procedure is this:
481 $ ./configure [32bit platform]
482 $ mv curl/curlbuild.h curl/curlbuild-32bit.h
483 $ ./configure [64bit platform]
484 $ mv curl/curlbuild.h curl/curlbuild-64bit.h
486 Then you make a toplevel curl/curlbuild.h replacement that only does this:
489 #include "curlbuild-32bit.h"
491 #include "curlbuild-64bit.h"
497 3.1 curl: (1) SSL is disabled, https: not supported
499 If you get this output when trying to get anything from a https:// server,
500 it means that the instance of curl/libcurl that you're using was built
501 without support for this protocol.
503 This could've happened if the configure script that was run at build time
504 couldn't find all libs and include files curl requires for SSL to work. If
505 the configure script fails to find them, curl is simply built without SSL
508 To get the https:// support into a curl that was previously built but that
509 reports that https:// is not supported, you should dig through the document
510 and logs and check out why the configure script doesn't find the SSL libs
511 and/or include files.
513 Also, check out the other paragraph in this FAQ labelled "configure doesn't
514 find OpenSSL even when it is installed".
516 3.2 How do I tell curl to resume a transfer?
518 Curl supports resumed transfers both ways on both FTP and HTTP.
521 3.3 Why doesn't my posting using -F work?
523 You can't simply use -F or -d at your choice. The web server that will
524 receive your post expects one of the formats. If the form you're trying to
525 submit uses the type 'multipart/form-data', then and only then you must use
526 the -F type. In all the most common cases, you should use -d which then
527 causes a posting with the type 'application/x-www-form-urlencoded'.
529 This is described in some detail in the MANUAL and TheArtOfHttpScripting
530 documents, and if you don't understand it the first time, read it again
531 before you post questions about this to the mailing list. Also, try reading
532 through the mailing list archives for old postings and questions regarding
535 3.4 How do I tell curl to run custom FTP commands?
537 You can tell curl to perform optional commands both before and/or after a
538 file transfer. Study the -Q/--quote option.
540 Since curl is used for file transfers, you don't normally use curl to
541 perform FTP commands without transferring anything. Therefore you must
542 always specify a URL to transfer to/from even when doing custom FTP
543 commands, or use -I which implies the "no body" option sent to libcurl.
545 3.5 How can I disable the Accept: */* header?
547 You can change all internally generated headers by adding a replacement with
548 the -H/--header option. By adding a header with empty contents you safely
549 disable that one. Use -H "Accept:" to disable that specific header.
551 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
553 To curl, all contents are alike. It doesn't matter how the page was
554 generated. It may be ASP, PHP, Perl, shell-script, SSI or plain HTML
555 files. There's no difference to curl and it doesn't even know what kind of
556 language that generated the page.
558 See also item 3.14 regarding javascript.
560 3.7 Can I use curl to delete/rename a file through FTP?
562 Yes. You specify custom FTP commands with -Q/--quote.
564 One example would be to delete a file after you have downloaded it:
566 curl -O ftp://download.com/coolfile -Q '-DELE coolfile'
568 or rename a file after upload:
570 curl -T infile ftp://upload.com/dir/ -Q "-RNFR infile" -Q "-RNTO newname"
572 3.8 How do I tell curl to follow HTTP redirects?
574 Curl does not follow so-called redirects by default. The Location: header
575 that informs the client about this is only interpreted if you're using the
576 -L/--location option. As in:
578 curl -L http://redirector.com
580 Not all redirects are HTTP ones, see 4.14
582 3.9 How do I use curl in my favorite programming language?
584 There exist many language interfaces/bindings for curl that integrates it
585 better with various languages. If you are fluid in a script language, you
586 may very well opt to use such an interface instead of using the command line
589 Find out more about which languages that support curl directly, and how to
590 install and use them, in the libcurl section of the curl web site:
591 https://curl.haxx.se/libcurl/
593 All the various bindings to libcurl are made by other projects and people,
594 outside of the cURL project. The cURL project itself only produces libcurl
595 with its plain C API. If you don't find anywhere else to ask you can ask
596 about bindings on the curl-library list too, but be prepared that people on
597 that list may not know anything about bindings.
599 In October 2009, there were interfaces available for the following
600 languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Dylan, Eiffel, Euphoria,
601 Ferite, Gambas, glib/GTK+, Haskell, ILE/RPG, Java, Lisp, Lua, Mono, .NET,
602 Object-Pascal, O'Caml, Pascal, Perl, PHP, PostgreSQL, Python, R, Rexx, Ruby,
603 Scheme, S-Lang, Smalltalk, SP-Forth, SPL, Tcl, Visual Basic, Visual FoxPro,
604 Q, wxwidgets and XBLite. By the time you read this, additional ones may have
607 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
609 Curl adheres to the HTTP spec, which basically means you can play with *any*
610 protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and
611 XML-RPC are all such ones. You can use -X to set custom requests and -H to
612 set custom headers (or replace internally generated ones).
614 Using libcurl is of course just as fine and you'd just use the proper
615 library options to do the same.
617 3.11 How do I POST with a different Content-Type?
619 You can always replace the internally generated headers with -H/--header.
620 To make a simple HTTP POST with text/xml as content-type, do something like:
622 curl -d "datatopost" -H "Content-Type: text/xml" [URL]
624 3.12 Why do FTP specific features over HTTP proxy fail?
626 Because when you use a HTTP proxy, the protocol spoken on the network will
627 be HTTP, even if you specify a FTP URL. This effectively means that you
628 normally can't use FTP specific features such as FTP upload and FTP quote
631 There is one exception to this rule, and that is if you can "tunnel through"
632 the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
633 and is generally not available as proxy admins usually disable tunneling to
634 other ports than 443 (which is used for HTTPS access through proxies).
636 3.13 Why does my single/double quotes fail?
638 To specify a command line option that includes spaces, you might need to
639 put the entire option within quotes. Like in:
641 curl -d " with spaces " url.com
645 curl -d ' with spaces ' url.com
647 Exactly what kind of quotes and how to do this is entirely up to the shell
648 or command line interpreter that you are using. For most unix shells, you
649 can more or less pick either single (') or double (") quotes. For
650 Windows/DOS prompts I believe you're forced to use double (") quotes.
652 Please study the documentation for your particular environment. Examples in
653 the curl docs will use a mix of both these ones as shown above. You must
654 adjust them to work in your environment.
656 Remember that curl works and runs on more operating systems than most single
657 individuals have ever tried.
659 3.14 Does curl support Javascript or PAC (automated proxy config)?
661 Many web pages do magic stuff using embedded Javascript. Curl and libcurl
662 have no built-in support for that, so it will be treated just like any other
665 .pac files are a netscape invention and are sometimes used by organizations
666 to allow them to differentiate which proxies to use. The .pac contents is
667 just a Javascript program that gets invoked by the browser and that returns
668 the name of the proxy to connect to. Since curl doesn't support Javascript,
669 it can't support .pac proxy configuration either.
671 Some workarounds usually suggested to overcome this Javascript dependency:
673 Depending on the Javascript complexity, write up a script that translates it
674 to another language and execute that.
676 Read the Javascript code and rewrite the same logic in another language.
678 Implement a Javascript interpreter, people have successfully used the
679 Mozilla Javascript engine in the past.
681 Ask your admins to stop this, for a static proxy setup or similar.
683 3.15 Can I do recursive fetches with curl?
685 No. curl itself has no code that performs recursive operations, such as
686 those performed by wget and similar tools.
688 There exist wrapper scripts with that functionality (for example the
689 curlmirror perl script), and you can write programs based on libcurl to do
690 it, but the command line tool curl itself cannot.
692 3.16 What certificates do I need when I use SSL?
694 There are three different kinds of "certificates" to keep track of when we
695 talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.
699 The server you communicate may require that you can provide this in order to
700 prove that you actually are who you claim to be. If the server doesn't
701 require this, you don't need a client certificate.
703 A client certificate is always used together with a private key, and the
704 private key has a pass phrase that protects it.
708 The server you communicate with has a server certificate. You can and should
709 verify this certificate to make sure that you are truly talking to the real
710 server and not a server impersonating it.
712 CERTIFICATE AUTHORITY CERTIFICATE ("CA cert")
714 You often have several CA certs in a CA cert bundle that can be used to
715 verify a server certificate that was signed by one of the authorities in the
716 bundle. curl does not come with a CA cert bundle but most curl installs
717 provide one. You can also override the default.
719 The server certificate verification process is made by using a Certificate
720 Authority certificate ("CA cert") that was used to sign the server
721 certificate. Server certificate verification is enabled by default in curl
722 and libcurl and is often the reason for problems as explained in FAQ entry
723 4.12 and the SSLCERTS document
724 (https://curl.haxx.se/docs/sslcerts.html). Server certificates that are
725 "self-signed" or otherwise signed by a CA that you do not have a CA cert
726 for, cannot be verified. If the verification during a connect fails, you are
727 refused access. You then need to explicitly disable the verification to
728 connect to the server.
730 3.17 How do I list the root dir of an FTP server?
732 There are two ways. The way defined in the RFC is to use an encoded slash
733 in the first path part. List the "/tmp" dir like this:
735 curl ftp://ftp.sunet.se/%2ftmp/
737 or the not-quite-kosher-but-more-readable way, by simply starting the path
738 section of the URL with a slash:
740 curl ftp://ftp.sunet.se//tmp/
742 3.18 Can I use curl to send a POST/PUT and not wait for a response?
746 But you could easily write your own program using libcurl to do such stunts.
748 3.19 How do I get HTTP from a host using a specific IP address?
750 For example, you may be trying out a web site installation that isn't yet in
751 the DNS. Or you have a site using multiple IP addresses for a given host
752 name and you want to address a specific one out of the set.
754 Set a custom Host: header that identifies the server name you want to reach
755 but use the target IP address in the URL:
757 curl --header "Host: www.example.com" http://127.0.0.1/
759 You can also opt to add faked host name entries to curl with the --resolve
760 option. That has the added benefit that things like redirects will also work
761 properly. The above operation would instead be done as:
763 curl --resolve www.example.com:80:127.0.0.1 http://www.example.com/
765 3.20 How to SFTP from my user's home directory?
767 Contrary to how FTP works, SFTP and SCP URLs specify the exact directory to
768 work with. It means that if you don't specify that you want the user's home
769 directory, you get the actual root directory.
771 To specify a file in your user's home directory, you need to use the correct
772 URL syntax which for sftp might look similar to:
774 curl -O -u user:password sftp://example.com/~/file.txt
776 and for SCP it is just a different protocol prefix:
778 curl -O -u user:password scp://example.com/~/file.txt
780 3.21 Protocol xxx not supported or disabled in libcurl
782 When passing on a URL to curl to use, it may respond that the particular
783 protocol is not supported or disabled. The particular way this error message
784 is phrased is because curl doesn't make a distinction internally of whether
785 a particular protocol is not supported (i.e. never got any code added that
786 knows how to speak that protocol) or if it was explicitly disabled. curl can
787 be built to only support a given set of protocols, and the rest would then
788 be disabled or not supported.
790 Note that this error will also occur if you pass a wrongly spelled protocol
791 part as in "htpt://example.com" or as in the less evident case if you prefix
792 the protocol part with a space as in " http://example.com/".
794 3.22 curl -X gives me HTTP problems
796 In normal circumstances, -X should hardly ever be used.
798 By default you use curl without explicitly saying which request method to
799 use when the URL identifies a HTTP transfer. If you just pass in a URL like
800 "curl http://example.com" it will use GET. If you use -d or -F curl will use
801 POST, -I will cause a HEAD and -T will make it a PUT.
803 If for whatever reason you're not happy with these default choices that curl
804 does for you, you can override those request methods by specifying -X
805 [WHATEVER]. This way you can for example send a DELETE by doing "curl -X
808 It is thus pointless to do "curl -XGET [URL]" as GET would be used
809 anyway. In the same vein it is pointless to do "curl -X POST -d data
810 [URL]"... But you can make a fun and somewhat rare request that sends a
811 request-body in a GET request with something like "curl -X GET -d data
814 Note that -X doesn't actually change curl's behavior as it only modifies the
815 actual string sent in the request, but that may of course trigger a
816 different set of events.
818 Accordingly, by using -XPOST on a command line that for example would follow
819 a 303 redirect, you will effectively prevent curl from behaving
825 4.1 Problems connecting to SSL servers.
827 It took a very long time before we could sort out why curl had problems to
828 connect to certain SSL servers when using SSLeay or OpenSSL v0.9+. The
829 error sometimes showed up similar to:
831 16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:
833 It turned out to be because many older SSL servers don't deal with SSLv3
834 requests properly. To correct this problem, tell curl to select SSLv2 from
835 the command line (-2/--sslv2).
837 There have also been examples where the remote server didn't like the SSLv2
838 request and instead you had to force curl to use SSLv3 with -3/--sslv3.
840 4.2 Why do I get problems when I use & or % in the URL?
842 In general unix shells, the & symbol is treated specially and when used, it
843 runs the specified command in the background. To safely send the & as a part
844 of a URL, you should quote the entire URL by using single (') or double (")
845 quotes around it. Similar problems can also occur on some shells with other
846 characters, including ?*!$~(){}<>\|;`. When in doubt, quote the URL.
848 An example that would invoke a remote CGI that uses &-symbols could be:
850 curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
852 In Windows, the standard DOS shell treats the percent sign specially and you
853 need to use TWO percent signs for each single one you want to use in the
856 If you want a literal percent sign to be part of the data you pass in a POST
857 using -d/--data you must encode it as '%25' (which then also needs the
858 percent sign doubled on Windows machines).
860 4.3 How can I use {, }, [ or ] to specify multiple URLs?
862 Because those letters have a special meaning to the shell, and to be used in
863 a URL specified to curl you must quote them.
865 An example that downloads two URLs (sequentially) would do:
867 curl '{curl,www}.haxx.se'
869 To be able to use those letters as actual parts of the URL (without using
870 them for the curl URL "globbing" system), use the -g/--globoff option:
872 curl -g 'www.site.com/weirdname[].html'
874 4.4 Why do I get downloaded data even though the web page doesn't exist?
876 Curl asks remote servers for the page you specify. If the page doesn't exist
877 at the server, the HTTP protocol defines how the server should respond and
878 that means that headers and a "page" will be returned. That's simply how
881 By using the --fail option you can tell curl explicitly to not get any data
882 if the HTTP return code doesn't say success.
884 4.5 Why do I get return code XXX from a HTTP server?
886 RFC2616 clearly explains the return codes. This is a short transcript. Go
887 read the RFC for exact details:
889 4.5.1 "400 Bad Request"
891 The request could not be understood by the server due to malformed
892 syntax. The client SHOULD NOT repeat the request without modifications.
894 4.5.2 "401 Unauthorized"
896 The request requires user authentication.
898 4.5.3 "403 Forbidden"
900 The server understood the request, but is refusing to fulfil it.
901 Authorization will not help and the request SHOULD NOT be repeated.
903 4.5.4 "404 Not Found"
905 The server has not found anything matching the Request-URI. No indication
906 is given of whether the condition is temporary or permanent.
908 4.5.5 "405 Method Not Allowed"
910 The method specified in the Request-Line is not allowed for the resource
911 identified by the Request-URI. The response MUST include an Allow header
912 containing a list of valid methods for the requested resource.
914 4.5.6 "301 Moved Permanently"
916 If you get this return code and an HTML output similar to this:
918 <H1>Moved Permanently</H1> The document has moved <A
919 HREF="http://same_url_now_with_a_trailing_slash/">here</A>.
921 it might be because you request a directory URL but without the trailing
922 slash. Try the same operation again _with_ the trailing URL, or use the
923 -L/--location option to follow the redirection.
925 4.6 Can you tell me what error code 142 means?
927 All curl error codes are described at the end of the man page, in the
928 section called "EXIT CODES".
930 Error codes that are larger than the highest documented error code means
931 that curl has exited due to a crash. This is a serious error, and we
932 appreciate a detailed bug report from you that describes how we could go
933 ahead and repeat this!
935 4.7 How do I keep user names and passwords secret in Curl command lines?
937 This problem has two sides:
939 The first part is to avoid having clear-text passwords in the command line
940 so that they don't appear in 'ps' outputs and similar. That is easily
941 avoided by using the "-K" option to tell curl to read parameters from a file
942 or stdin to which you can pass the secret info. curl itself will also
943 attempt to "hide" the given password by blanking out the option - this
944 doesn't work on all platforms.
946 To keep the passwords in your account secret from the rest of the world is
947 not a task that curl addresses. You could of course encrypt them somehow to
948 at least hide them from being read by human eyes, but that is not what
949 anyone would call security.
951 Also note that regular HTTP (using Basic authentication) and FTP passwords
952 are sent in clear across the network. All it takes for anyone to fetch them
953 is to listen on the network. Eavesdropping is very easy. Use more secure
954 authentication methods (like Digest, Negotiate or even NTLM) or consider the
955 SSL-based alternatives HTTPS and FTPS.
959 It is not a bug if the behavior is documented. Read the docs first.
960 Especially check out the KNOWN_BUGS file, it may be a documented bug!
962 If it is a problem with a binary you've downloaded or a package for your
963 particular platform, try contacting the person who built the package/archive
966 If there is a bug, read the BUGS document first. Then report it as described
969 4.9 Curl can't authenticate to the server that requires NTLM?
971 NTLM support requires OpenSSL, GnuTLS, mbedTLS, NSS, Secure Transport, or
972 Microsoft Windows libraries at build-time to provide this functionality.
974 NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You
975 should not use such ones.
977 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
979 Many web servers allow or demand that the administrator configures the
980 server properly for these requests to work on the web server.
982 Some servers seem to support HEAD only on certain kinds of URLs.
984 To fully grasp this, try the documentation for the particular server
985 software you're trying to interact with. This is not anything curl can do
988 4.11 Why does my HTTP range requests return the full document?
990 Because the range may not be supported by the server, or the server may
991 choose to ignore it and return the full document anyway.
993 4.12 Why do I get "certificate verify failed" ?
995 You invoke curl 7.10 or later to communicate on a https:// URL and get an
996 error back looking something similar to this:
998 curl: (35) SSL: error:14090086:SSL routines:
999 SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
1001 Then it means that curl couldn't verify that the server's certificate was
1002 good. Curl verifies the certificate using the CA cert bundle that comes with
1003 the curl installation.
1005 To disable the verification (which makes it act like curl did before 7.10),
1006 use -k. This does however enable man-in-the-middle attacks.
1008 If you get this failure but are having a CA cert bundle installed and used,
1009 the server's certificate is not signed by one of the CA's in the bundle. It
1010 might for example be self-signed. You then correct this problem by obtaining
1011 a valid CA cert for the server. Or again, decrease the security by disabling
1014 Details are also in the SSLCERTS file in the release archives, found online
1015 here: https://curl.haxx.se/docs/sslcerts.html
1017 4.13 Why is curl -R on Windows one hour off?
1019 During daylight savings time, when -R is used, curl will set a time that
1020 appears one hour off. This happens due to a flaw in how Windows stores and
1021 uses file modification times and it is not easily worked around. For details
1022 on this problem, read this: http://www.codeproject.com/datetime/dstbugs.asp
1024 4.14 Redirects work in browser but not with curl!
1026 curl supports HTTP redirects fine (see item 3.8). Browsers generally support
1027 at least two other ways to perform redirects that curl does not:
1029 Meta tags. You can write a HTML tag that will cause the browser to redirect
1030 to another given URL after a certain time.
1032 Javascript. You can write a Javascript program embedded in a HTML page that
1033 redirects the browser to another given URL.
1035 There is no way to make curl follow these redirects. You must either
1036 manually figure out what the page is set to do, or you write a script that
1037 parses the results and fetches the new URL.
1039 4.15 FTPS doesn't work
1041 curl supports FTPS (sometimes known as FTP-SSL) both implicit and explicit
1044 When a URL is used that starts with FTPS://, curl assumes implicit SSL on
1045 the control connection and will therefore immediately connect and try to
1046 speak SSL. FTPS:// connections default to port 990.
1048 To use explicit FTPS, you use a FTP:// URL and the --ftp-ssl option (or one
1049 of its related flavours). This is the most common method, and the one
1050 mandated by RFC4217. This kind of connection then of course uses the
1051 standard FTP port 21 by default.
1053 4.16 My HTTP POST or PUT requests are slow!
1055 libcurl makes all POST and PUT requests (except for POST requests with a
1056 very tiny request body) use the "Expect: 100-continue" header. This header
1057 allows the server to deny the operation early so that libcurl can bail out
1058 already before having to send any data. This is useful in authentication
1061 However, many servers don't implement the Expect: stuff properly and if the
1062 server doesn't respond (positively) within 1 second libcurl will continue
1063 and send off the data anyway.
1065 You can disable libcurl's use of the Expect: header the same way you disable
1066 any header, using -H / CURLOPT_HTTPHEADER, or by forcing it to use HTTP 1.0.
1068 4.17 Non-functional connect timeouts
1070 In most Windows setups having a timeout longer than 21 seconds make no
1071 difference, as it will only send 3 TCP SYN packets and no more. The second
1072 packet sent three seconds after the first and the third six seconds after
1073 the second. No more than three packets are sent, no matter how long the
1076 See option TcpMaxConnectRetransmissions on this page:
1077 https://support.microsoft.com/en-us/kb/175523/en-us
1079 Also, even on non-Windows systems there may run a firewall or anti-virus
1080 software or similar that accepts the connection but does not actually do
1081 anything else. This will make (lib)curl to consider the connection connected
1082 and thus the connect timeout won't trigger.
1084 4.18 file:// URLs containing drive letters (Windows, NetWare)
1086 When using cURL to try to download a local file, one might use a URL
1091 You'll find that even if D:\blah.txt does exist, cURL returns a 'file
1094 According to RFC 1738 (https://www.ietf.org/rfc/rfc1738.txt),
1095 file:// URLs must contain a host component, but it is ignored by
1096 most implementations. In the above example, 'D:' is treated as the
1097 host component, and is taken away. Thus, cURL tries to open '/blah.txt'.
1098 If your system is installed to drive C:, that will resolve to 'C:\blah.txt',
1099 and if that doesn't exist you will get the not found error.
1101 To fix this problem, use file:// URLs with *three* leading slashes:
1105 Alternatively, if it makes more sense, specify 'localhost' as the host
1108 file://localhost/D:/blah.txt
1110 In either case, cURL should now be looking for the correct file.
1112 4.19 Why doesn't cURL return an error when the network cable is unplugged?
1114 Unplugging a cable is not an error situation. The TCP/IP protocol stack
1115 was designed to be fault tolerant, so even though there may be a physical
1116 break somewhere the connection shouldn't be affected, just possibly
1117 delayed. Eventually, the physical break will be fixed or the data will be
1118 re-routed around the physical problem through another path.
1120 In such cases, the TCP/IP stack is responsible for detecting when the
1121 network connection is irrevocably lost. Since with some protocols it is
1122 perfectly legal for the client to wait indefinitely for data, the stack may
1123 never report a problem, and even when it does, it can take up to 20 minutes
1124 for it to detect an issue. The curl option --keepalive-time enables
1125 keep-alive support in the TCP/IP stack which makes it periodically probe the
1126 connection to make sure it is still available to send data. That should
1127 reliably detect any TCP/IP network failure.
1129 But even that won't detect the network going down before the TCP/IP
1130 connection is established (e.g. during a DNS lookup) or using protocols that
1131 don't use TCP. To handle those situations, curl offers a number of timeouts
1132 on its own. --speed-limit/--speed-time will abort if the data transfer rate
1133 falls too low, and --connect-timeout and --max-time can be used to put an
1134 overall timeout on the connection phase or the entire transfer.
1136 A libcurl-using application running in a known physical environment (e.g.
1137 an embedded device with only a single network connection) may want to act
1138 immediately if its lone network connection goes down. That can be achieved
1139 by having the application monitor the network connection on its own using an
1140 OS-specific mechanism, then signalling libcurl to abort (see also item 5.13).
1142 4.20 curl doesn't return error for HTTP non-200 responses!
1144 Correct. Unless you use -f (--fail).
1146 When doing HTTP transfers, curl will perform exactly what you're asking it
1147 to do and if successful it will not return an error. You can use curl to
1148 test your web server's "file not found" page (that gets 404 back), you can
1149 use it to check your authentication protected web pages (that get a 401
1152 The specific HTTP response code does not constitute a problem or error for
1153 curl. It simply sends and delivers HTTP as you asked and if that worked,
1154 everything is fine and dandy. The response code is generally providing more
1155 higher level error information that curl doesn't care about. The error was
1156 not in the HTTP transfer.
1158 If you want your command line to treat error codes in the 400 and up range
1159 as errors and thus return a non-zero value and possibly show an error
1160 message, curl has a dedicated option for that: -f (CURLOPT_FAILONERROR in
1163 You can also use the -w option and the variable %{response_code} to extract
1164 the exact response code that was return in the response.
1166 4.21 Why is there a HTTP/1.1 in my HTTP/2 request?
1168 If you use verbose to see the HTTP request when you send off a HTTP/2
1169 request, it will still say 1.1.
1171 The reason for this is that we first generate the request to send using the
1172 old 1.1 style and show that request in the verbose output, and then we
1173 convert it over to the binary header-compressed HTTP/2 style. The actual
1174 "1.1" part from that request is then not actually used in the transfer. The
1175 binary HTTP/2 headers are not human readable.
1179 5.1 Is libcurl thread-safe?
1183 We have written the libcurl code specifically adjusted for multi-threaded
1184 programs. libcurl will use thread-safe functions instead of non-safe ones if
1185 your system has such. Note that you must never share the same handle in
1188 There may be some exceptions to thread safety depending on how libcurl was
1189 built. Please review the guidelines for thread safety to learn more:
1190 https://curl.haxx.se/libcurl/c/threadsafe.html
1192 5.2 How can I receive all data into a large memory chunk?
1194 [ See also the examples/getinmemory.c source ]
1196 You are in full control of the callback function that gets called every time
1197 there is data received from the remote server. You can make that callback do
1198 whatever you want. You do not have to write the received data to a file.
1200 One solution to this problem could be to have a pointer to a struct that you
1201 pass to the callback function. You set the pointer using the
1202 CURLOPT_WRITEDATA option. Then that pointer will be passed to the callback
1203 instead of a FILE * to a file:
1205 /* imaginary struct */
1206 struct MemoryStruct {
1211 /* imaginary callback function */
1213 WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
1215 size_t realsize = size * nmemb;
1216 struct MemoryStruct *mem = (struct MemoryStruct *)data;
1218 mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
1220 memcpy(&(mem->memory[mem->size]), ptr, realsize);
1221 mem->size += realsize;
1222 mem->memory[mem->size] = 0;
1227 5.3 How do I fetch multiple files with libcurl?
1229 libcurl has excellent support for transferring multiple files. You should
1230 just repeatedly set new URLs with curl_easy_setopt() and then transfer it
1231 with curl_easy_perform(). The handle you get from curl_easy_init() is not
1232 only reusable, but you're even encouraged to reuse it if you can, as that
1233 will enable libcurl to use persistent connections.
1235 5.4 Does libcurl do Winsock initialization on win32 systems?
1237 Yes, if told to in the curl_global_init() call.
1239 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
1241 Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
1242 that DLL use the FILE * (as the DLL and the client application cannot access
1243 each others' variable memory areas). If you set CURLOPT_WRITEDATA you must
1244 also use CURLOPT_WRITEFUNCTION as well to set a function that writes the
1245 file, even if that simply writes the data to the specified FILE *.
1246 Similarly, if you use CURLOPT_READDATA you must also specify
1247 CURLOPT_READFUNCTION.
1249 5.6 What about Keep-Alive or persistent connections?
1251 curl and libcurl have excellent support for persistent connections when
1252 transferring several files from the same server. Curl will attempt to reuse
1253 connections for all URLs specified on the same command line/config file, and
1254 libcurl will reuse connections for all transfers that are made using the
1255 same libcurl handle.
1257 When you use the easy interface, the connection cache is kept within the
1258 easy handle. If you instead use the multi interface, the connection cache
1259 will be kept within the multi handle and will be shared among all the easy
1260 handles that are used within the same multi handle.
1262 5.7 Link errors when building libcurl on Windows!
1264 You need to make sure that your project, and all the libraries (both static
1265 and dynamic) that it links against, are compiled/linked against the same run
1268 This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
1269 options to the command line compiler. /MD (linking against MSVCRT dll) seems
1270 to be the most commonly used option.
1272 When building an application that uses the static libcurl library, you must
1273 add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for
1274 dynamic import symbols. If you're using Visual Studio, you need to instead
1275 add CURL_STATICLIB in the "Preprocessor Definitions" section.
1277 If you get linker error like "unknown symbol __imp__curl_easy_init ..." you
1278 have linked against the wrong (static) library. If you want to use the
1279 libcurl.dll and import lib, you don't need any extra CFLAGS, but use one of
1280 the import libraries below. These are the libraries produced by the various
1281 lib/Makefile.* files:
1283 Target: static lib. import lib for libcurl*.dll.
1284 -----------------------------------------------------------
1285 MingW: libcurl.a libcurldll.a
1286 MSVC (release): libcurl.lib libcurl_imp.lib
1287 MSVC (debug): libcurld.lib libcurld_imp.lib
1288 Borland: libcurl.lib libcurl_imp.lib
1290 5.8 libcurl.so.X: open failed: No such file or directory
1292 This is an error message you might get when you try to run a program linked
1293 with a shared version of libcurl and your run-time linker (ld.so) couldn't
1294 find the shared library named libcurl.so.X. (Where X is the number of the
1295 current libcurl ABI, typically 3 or 4).
1297 You need to make sure that ld.so finds libcurl.so.X. You can do that
1298 multiple ways, and it differs somewhat between different operating systems,
1299 but they are usually:
1301 * Add an option to the linker command line that specify the hard-coded path
1302 the run-time linker should check for the lib (usually -R)
1304 * Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
1305 should check for libs
1307 * Adjust the system's config to check for libs in the directory where you've
1308 put the dir (like Linux's /etc/ld.so.conf)
1310 'man ld.so' and 'man ld' will tell you more details
1312 5.9 How does libcurl resolve host names?
1314 libcurl supports a large a number of different name resolve functions. One
1315 of them is picked at build-time and will be used unconditionally. Thus, if
1316 you want to change name resolver function you must rebuild libcurl and tell
1317 it to use a different function.
1319 - The non-IPv6 resolver that can use one out of four host name resolve calls
1320 (depending on what your system supports):
1323 B - gethostbyname_r() with 3 arguments
1324 C - gethostbyname_r() with 5 arguments
1325 D - gethostbyname_r() with 6 arguments
1327 - The IPv6-resolver that uses getaddrinfo()
1329 - The c-ares based name resolver that uses the c-ares library for resolves.
1330 Using this offers asynchronous name resolves.
1332 - The threaded resolver (default option on Windows). It uses:
1334 A - gethostbyname() on plain IPv4 hosts
1335 B - getaddrinfo() on IPv6 enabled hosts
1337 Also note that libcurl never resolves or reverse-lookups addresses given as
1338 pure numbers, such as 127.0.0.1 or ::1.
1340 5.10 How do I prevent libcurl from writing the response to stdout?
1342 libcurl provides a default built-in write function that writes received data
1343 to stdout. Set the CURLOPT_WRITEFUNCTION to receive the data, or possibly
1344 set CURLOPT_WRITEDATA to a different FILE * handle.
1346 5.11 How do I make libcurl not receive the whole HTTP response?
1348 You make the write callback (or progress callback) return an error and
1349 libcurl will then abort the transfer.
1351 5.12 Can I make libcurl fake or hide my real IP address?
1353 No. libcurl operates on a higher level. Besides, faking IP address would
1354 imply sending IP packet with a made-up source address, and then you normally
1355 get a problem with receiving the packet sent back as they would then not be
1358 If you use a proxy to access remote sites, the sites will not see your local
1359 IP address but instead the address of the proxy.
1361 Also note that on many networks NATs or other IP-munging techniques are used
1362 that makes you see and use a different IP address locally than what the
1363 remote server will see you coming from. You may also consider using
1364 https://www.torproject.org/ .
1366 5.13 How do I stop an ongoing transfer?
1368 With the easy interface you make sure to return the correct error code from
1369 one of the callbacks, but none of them are instant. There is no function you
1370 can call from another thread or similar that will stop it immediately.
1371 Instead, you need to make sure that one of the callbacks you use returns an
1372 appropriate value that will stop the transfer. Suitable callbacks that you
1373 can do this with include the progress callback, the read callback and the
1376 If you're using the multi interface, you can also stop a transfer by
1377 removing the particular easy handle from the multi stack at any moment you
1378 think the transfer is done or when you wish to abort the transfer.
1380 5.14 Using C++ non-static functions for callbacks?
1382 libcurl is a C library, it doesn't know anything about C++ member functions.
1384 You can overcome this "limitation" with a relative ease using a static
1385 member function that is passed a pointer to the class:
1387 // f is the pointer to your object.
1388 static YourClass::func(void *buffer, size_t sz, size_t n, void *f)
1390 // Call non-static member function.
1391 static_cast<YourClass*>(f)->nonStaticFunction();
1394 // This is how you pass pointer to the static function:
1395 curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass:func);
1396 curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);
1398 5.15 How do I get an FTP directory listing?
1400 If you end the FTP URL you request with a slash, libcurl will provide you
1401 with a directory listing of that given directory. You can also set
1402 CURLOPT_CUSTOMREQUEST to alter what exact listing command libcurl would use
1405 The follow-up question tends to be how is a program supposed to parse the
1406 directory listing. How does it know what's a file and what's a dir and what's
1407 a symlink etc. If the FTP server supports the MLSD command then it will
1408 return data in a machine-readable format that can be parsed for type. The
1409 types are specified by RFC3659 section 7.5.1. If MLSD is not supported then
1410 you have to work with what you're given. The LIST output format is entirely
1411 at the server's own liking and the NLST output doesn't reveal any types and
1412 in many cases doesn't even include all the directory entries. Also, both LIST
1413 and NLST tend to hide unix-style hidden files (those that start with a dot)
1414 by default so you need to do "LIST -a" or similar to see them.
1416 Example - List only directories.
1417 ftp.funet.fi supports MLSD and ftp.kernel.org does not:
1419 curl -s ftp.funet.fi/pub/ -X MLSD | \
1420 perl -lne 'print if s/(?:^|;)type=dir;[^ ]+ (.+)$/$1/'
1422 curl -s ftp.kernel.org/pub/linux/kernel/ | \
1423 perl -lne 'print if s/^d[-rwx]{9}(?: +[^ ]+){7} (.+)$/$1/'
1425 If you need to parse LIST output in libcurl one such existing
1426 list parser is available at https://cr.yp.to/ftpparse.html Versions of
1427 libcurl since 7.21.0 also provide the ability to specify a wildcard to
1428 download multiple files from one FTP directory.
1430 5.16 I want a different time-out!
1432 Time and time again users realize that CURLOPT_TIMEOUT and
1433 CURLOPT_CONNECTIMEOUT are not sufficiently advanced or flexible to cover all
1434 the various use cases and scenarios applications end up with.
1436 libcurl offers many more ways to time-out operations. A common alternative
1437 is to use the CURLOPT_LOW_SPEED_LIMIT and CURLOPT_LOW_SPEED_TIME options to
1438 specify the lowest possible speed to accept before to consider the transfer
1441 The most flexible way is by writing your own time-out logic and using
1442 CURLOPT_PROGRESSFUNCTION (perhaps in combination with other callbacks) and
1443 use that to figure out exactly when the right condition is met when the
1444 transfer should get stopped.
1446 5.17 Can I write a server with libcurl?
1448 No. libcurl offers no functions or building blocks to build any kind of
1449 internet protocol server. libcurl is only a client-side library. For server
1450 libraries, you need to continue your search elsewhere but there exist many
1451 good open source ones out there for most protocols you could possibly want a
1452 server for. And there are really good stand-alone ones that have been tested
1453 and proven for many years. There's no need for you to reinvent them!
1455 5.18 Does libcurl use threads?
1457 Put simply: no, libcurl will execute in the same thread you call it in. All
1458 callbacks will be called in the same thread as the one you call libcurl in.
1460 If you want to avoid your thread to be blocked by the libcurl call, you make
1461 sure you use the non-blocking API which will do transfers asynchronously -
1462 but still in the same single thread.
1464 libcurl will potentially internally use threads for name resolving, if it
1465 was built to work like that, but in those cases it'll create the child
1466 threads by itself and they will only be used and then killed internally by
1467 libcurl and never exposed to the outside.
1471 Curl and libcurl are released under a MIT/X derivate license. The license is
1472 very liberal and should not impose a problem for your project. This section
1473 is just a brief summary for the cases we get the most questions. (Parts of
1474 this section was much enhanced by Bjorn Reese.)
1476 We are not lawyers and this is not legal advice. You should probably consult
1477 one if you want true and accurate legal insights without our prejudice. Note
1478 especially that this section concerns the libcurl license only; compiling in
1479 features of libcurl that depend on other libraries (e.g. OpenSSL) may affect
1480 the licensing obligations of your application.
1482 6.1 I have a GPL program, can I use the libcurl library?
1486 Since libcurl may be distributed under the MIT/X derivate license, it can be
1487 used together with GPL in any software.
1489 6.2 I have a closed-source program, can I use the libcurl library?
1493 libcurl does not put any restrictions on the program that uses the library.
1495 6.3 I have a BSD licensed program, can I use the libcurl library?
1499 libcurl does not put any restrictions on the program that uses the library.
1501 6.4 I have a program that uses LGPL libraries, can I use libcurl?
1505 The LGPL license doesn't clash with other licenses.
1507 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
1511 The MIT/X derivate license practically allows you to do almost anything with
1512 the sources, on the condition that the copyright texts in the sources are
1515 6.6 Can you please change the curl/libcurl license to XXXX?
1519 We have carefully picked this license after years of development and
1520 discussions and a large amount of people have contributed with source code
1521 knowing that this is the license we use. This license puts the restrictions
1522 we want on curl/libcurl and it does not spread to other programs or
1523 libraries that use it. It should be possible for everyone to use libcurl or
1524 curl in their projects, no matter what license they already have in use.
1526 6.7 What are my obligations when using libcurl in my commercial apps?
1528 Next to none. All you need to adhere to is the MIT-style license (stated in
1529 the COPYING file) which basically says you have to include the copyright
1530 notice in "all copies" and that you may not use the copyright holder's name
1531 when promoting your software.
1533 You do not have to release any of your source code.
1535 You do not have to reveal or make public any changes to the libcurl source
1538 You do not have to broadcast to the world that you are using libcurl within
1541 All we ask is that you disclose "the copyright notice and this permission
1542 notice" somewhere. Most probably like in the documentation or in the section
1543 where other third party dependencies already are mentioned and acknowledged.
1545 As can be seen here: https://curl.haxx.se/docs/companies.html and elsewhere,
1546 more and more companies are discovering the power of libcurl and take
1547 advantage of it even in commercial environments.
1552 7.1 What is PHP/CURL?
1554 The module for PHP that makes it possible for PHP programs to access curl-
1555 functions from within PHP.
1557 In the cURL project we call this module PHP/CURL to differentiate it from
1558 curl the command line tool and libcurl the library. The PHP team however
1559 does not refer to it like this (for unknown reasons). They call it plain
1560 CURL (often using all caps) or sometimes ext/curl, but both cause much
1561 confusion to users which in turn gives us a higher question load.
1563 7.2 Who wrote PHP/CURL?
1565 PHP/CURL was initially written by Sterling Hughes.
1567 7.3 Can I perform multiple requests using the same handle?
1569 Yes - at least in PHP version 4.3.8 and later (this has been known to not
1570 work in earlier versions, but the exact version when it started to work is
1573 After a transfer, you just set new options in the handle and make another
1574 transfer. This will make libcurl re-use the same connection if it can.
1576 7.4 Does PHP/CURL have dependencies?
1578 PHP/CURL is a module that comes with the regular PHP package. It depends on
1579 and uses libcurl, so you need to have libcurl installed properly before
1580 PHP/CURL can be used.