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 well at the side. Curl's output can be piped into another program or
219 redirected 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 good
223 we will agree. If you wanna add more protocols, we may very well agree.
225 If you want someone else to do 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 effort 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 better 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 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 full time. We do this voluntarily, mostly in our spare time.
251 Occasionally companies pay individual developers to work on curl, but that's
252 up to each company and developer. This 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 with coding, documenting or testing etc.
265 1.7 What about CURL from curl.com?
267 During the summer of 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 whom 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, to contribute their
289 suggestions and to 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 be shipped with curl was very outdated and
328 must 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 that ever
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 good
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 most 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 gets 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 arbitrarily use -F or -d, the choice between -F or -d depends on the
524 HTTP operation you need curl to do and what the web server that will receive
527 If the form you're trying to submit uses the type 'multipart/form-data', then
528 and only then you must use the -F type. In all the most common cases, you
529 should use -d which then causes a posting with the type
530 'application/x-www-form-urlencoded'.
532 This is described in some detail in the MANUAL and TheArtOfHttpScripting
533 documents, and if you don't understand it the first time, read it again
534 before you post questions about this to the mailing list. Also, try reading
535 through the mailing list archives for old postings and questions regarding
538 3.4 How do I tell curl to run custom FTP commands?
540 You can tell curl to perform optional commands both before and/or after a
541 file transfer. Study the -Q/--quote option.
543 Since curl is used for file transfers, you don't normally use curl to
544 perform FTP commands without transferring anything. Therefore you must
545 always specify a URL to transfer to/from even when doing custom FTP
546 commands, or use -I which implies the "no body" option sent to libcurl.
548 3.5 How can I disable the Accept: */* header?
550 You can change all internally generated headers by adding a replacement with
551 the -H/--header option. By adding a header with empty contents you safely
552 disable that one. Use -H "Accept:" to disable that specific header.
554 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
556 To curl, all contents are alike. It doesn't matter how the page was
557 generated. It may be ASP, PHP, Perl, shell-script, SSI or plain HTML
558 files. There's no difference to curl and it doesn't even know what kind of
559 language that generated the page.
561 See also item 3.14 regarding javascript.
563 3.7 Can I use curl to delete/rename a file through FTP?
565 Yes. You specify custom FTP commands with -Q/--quote.
567 One example would be to delete a file after you have downloaded it:
569 curl -O ftp://download.com/coolfile -Q '-DELE coolfile'
571 or rename a file after upload:
573 curl -T infile ftp://upload.com/dir/ -Q "-RNFR infile" -Q "-RNTO newname"
575 3.8 How do I tell curl to follow HTTP redirects?
577 Curl does not follow so-called redirects by default. The Location: header
578 that informs the client about this is only interpreted if you're using the
579 -L/--location option. As in:
581 curl -L http://redirector.com
583 Not all redirects are HTTP ones, see 4.14
585 3.9 How do I use curl in my favorite programming language?
587 There exist many language interfaces/bindings for curl that integrates it
588 better with various languages. If you are fluid in a script language, you
589 may very well opt to use such an interface instead of using the command line
592 Find out more about which languages that support curl directly, and how to
593 install and use them, in the libcurl section of the curl web site:
594 https://curl.haxx.se/libcurl/
596 All the various bindings to libcurl are made by other projects and people,
597 outside of the cURL project. The cURL project itself only produces libcurl
598 with its plain C API. If you don't find anywhere else to ask you can ask
599 about bindings on the curl-library list too, but be prepared that people on
600 that list may not know anything about bindings.
602 In October 2009, there were interfaces available for the following
603 languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Dylan, Eiffel, Euphoria,
604 Ferite, Gambas, glib/GTK+, Haskell, ILE/RPG, Java, Lisp, Lua, Mono, .NET,
605 Object-Pascal, O'Caml, Pascal, Perl, PHP, PostgreSQL, Python, R, Rexx, Ruby,
606 Scheme, S-Lang, Smalltalk, SP-Forth, SPL, Tcl, Visual Basic, Visual FoxPro,
607 Q, wxwidgets and XBLite. By the time you read this, additional ones may have
610 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
612 Curl adheres to the HTTP spec, which basically means you can play with *any*
613 protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and
614 XML-RPC are all such ones. You can use -X to set custom requests and -H to
615 set custom headers (or replace internally generated ones).
617 Using libcurl is of course just as good and you'd just use the proper
618 library options to do the same.
620 3.11 How do I POST with a different Content-Type?
622 You can always replace the internally generated headers with -H/--header.
623 To make a simple HTTP POST with text/xml as content-type, do something like:
625 curl -d "datatopost" -H "Content-Type: text/xml" [URL]
627 3.12 Why do FTP specific features over HTTP proxy fail?
629 Because when you use a HTTP proxy, the protocol spoken on the network will
630 be HTTP, even if you specify a FTP URL. This effectively means that you
631 normally can't use FTP specific features such as FTP upload and FTP quote
634 There is one exception to this rule, and that is if you can "tunnel through"
635 the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
636 and is generally not available as proxy admins usually disable tunneling to
637 ports other than 443 (which is used for HTTPS access through proxies).
639 3.13 Why does my single/double quotes fail?
641 To specify a command line option that includes spaces, you might need to
642 put the entire option within quotes. Like in:
644 curl -d " with spaces " url.com
648 curl -d ' with spaces ' url.com
650 Exactly what kind of quotes and how to do this is entirely up to the shell
651 or command line interpreter that you are using. For most unix shells, you
652 can more or less pick either single (') or double (") quotes. For
653 Windows/DOS prompts I believe you're forced to use double (") quotes.
655 Please study the documentation for your particular environment. Examples in
656 the curl docs will use a mix of both of these as shown above. You must
657 adjust them to work in your environment.
659 Remember that curl works and runs on more operating systems than most single
660 individuals have ever tried.
662 3.14 Does curl support Javascript or PAC (automated proxy config)?
664 Many web pages do magic stuff using embedded Javascript. Curl and libcurl
665 have no built-in support for that, so it will be treated just like any other
668 .pac files are a netscape invention and are sometimes used by organizations
669 to allow them to differentiate which proxies to use. The .pac contents is
670 just a Javascript program that gets invoked by the browser and that returns
671 the name of the proxy to connect to. Since curl doesn't support Javascript,
672 it can't support .pac proxy configuration either.
674 Some workarounds usually suggested to overcome this Javascript dependency:
676 Depending on the Javascript complexity, write up a script that translates it
677 to another language and execute that.
679 Read the Javascript code and rewrite the same logic in another language.
681 Implement a Javascript interpreter, people have successfully used the
682 Mozilla Javascript engine in the past.
684 Ask your admins to stop this, for a static proxy setup or similar.
686 3.15 Can I do recursive fetches with curl?
688 No. curl itself has no code that performs recursive operations, such as
689 those performed by wget and similar tools.
691 There exists wrapper scripts with that functionality (for example the
692 curlmirror perl script), and you can write programs based on libcurl to do
693 it, but the command line tool curl itself cannot.
695 3.16 What certificates do I need when I use SSL?
697 There are three different kinds of "certificates" to keep track of when we
698 talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.
702 The server you communicate with may require that you can provide this in
703 order to prove that you actually are who you claim to be. If the server
704 doesn't require this, you don't need a client certificate.
706 A client certificate is always used together with a private key, and the
707 private key has a pass phrase that protects it.
711 The server you communicate with has a server certificate. You can and should
712 verify this certificate to make sure that you are truly talking to the real
713 server and not a server impersonating it.
715 CERTIFICATE AUTHORITY CERTIFICATE ("CA cert")
717 You often have several CA certs in a CA cert bundle that can be used to
718 verify a server certificate that was signed by one of the authorities in the
719 bundle. curl does not come with a CA cert bundle but most curl installs
720 provide one. You can also override the default.
722 The server certificate verification process is made by using a Certificate
723 Authority certificate ("CA cert") that was used to sign the server
724 certificate. Server certificate verification is enabled by default in curl
725 and libcurl and is often the reason for problems as explained in FAQ entry
726 4.12 and the SSLCERTS document
727 (https://curl.haxx.se/docs/sslcerts.html). Server certificates that are
728 "self-signed" or otherwise signed by a CA that you do not have a CA cert
729 for, cannot be verified. If the verification during a connect fails, you are
730 refused access. You then need to explicitly disable the verification to
731 connect to the server.
733 3.17 How do I list the root dir of an FTP server?
735 There are two ways. The way defined in the RFC is to use an encoded slash
736 in the first path part. List the "/tmp" dir like this:
738 curl ftp://ftp.sunet.se/%2ftmp/
740 or the not-quite-kosher-but-more-readable way, by simply starting the path
741 section of the URL with a slash:
743 curl ftp://ftp.sunet.se//tmp/
745 3.18 Can I use curl to send a POST/PUT and not wait for a response?
749 But you could easily write your own program using libcurl to do such stunts.
751 3.19 How do I get HTTP from a host using a specific IP address?
753 For example, you may be trying out a web site installation that isn't yet in
754 the DNS. Or you have a site using multiple IP addresses for a given host
755 name and you want to address a specific one out of the set.
757 Set a custom Host: header that identifies the server name you want to reach
758 but use the target IP address in the URL:
760 curl --header "Host: www.example.com" http://127.0.0.1/
762 You can also opt to add faked host name entries to curl with the --resolve
763 option. That has the added benefit that things like redirects will also work
764 properly. The above operation would instead be done as:
766 curl --resolve www.example.com:80:127.0.0.1 http://www.example.com/
768 3.20 How to SFTP from my user's home directory?
770 Contrary to how FTP works, SFTP and SCP URLs specify the exact directory to
771 work with. It means that if you don't specify that you want the user's home
772 directory, you get the actual root directory.
774 To specify a file in your user's home directory, you need to use the correct
775 URL syntax which for sftp might look similar to:
777 curl -O -u user:password sftp://example.com/~/file.txt
779 and for SCP it is just a different protocol prefix:
781 curl -O -u user:password scp://example.com/~/file.txt
783 3.21 Protocol xxx not supported or disabled in libcurl
785 When passing on a URL to curl to use, it may respond that the particular
786 protocol is not supported or disabled. The particular way this error message
787 is phrased is because curl doesn't make a distinction internally of whether
788 a particular protocol is not supported (i.e. never got any code added that
789 knows how to speak that protocol) or if it was explicitly disabled. curl can
790 be built to only support a given set of protocols, and the rest would then
791 be disabled or not supported.
793 Note that this error will also occur if you pass a wrongly spelled protocol
794 part as in "htpt://example.com" or as in the less evident case if you prefix
795 the protocol part with a space as in " http://example.com/".
797 3.22 curl -X gives me HTTP problems
799 In normal circumstances, -X should hardly ever be used.
801 By default you use curl without explicitly saying which request method to
802 use when the URL identifies a HTTP transfer. If you just pass in a URL like
803 "curl http://example.com" it will use GET. If you use -d or -F curl will use
804 POST, -I will cause a HEAD and -T will make it a PUT.
806 If for whatever reason you're not happy with these default choices that curl
807 does for you, you can override those request methods by specifying -X
808 [WHATEVER]. This way you can for example send a DELETE by doing "curl -X
811 It is thus pointless to do "curl -XGET [URL]" as GET would be used
812 anyway. In the same vein it is pointless to do "curl -X POST -d data
813 [URL]"... But you can make a fun and somewhat rare request that sends a
814 request-body in a GET request with something like "curl -X GET -d data
817 Note that -X doesn't actually change curl's behavior as it only modifies the
818 actual string sent in the request, but that may of course trigger a
819 different set of events.
821 Accordingly, by using -XPOST on a command line that for example would follow
822 a 303 redirect, you will effectively prevent curl from behaving
828 4.1 Problems connecting to SSL servers.
830 It took a very long time before we could sort out why curl had problems to
831 connect to certain SSL servers when using SSLeay or OpenSSL v0.9+. The
832 error sometimes showed up similar to:
834 16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:
836 It turned out to be because many older SSL servers don't deal with SSLv3
837 requests properly. To correct this problem, tell curl to select SSLv2 from
838 the command line (-2/--sslv2).
840 There have also been examples where the remote server didn't like the SSLv2
841 request and instead you had to force curl to use SSLv3 with -3/--sslv3.
843 4.2 Why do I get problems when I use & or % in the URL?
845 In general unix shells, the & symbol is treated specially and when used, it
846 runs the specified command in the background. To safely send the & as a part
847 of a URL, you should quote the entire URL by using single (') or double (")
848 quotes around it. Similar problems can also occur on some shells with other
849 characters, including ?*!$~(){}<>\|;`. When in doubt, quote the URL.
851 An example that would invoke a remote CGI that uses &-symbols could be:
853 curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
855 In Windows, the standard DOS shell treats the percent sign specially and you
856 need to use TWO percent signs for each single one you want to use in the
859 If you want a literal percent sign to be part of the data you pass in a POST
860 using -d/--data you must encode it as '%25' (which then also needs the
861 percent sign doubled on Windows machines).
863 4.3 How can I use {, }, [ or ] to specify multiple URLs?
865 Because those letters have a special meaning to the shell, to be used in
866 a URL specified to curl you must quote them.
868 An example that downloads two URLs (sequentially) would be:
870 curl '{curl,www}.haxx.se'
872 To be able to use those characters as actual parts of the URL (without using
873 them for the curl URL "globbing" system), use the -g/--globoff option:
875 curl -g 'www.site.com/weirdname[].html'
877 4.4 Why do I get downloaded data even though the web page doesn't exist?
879 Curl asks remote servers for the page you specify. If the page doesn't exist
880 at the server, the HTTP protocol defines how the server should respond and
881 that means that headers and a "page" will be returned. That's simply how
884 By using the --fail option you can tell curl explicitly to not get any data
885 if the HTTP return code doesn't say success.
887 4.5 Why do I get return code XXX from a HTTP server?
889 RFC2616 clearly explains the return codes. This is a short transcript. Go
890 read the RFC for exact details:
892 4.5.1 "400 Bad Request"
894 The request could not be understood by the server due to malformed
895 syntax. The client SHOULD NOT repeat the request without modifications.
897 4.5.2 "401 Unauthorized"
899 The request requires user authentication.
901 4.5.3 "403 Forbidden"
903 The server understood the request, but is refusing to fulfil it.
904 Authorization will not help and the request SHOULD NOT be repeated.
906 4.5.4 "404 Not Found"
908 The server has not found anything matching the Request-URI. No indication
909 is given of whether the condition is temporary or permanent.
911 4.5.5 "405 Method Not Allowed"
913 The method specified in the Request-Line is not allowed for the resource
914 identified by the Request-URI. The response MUST include an Allow header
915 containing a list of valid methods for the requested resource.
917 4.5.6 "301 Moved Permanently"
919 If you get this return code and an HTML output similar to this:
921 <H1>Moved Permanently</H1> The document has moved <A
922 HREF="http://same_url_now_with_a_trailing_slash/">here</A>.
924 it might be because you request a directory URL but without the trailing
925 slash. Try the same operation again _with_ the trailing URL, or use the
926 -L/--location option to follow the redirection.
928 4.6 Can you tell me what error code 142 means?
930 All curl error codes are described at the end of the man page, in the
931 section called "EXIT CODES".
933 Error codes that are larger than the highest documented error code means
934 that curl has exited due to a crash. This is a serious error, and we
935 appreciate a detailed bug report from you that describes how we could go
936 ahead and repeat this!
938 4.7 How do I keep user names and passwords secret in Curl command lines?
940 This problem has two sides:
942 The first part is to avoid having clear-text passwords in the command line
943 so that they don't appear in 'ps' outputs and similar. That is easily
944 avoided by using the "-K" option to tell curl to read parameters from a file
945 or stdin to which you can pass the secret info. curl itself will also
946 attempt to "hide" the given password by blanking out the option - this
947 doesn't work on all platforms.
949 To keep the passwords in your account secret from the rest of the world is
950 not a task that curl addresses. You could of course encrypt them somehow to
951 at least hide them from being read by human eyes, but that is not what
952 anyone would call security.
954 Also note that regular HTTP (using Basic authentication) and FTP passwords
955 are sent in clear across the network. All it takes for anyone to fetch them
956 is to listen on the network. Eavesdropping is very easy. Use more secure
957 authentication methods (like Digest, Negotiate or even NTLM) or consider the
958 SSL-based alternatives HTTPS and FTPS.
962 It is not a bug if the behavior is documented. Read the docs first.
963 Especially check out the KNOWN_BUGS file, it may be a documented bug!
965 If it is a problem with a binary you've downloaded or a package for your
966 particular platform, try contacting the person who built the package/archive
969 If there is a bug, read the BUGS document first. Then report it as described
972 4.9 Curl can't authenticate to the server that requires NTLM?
974 NTLM support requires OpenSSL, GnuTLS, mbedTLS, NSS, Secure Transport, or
975 Microsoft Windows libraries at build-time to provide this functionality.
977 NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You
978 should not use such ones.
980 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
982 Many web servers allow or demand that the administrator configures the
983 server properly for these requests to work on the web server.
985 Some servers seem to support HEAD only on certain kinds of URLs.
987 To fully grasp this, try the documentation for the particular server
988 software you're trying to interact with. This is not anything curl can do
991 4.11 Why does my HTTP range requests return the full document?
993 Because the range may not be supported by the server, or the server may
994 choose to ignore it and return the full document anyway.
996 4.12 Why do I get "certificate verify failed" ?
998 You invoke curl 7.10 or later to communicate on a https:// URL and get an
999 error back looking something similar to this:
1001 curl: (35) SSL: error:14090086:SSL routines:
1002 SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
1004 Then it means that curl couldn't verify that the server's certificate was
1005 good. Curl verifies the certificate using the CA cert bundle that comes with
1006 the curl installation.
1008 To disable the verification (which makes it act like curl did before 7.10),
1009 use -k. This does however enable man-in-the-middle attacks.
1011 If you get this failure but are having a CA cert bundle installed and used,
1012 the server's certificate is not signed by one of the CA's in the bundle. It
1013 might for example be self-signed. You then correct this problem by obtaining
1014 a valid CA cert for the server. Or again, decrease the security by disabling
1017 Details are also in the SSLCERTS file in the release archives, found online
1018 here: https://curl.haxx.se/docs/sslcerts.html
1020 4.13 Why is curl -R on Windows one hour off?
1022 Since curl 7.53.0 this issue should be fixed as long as curl was built with
1023 any modern compiler that allows for a 64-bit curl_off_t type. For older
1024 compilers or prior curl versions it may set a time that appears one hour off.
1025 This happens due to a flaw in how Windows stores and uses file modification
1026 times and it is not easily worked around. For more details read this:
1027 http://www.codeproject.com/datetime/dstbugs.asp
1029 4.14 Redirects work in browser but not with curl!
1031 curl supports HTTP redirects well (see item 3.8). Browsers generally support
1032 at least two other ways to perform redirects that curl does not:
1034 Meta tags. You can write a HTML tag that will cause the browser to redirect
1035 to another given URL after a certain time.
1037 Javascript. You can write a Javascript program embedded in a HTML page that
1038 redirects the browser to another given URL.
1040 There is no way to make curl follow these redirects. You must either
1041 manually figure out what the page is set to do, or you write a script that
1042 parses the results and fetches the new URL.
1044 4.15 FTPS doesn't work
1046 curl supports FTPS (sometimes known as FTP-SSL) both implicit and explicit
1049 When a URL is used that starts with FTPS://, curl assumes implicit SSL on
1050 the control connection and will therefore immediately connect and try to
1051 speak SSL. FTPS:// connections default to port 990.
1053 To use explicit FTPS, you use a FTP:// URL and the --ftp-ssl option (or one
1054 of its related flavours). This is the most common method, and the one
1055 mandated by RFC4217. This kind of connection will then of course use the
1056 standard FTP port 21 by default.
1058 4.16 My HTTP POST or PUT requests are slow!
1060 libcurl makes all POST and PUT requests (except for POST requests with a
1061 very tiny request body) use the "Expect: 100-continue" header. This header
1062 allows the server to deny the operation early so that libcurl can bail out
1063 before having to send any data. This is useful in authentication
1066 However, many servers don't implement the Expect: stuff properly and if the
1067 server doesn't respond (positively) within 1 second libcurl will continue
1068 and send off the data anyway.
1070 You can disable libcurl's use of the Expect: header the same way you disable
1071 any header, using -H / CURLOPT_HTTPHEADER, or by forcing it to use HTTP 1.0.
1073 4.17 Non-functional connect timeouts
1075 In most Windows setups having a timeout longer than 21 seconds make no
1076 difference, as it will only send 3 TCP SYN packets and no more. The second
1077 packet sent three seconds after the first and the third six seconds after
1078 the second. No more than three packets are sent, no matter how long the
1081 See option TcpMaxConnectRetransmissions on this page:
1082 https://support.microsoft.com/en-us/kb/175523/en-us
1084 Also, even on non-Windows systems there may run a firewall or anti-virus
1085 software or similar that accepts the connection but does not actually do
1086 anything else. This will make (lib)curl to consider the connection connected
1087 and thus the connect timeout won't trigger.
1089 4.18 file:// URLs containing drive letters (Windows, NetWare)
1091 When using curl to try to download a local file, one might use a URL
1096 You'll find that even if D:\blah.txt does exist, curl returns a 'file
1099 According to RFC 1738 (https://www.ietf.org/rfc/rfc1738.txt),
1100 file:// URLs must contain a host component, but it is ignored by
1101 most implementations. In the above example, 'D:' is treated as the
1102 host component, and is taken away. Thus, curl tries to open '/blah.txt'.
1103 If your system is installed to drive C:, that will resolve to 'C:\blah.txt',
1104 and if that doesn't exist you will get the not found error.
1106 To fix this problem, use file:// URLs with *three* leading slashes:
1110 Alternatively, if it makes more sense, specify 'localhost' as the host
1113 file://localhost/D:/blah.txt
1115 In either case, curl should now be looking for the correct file.
1117 4.19 Why doesn't curl return an error when the network cable is unplugged?
1119 Unplugging a cable is not an error situation. The TCP/IP protocol stack
1120 was designed to be fault tolerant, so even though there may be a physical
1121 break somewhere the connection shouldn't be affected, just possibly
1122 delayed. Eventually, the physical break will be fixed or the data will be
1123 re-routed around the physical problem through another path.
1125 In such cases, the TCP/IP stack is responsible for detecting when the
1126 network connection is irrevocably lost. Since with some protocols it is
1127 perfectly legal for the client to wait indefinitely for data, the stack may
1128 never report a problem, and even when it does, it can take up to 20 minutes
1129 for it to detect an issue. The curl option --keepalive-time enables
1130 keep-alive support in the TCP/IP stack which makes it periodically probe the
1131 connection to make sure it is still available to send data. That should
1132 reliably detect any TCP/IP network failure.
1134 But even that won't detect the network going down before the TCP/IP
1135 connection is established (e.g. during a DNS lookup) or using protocols that
1136 don't use TCP. To handle those situations, curl offers a number of timeouts
1137 on its own. --speed-limit/--speed-time will abort if the data transfer rate
1138 falls too low, and --connect-timeout and --max-time can be used to put an
1139 overall timeout on the connection phase or the entire transfer.
1141 A libcurl-using application running in a known physical environment (e.g.
1142 an embedded device with only a single network connection) may want to act
1143 immediately if its lone network connection goes down. That can be achieved
1144 by having the application monitor the network connection on its own using an
1145 OS-specific mechanism, then signalling libcurl to abort (see also item 5.13).
1147 4.20 curl doesn't return error for HTTP non-200 responses!
1149 Correct. Unless you use -f (--fail).
1151 When doing HTTP transfers, curl will perform exactly what you're asking it
1152 to do and if successful it will not return an error. You can use curl to
1153 test your web server's "file not found" page (that gets 404 back), you can
1154 use it to check your authentication protected web pages (that gets a 401
1157 The specific HTTP response code does not constitute a problem or error for
1158 curl. It simply sends and delivers HTTP as you asked and if that worked,
1159 everything is fine and dandy. The response code is generally providing more
1160 higher level error information that curl doesn't care about. The error was
1161 not in the HTTP transfer.
1163 If you want your command line to treat error codes in the 400 and up range
1164 as errors and thus return a non-zero value and possibly show an error
1165 message, curl has a dedicated option for that: -f (CURLOPT_FAILONERROR in
1168 You can also use the -w option and the variable %{response_code} to extract
1169 the exact response code that was returned in the response.
1171 4.21 Why is there a HTTP/1.1 in my HTTP/2 request?
1173 If you use verbose to see the HTTP request when you send off a HTTP/2
1174 request, it will still say 1.1.
1176 The reason for this is that we first generate the request to send using the
1177 old 1.1 style and show that request in the verbose output, and then we
1178 convert it over to the binary header-compressed HTTP/2 style. The actual
1179 "1.1" part from that request is then not actually used in the transfer.
1180 The binary HTTP/2 headers are not human readable.
1184 5.1 Is libcurl thread-safe?
1188 We have written the libcurl code specifically adjusted for multi-threaded
1189 programs. libcurl will use thread-safe functions instead of non-safe ones if
1190 your system has such. Note that you must never share the same handle in
1193 There may be some exceptions to thread safety depending on how libcurl was
1194 built. Please review the guidelines for thread safety to learn more:
1195 https://curl.haxx.se/libcurl/c/threadsafe.html
1197 5.2 How can I receive all data into a large memory chunk?
1199 [ See also the examples/getinmemory.c source ]
1201 You are in full control of the callback function that gets called every time
1202 there is data received from the remote server. You can make that callback do
1203 whatever you want. You do not have to write the received data to a file.
1205 One solution to this problem could be to have a pointer to a struct that you
1206 pass to the callback function. You set the pointer using the
1207 CURLOPT_WRITEDATA option. Then that pointer will be passed to the callback
1208 instead of a FILE * to a file:
1210 /* imaginary struct */
1211 struct MemoryStruct {
1216 /* imaginary callback function */
1218 WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
1220 size_t realsize = size * nmemb;
1221 struct MemoryStruct *mem = (struct MemoryStruct *)data;
1223 mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
1225 memcpy(&(mem->memory[mem->size]), ptr, realsize);
1226 mem->size += realsize;
1227 mem->memory[mem->size] = 0;
1232 5.3 How do I fetch multiple files with libcurl?
1234 libcurl has excellent support for transferring multiple files. You should
1235 just repeatedly set new URLs with curl_easy_setopt() and then transfer it
1236 with curl_easy_perform(). The handle you get from curl_easy_init() is not
1237 only reusable, but you're even encouraged to reuse it if you can, as that
1238 will enable libcurl to use persistent connections.
1240 5.4 Does libcurl do Winsock initialization on win32 systems?
1242 Yes, if told to in the curl_global_init() call.
1244 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
1246 Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
1247 that DLL use the FILE * (as the DLL and the client application cannot access
1248 each others' variable memory areas). If you set CURLOPT_WRITEDATA you must
1249 also use CURLOPT_WRITEFUNCTION as well to set a function that writes the
1250 file, even if that simply writes the data to the specified FILE *.
1251 Similarly, if you use CURLOPT_READDATA you must also specify
1252 CURLOPT_READFUNCTION.
1254 5.6 What about Keep-Alive or persistent connections?
1256 curl and libcurl have excellent support for persistent connections when
1257 transferring several files from the same server. Curl will attempt to reuse
1258 connections for all URLs specified on the same command line/config file, and
1259 libcurl will reuse connections for all transfers that are made using the
1260 same libcurl handle.
1262 When you use the easy interface the connection cache is kept within the easy
1263 handle. If you instead use the multi interface, the connection cache will be
1264 kept within the multi handle and will be shared among all the easy handles
1265 that are used within the same multi handle.
1267 5.7 Link errors when building libcurl on Windows!
1269 You need to make sure that your project, and all the libraries (both static
1270 and dynamic) that it links against, are compiled/linked against the same run
1273 This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
1274 options to the command line compiler. /MD (linking against MSVCRT dll) seems
1275 to be the most commonly used option.
1277 When building an application that uses the static libcurl library, you must
1278 add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for
1279 dynamic import symbols. If you're using Visual Studio, you need to instead
1280 add CURL_STATICLIB in the "Preprocessor Definitions" section.
1282 If you get linker error like "unknown symbol __imp__curl_easy_init ..." you
1283 have linked against the wrong (static) library. If you want to use the
1284 libcurl.dll and import lib, you don't need any extra CFLAGS, but use one of
1285 the import libraries below. These are the libraries produced by the various
1286 lib/Makefile.* files:
1288 Target: static lib. import lib for libcurl*.dll.
1289 -----------------------------------------------------------
1290 MingW: libcurl.a libcurldll.a
1291 MSVC (release): libcurl.lib libcurl_imp.lib
1292 MSVC (debug): libcurld.lib libcurld_imp.lib
1293 Borland: libcurl.lib libcurl_imp.lib
1295 5.8 libcurl.so.X: open failed: No such file or directory
1297 This is an error message you might get when you try to run a program linked
1298 with a shared version of libcurl and your run-time linker (ld.so) couldn't
1299 find the shared library named libcurl.so.X. (Where X is the number of the
1300 current libcurl ABI, typically 3 or 4).
1302 You need to make sure that ld.so finds libcurl.so.X. You can do that
1303 multiple ways, and it differs somewhat between different operating systems,
1304 but they are usually:
1306 * Add an option to the linker command line that specify the hard-coded path
1307 the run-time linker should check for the lib (usually -R)
1309 * Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
1310 should check for libs
1312 * Adjust the system's config to check for libs in the directory where you've
1313 put the dir (like Linux's /etc/ld.so.conf)
1315 'man ld.so' and 'man ld' will tell you more details
1317 5.9 How does libcurl resolve host names?
1319 libcurl supports a large a number of different name resolve functions. One
1320 of them is picked at build-time and will be used unconditionally. Thus, if
1321 you want to change name resolver function you must rebuild libcurl and tell
1322 it to use a different function.
1324 - The non-IPv6 resolver that can use one of four different host name resolve
1325 calls (depending on what your system supports):
1328 B - gethostbyname_r() with 3 arguments
1329 C - gethostbyname_r() with 5 arguments
1330 D - gethostbyname_r() with 6 arguments
1332 - The IPv6-resolver that uses getaddrinfo()
1334 - The c-ares based name resolver that uses the c-ares library for resolves.
1335 Using this offers asynchronous name resolves.
1337 - The threaded resolver (default option on Windows). It uses:
1339 A - gethostbyname() on plain IPv4 hosts
1340 B - getaddrinfo() on IPv6 enabled hosts
1342 Also note that libcurl never resolves or reverse-lookups addresses given as
1343 pure numbers, such as 127.0.0.1 or ::1.
1345 5.10 How do I prevent libcurl from writing the response to stdout?
1347 libcurl provides a default built-in write function that writes received data
1348 to stdout. Set the CURLOPT_WRITEFUNCTION to receive the data, or possibly
1349 set CURLOPT_WRITEDATA to a different FILE * handle.
1351 5.11 How do I make libcurl not receive the whole HTTP response?
1353 You make the write callback (or progress callback) return an error and
1354 libcurl will then abort the transfer.
1356 5.12 Can I make libcurl fake or hide my real IP address?
1358 No. libcurl operates on a higher level. Besides, faking IP address would
1359 imply sending IP packets with a made-up source address, and then you normally
1360 get a problem with receiving the packet sent back as they would then not be
1363 If you use a proxy to access remote sites, the sites will not see your local
1364 IP address but instead the address of the proxy.
1366 Also note that on many networks NATs or other IP-munging techniques are used
1367 that makes you see and use a different IP address locally than what the
1368 remote server will see you coming from. You may also consider using
1369 https://www.torproject.org/ .
1371 5.13 How do I stop an ongoing transfer?
1373 With the easy interface you make sure to return the correct error code from
1374 one of the callbacks, but none of them are instant. There is no function you
1375 can call from another thread or similar that will stop it immediately.
1376 Instead, you need to make sure that one of the callbacks you use returns an
1377 appropriate value that will stop the transfer. Suitable callbacks that you
1378 can do this with include the progress callback, the read callback and the
1381 If you're using the multi interface, you can also stop a transfer by
1382 removing the particular easy handle from the multi stack at any moment you
1383 think the transfer is done or when you wish to abort the transfer.
1385 5.14 Using C++ non-static functions for callbacks?
1387 libcurl is a C library, it doesn't know anything about C++ member functions.
1389 You can overcome this "limitation" with relative ease using a static
1390 member function that is passed a pointer to the class:
1392 // f is the pointer to your object.
1393 static size_t YourClass::func(void *buffer, size_t sz, size_t n, void *f)
1395 // Call non-static member function.
1396 static_cast<YourClass*>(f)->nonStaticFunction();
1399 // This is how you pass pointer to the static function:
1400 curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass::func);
1401 curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);
1403 5.15 How do I get an FTP directory listing?
1405 If you end the FTP URL you request with a slash, libcurl will provide you
1406 with a directory listing of that given directory. You can also set
1407 CURLOPT_CUSTOMREQUEST to alter what exact listing command libcurl would use
1410 The follow-up question tends to be how is a program supposed to parse the
1411 directory listing. How does it know what's a file and what's a dir and what's
1412 a symlink etc. If the FTP server supports the MLSD command then it will
1413 return data in a machine-readable format that can be parsed for type. The
1414 types are specified by RFC3659 section 7.5.1. If MLSD is not supported then
1415 you have to work with what you're given. The LIST output format is entirely
1416 at the server's own liking and the NLST output doesn't reveal any types and
1417 in many cases doesn't even include all the directory entries. Also, both LIST
1418 and NLST tend to hide unix-style hidden files (those that start with a dot)
1419 by default so you need to do "LIST -a" or similar to see them.
1421 Example - List only directories.
1422 ftp.funet.fi supports MLSD and ftp.kernel.org does not:
1424 curl -s ftp.funet.fi/pub/ -X MLSD | \
1425 perl -lne 'print if s/(?:^|;)type=dir;[^ ]+ (.+)$/$1/'
1427 curl -s ftp.kernel.org/pub/linux/kernel/ | \
1428 perl -lne 'print if s/^d[-rwx]{9}(?: +[^ ]+){7} (.+)$/$1/'
1430 If you need to parse LIST output in libcurl one such existing
1431 list parser is available at https://cr.yp.to/ftpparse.html Versions of
1432 libcurl since 7.21.0 also provide the ability to specify a wildcard to
1433 download multiple files from one FTP directory.
1435 5.16 I want a different time-out!
1437 Time and time again users realize that CURLOPT_TIMEOUT and
1438 CURLOPT_CONNECTIMEOUT are not sufficiently advanced or flexible to cover all
1439 the various use cases and scenarios applications end up with.
1441 libcurl offers many more ways to time-out operations. A common alternative
1442 is to use the CURLOPT_LOW_SPEED_LIMIT and CURLOPT_LOW_SPEED_TIME options to
1443 specify the lowest possible speed to accept before to consider the transfer
1446 The most flexible way is by writing your own time-out logic and using
1447 CURLOPT_PROGRESSFUNCTION (perhaps in combination with other callbacks) and
1448 use that to figure out exactly when the right condition is met when the
1449 transfer should get stopped.
1451 5.17 Can I write a server with libcurl?
1453 No. libcurl offers no functions or building blocks to build any kind of
1454 internet protocol server. libcurl is only a client-side library. For server
1455 libraries, you need to continue your search elsewhere but there exist many
1456 good open source ones out there for most protocols you could possibly want a
1457 server for. And there are really good stand-alone ones that have been tested
1458 and proven for many years. There's no need for you to reinvent them!
1460 5.18 Does libcurl use threads?
1462 Put simply: no, libcurl will execute in the same thread you call it in. All
1463 callbacks will be called in the same thread as the one you call libcurl in.
1465 If you want to avoid your thread to be blocked by the libcurl call, you make
1466 sure you use the non-blocking API which will do transfers asynchronously -
1467 but still in the same single thread.
1469 libcurl will potentially internally use threads for name resolving, if it
1470 was built to work like that, but in those cases it'll create the child
1471 threads by itself and they will only be used and then killed internally by
1472 libcurl and never exposed to the outside.
1476 Curl and libcurl are released under a MIT/X derivate license. The license is
1477 very liberal and should not impose a problem for your project. This section
1478 is just a brief summary for the cases we get the most questions. (Parts of
1479 this section was much enhanced by Bjorn Reese.)
1481 We are not lawyers and this is not legal advice. You should probably consult
1482 one if you want true and accurate legal insights without our prejudice. Note
1483 especially that this section concerns the libcurl license only; compiling in
1484 features of libcurl that depend on other libraries (e.g. OpenSSL) may affect
1485 the licensing obligations of your application.
1487 6.1 I have a GPL program, can I use the libcurl library?
1491 Since libcurl may be distributed under the MIT/X derivate license, it can be
1492 used together with GPL in any software.
1494 6.2 I have a closed-source program, can I use the libcurl library?
1498 libcurl does not put any restrictions on the program that uses the library.
1500 6.3 I have a BSD licensed program, can I use the libcurl library?
1504 libcurl does not put any restrictions on the program that uses the library.
1506 6.4 I have a program that uses LGPL libraries, can I use libcurl?
1510 The LGPL license doesn't clash with other licenses.
1512 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
1516 The MIT/X derivate license practically allows you to do almost anything with
1517 the sources, on the condition that the copyright texts in the sources are
1520 6.6 Can you please change the curl/libcurl license to XXXX?
1524 We have carefully picked this license after years of development and
1525 discussions and a large amount of people have contributed with source code
1526 knowing that this is the license we use. This license puts the restrictions
1527 we want on curl/libcurl and it does not spread to other programs or
1528 libraries that use it. It should be possible for everyone to use libcurl or
1529 curl in their projects, no matter what license they already have in use.
1531 6.7 What are my obligations when using libcurl in my commercial apps?
1533 Next to none. All you need to adhere to is the MIT-style license (stated in
1534 the COPYING file) which basically says you have to include the copyright
1535 notice in "all copies" and that you may not use the copyright holder's name
1536 when promoting your software.
1538 You do not have to release any of your source code.
1540 You do not have to reveal or make public any changes to the libcurl source
1543 You do not have to broadcast to the world that you are using libcurl within
1546 All we ask is that you disclose "the copyright notice and this permission
1547 notice" somewhere. Most probably like in the documentation or in the section
1548 where other third party dependencies already are mentioned and acknowledged.
1550 As can be seen here: https://curl.haxx.se/docs/companies.html and elsewhere,
1551 more and more companies are discovering the power of libcurl and take
1552 advantage of it even in commercial environments.
1557 7.1 What is PHP/CURL?
1559 The module for PHP that makes it possible for PHP programs to access curl-
1560 functions from within PHP.
1562 In the cURL project we call this module PHP/CURL to differentiate it from
1563 curl the command line tool and libcurl the library. The PHP team however
1564 does not refer to it like this (for unknown reasons). They call it plain
1565 CURL (often using all caps) or sometimes ext/curl, but both cause much
1566 confusion to users which in turn gives us a higher question load.
1568 7.2 Who wrote PHP/CURL?
1570 PHP/CURL was initially written by Sterling Hughes.
1572 7.3 Can I perform multiple requests using the same handle?
1574 Yes - at least in PHP version 4.3.8 and later (this has been known to not
1575 work in earlier versions, but the exact version when it started to work is
1578 After a transfer, you just set new options in the handle and make another
1579 transfer. This will make libcurl re-use the same connection if it can.
1581 7.4 Does PHP/CURL have dependencies?
1583 PHP/CURL is a module that comes with the regular PHP package. It depends on
1584 and uses libcurl, so you need to have libcurl installed properly before
1585 PHP/CURL can be used.