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ADD: MuPDF v1.26.7: the MuPDF source as downloaded by a default build of PyMuPDF 1.26.4. The directory name has changed: no version number in the expanded directory now.
author Franz Glasner <fzglas.hg@dom66.de>
date Mon, 15 Sep 2025 11:43:07 +0200
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6
7 FAQ
8
9 1. Philosophy
10 1.1 What is cURL?
11 1.2 What is libcurl?
12 1.3 What is curl not?
13 1.4 When will you make curl do XXXX ?
14 1.5 Who makes curl?
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?
25
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
34 3. Usage Problems
35 3.1 curl: (1) SSL is disabled, https: not supported
36 3.2 How do I tell curl to resume a transfer?
37 3.3 Why doesn't my posting using -F work?
38 3.4 How do I tell curl to run custom FTP commands?
39 3.5 How can I disable the Accept: */* header?
40 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
41 3.7 Can I use curl to delete/rename a file through FTP?
42 3.8 How do I tell curl to follow HTTP redirects?
43 3.9 How do I use curl in my favorite programming language?
44 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
45 3.11 How do I POST with a different Content-Type?
46 3.12 Why do FTP-specific features over HTTP proxy fail?
47 3.13 Why do my single/double quotes fail?
48 3.14 Does curl support Javascript or PAC (automated proxy config)?
49 3.15 Can I do recursive fetches with curl?
50 3.16 What certificates do I need when I use SSL?
51 3.17 How do I list the root dir of an FTP server?
52 3.18 Can I use curl to send a POST/PUT and not wait for a response?
53 3.19 How do I get HTTP from a host using a specific IP address?
54 3.20 How to SFTP from my user's home directory?
55 3.21 Protocol xxx not supported or disabled in libcurl
56 3.22 curl -X gives me HTTP problems
57
58 4. Running Problems
59 4.1 Problems connecting to SSL servers.
60 4.2 Why do I get problems when I use & or % in the URL?
61 4.3 How can I use {, }, [ or ] to specify multiple URLs?
62 4.4 Why do I get downloaded data even though the web page doesn't exist?
63 4.5 Why do I get return code XXX from a HTTP server?
64 4.5.1 "400 Bad Request"
65 4.5.2 "401 Unauthorized"
66 4.5.3 "403 Forbidden"
67 4.5.4 "404 Not Found"
68 4.5.5 "405 Method Not Allowed"
69 4.5.6 "301 Moved Permanently"
70 4.6 Can you tell me what error code 142 means?
71 4.7 How do I keep user names and passwords secret in Curl command lines?
72 4.8 I found a bug!
73 4.9 Curl can't authenticate to the server that requires NTLM?
74 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
75 4.11 Why do my HTTP range requests return the full document?
76 4.12 Why do I get "certificate verify failed" ?
77 4.13 Why is curl -R on Windows one hour off?
78 4.14 Redirects work in browser but not with curl!
79 4.15 FTPS doesn't work
80 4.16 My HTTP POST or PUT requests are slow!
81 4.17 Non-functional connect timeouts on Windows
82 4.18 file:// URLs containing drive letters (Windows, NetWare)
83 4.19 Why doesn't curl return an error when the network cable is unplugged?
84 4.20 curl doesn't return error for HTTP non-200 responses!
85 4.21 Why is there a HTTP/1.1 in my HTTP/2 request?
86
87 5. libcurl Issues
88 5.1 Is libcurl thread-safe?
89 5.2 How can I receive all data into a large memory chunk?
90 5.3 How do I fetch multiple files with libcurl?
91 5.4 Does libcurl do Winsock initing on win32 systems?
92 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
93 5.6 What about Keep-Alive or persistent connections?
94 5.7 Link errors when building libcurl on Windows!
95 5.8 libcurl.so.X: open failed: No such file or directory
96 5.9 How does libcurl resolve host names?
97 5.10 How do I prevent libcurl from writing the response to stdout?
98 5.11 How do I make libcurl not receive the whole HTTP response?
99 5.12 Can I make libcurl fake or hide my real IP address?
100 5.13 How do I stop an ongoing transfer?
101 5.14 Using C++ non-static functions for callbacks?
102 5.15 How do I get an FTP directory listing?
103 5.16 I want a different time-out!
104 5.17 Can I write a server with libcurl?
105 5.18 Does libcurl use threads?
106
107 6. License Issues
108 6.1 I have a GPL program, can I use the libcurl library?
109 6.2 I have a closed-source program, can I use the libcurl library?
110 6.3 I have a BSD licensed program, can I use the libcurl library?
111 6.4 I have a program that uses LGPL libraries, can I use libcurl?
112 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
113 6.6 Can you please change the curl/libcurl license to XXXX?
114 6.7 What are my obligations when using libcurl in my commercial apps?
115
116 7. PHP/CURL Issues
117 7.1 What is PHP/CURL?
118 7.2 Who wrote PHP/CURL?
119 7.3 Can I perform multiple requests using the same handle?
120 7.4 Does PHP/CURL have dependencies?
121
122 ==============================================================================
123
124 1. Philosophy
125
126 1.1 What is cURL?
127
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".
133
134 The cURL project produces two products:
135
136 libcurl
137
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.
141
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!
145
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...
151
152 libcurl is free, thread-safe, IPv6 compatible, feature rich, well
153 supported and fast.
154
155 curl
156
157 A command line tool for getting or sending files using URL syntax.
158
159 Since curl uses libcurl, curl supports the same wide range of common
160 Internet protocols that libcurl does.
161
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:
164
165 https://media.merriam-webster.com/soundc11/c/curl0001.wav
166
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.)
172
173 1.2 What is libcurl?
174
175 libcurl is a reliable and portable library which provides you with an easy
176 interface to a range of common Internet protocols.
177
178 You can use libcurl for free in your application, be it open source,
179 commercial or closed-source.
180
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.
184
185 1.3 What is curl not?
186
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.
190
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).
194
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.
198
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).
201
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,
205 OS X, QNX etc.
206
207 1.4 When will you make curl do XXXX ?
208
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
211 curl:
212
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.
216
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.
220
221 We focus on protocol related issues and improvements. If you want to do more
222 magic with the supported protocols than curl currently does, chances are good
223 we will agree. If you want to add more protocols, we may very well agree.
224
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.
232
233 If you write the code, chances are better that it will get into curl faster.
234
235 1.5 Who makes curl?
236
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).
242
243 The full list of all contributors is found in the docs/THANKS file.
244
245 curl is developed by a community, with Daniel at the wheel.
246
247 1.6 What do you get for making curl?
248
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.
254
255 We still get help from companies. Haxx provides web site, bandwidth, mailing
256 lists etc, GitHub hosts the primary git repository and other services like
257 the bug tracker at https://github.com/curl/curl. Also again, some companies
258 have sponsored certain parts of the development in the past and I hope some
259 will continue to do so in the future.
260
261 If you want to support our project, consider a donation or a banner-program
262 or even better: by helping us with coding, documenting or testing etc.
263
264 1.7 What about CURL from curl.com?
265
266 During the summer of 2001, curl.com was busy advertising their client-side
267 programming language for the web, named CURL.
268
269 We are in no way associated with curl.com or their CURL programming
270 language.
271
272 Our project name curl has been in effective use since 1998. We were not the
273 first computer related project to use the name "curl" and do not claim any
274 rights to the name.
275
276 We recognize that we will be living in parallel with curl.com and wish them
277 every success.
278
279 1.8 I have a problem whom do I mail?
280
281 Please do not mail any single individual unless you really need to. Keep
282 curl-related questions on a suitable mailing list. All available mailing
283 lists are listed in the MANUAL document and online at
284 https://curl.haxx.se/mail/
285
286 Keeping curl-related questions and discussions on mailing lists allows
287 others to join in and help, to share their ideas, to contribute their
288 suggestions and to spread their wisdom. Keeping discussions on public mailing
289 lists also allows for others to learn from this (both current and future
290 users thanks to the web based archives of the mailing lists), thus saving us
291 from having to repeat ourselves even more. Thanks for respecting this.
292
293 If you have found or simply suspect a security problem in curl or libcurl,
294 mail curl-security at haxx.se (closed list of receivers, mails are not
295 disclosed) and tell. Then we can produce a fix in a timely manner before the
296 flaw is announced to the world, thus lessen the impact the problem will have
297 on existing users.
298
299 1.9 Where do I buy commercial support for curl?
300
301 curl is fully open source. It means you can hire any skilled engineer to fix
302 your curl-related problems.
303
304 We list available alternatives on the curl web site:
305 https://curl.haxx.se/support.html
306
307 1.10 How many are using curl?
308
309 It is impossible to tell.
310
311 We don't know how many users that knowingly have installed and use curl.
312
313 We don't know how many users that use curl without knowing that they are in
314 fact using it.
315
316 We don't know how many users that downloaded or installed curl and then
317 never use it.
318
319 In May 2012 Daniel did a counting game and came up with a number that may
320 be completely wrong or somewhat accurate. Over 500 million!
321
322 See https://daniel.haxx.se/blog/2012/05/16/300m-users/
323
324 1.11 Why don't you update ca-bundle.crt
325
326 The ca cert bundle that used to be shipped with curl was very outdated and
327 must be replaced with an up-to-date version by anyone who wants to verify
328 peers. It is no longer provided by curl. The last curl release that ever
329 shipped a ca cert bundle was curl 7.18.0.
330
331 In the cURL project we've decided not to attempt to keep this file updated
332 (or even present anymore) since deciding what to add to a ca cert bundle is
333 an undertaking we've not been ready to accept, and the one we can get from
334 Mozilla is perfectly fine so there's no need to duplicate that work.
335
336 Today, with many services performed over HTTPS, every operating system
337 should come with a default ca cert bundle that can be deemed somewhat
338 trustworthy and that collection (if reasonably updated) should be deemed to
339 be a lot better than a private curl version.
340
341 If you want the most recent collection of ca certs that Mozilla Firefox
342 uses, we recommend that you extract the collection yourself from Mozilla
343 Firefox (by running 'make ca-bundle), or by using our online service setup
344 for this purpose: https://curl.haxx.se/docs/caextract.html
345
346 1.12 I have a problem who can I chat with?
347
348 There's a bunch of friendly people hanging out in the #curl channel on the
349 IRC network irc.freenode.net. If you're polite and nice, chances are good
350 that you can get -- or provide -- help instantly.
351
352 1.13 curl's ECCN number?
353
354 The US government restricts exports of software that contains or uses
355 cryptography. When doing so, the Export Control Classification Number (ECCN)
356 is used to identify the level of export control etc.
357
358 Apache Software Foundation gives a good explanation of ECCNs at
359 https://www.apache.org/dev/crypto.html
360
361 We believe curl's number might be ECCN 5D002, another possibility is
362 5D992. It seems necessary to write them (the authority that administers ECCN
363 numbers), asking to confirm.
364
365 Comprehensible explanations of the meaning of such numbers and how to obtain
366 them (resp.) are here
367
368 https://www.bis.doc.gov/licensing/exportingbasics.htm
369 https://www.bis.doc.gov/licensing/do_i_needaneccn.html
370
371 An incomprehensible description of the two numbers above is here
372 https://www.bis.doc.gov/index.php/documents/new-encryption/1653-ccl5-pt2-3
373
374 1.14 How do I submit my patch?
375
376 When you have made a patch or a change of whatever sort, and want to submit
377 that to the project, there are a few different ways we prefer:
378
379 o send a patch to the curl-library mailing list. We're many subscribers
380 there and there are lots of people who can review patches, comment on them
381 and "receive" them properly.
382
383 o if your patch changes or fixes a bug, you can also opt to submit a bug
384 report in the bug tracker and attach your patch there. There are less
385 people involved there.
386
387 Lots of more details are found in the CONTRIBUTE and INTERNALS docs.
388
389 1.15 How do I port libcurl to my OS?
390
391 Here's a rough step-by-step:
392
393 1. copy a suitable lib/config-*.h file as a start to lib/config-[youros].h
394
395 2. edit lib/config-[youros].h to match your OS and setup
396
397 3. edit lib/curl_setup.h to include config-[youros].h when your OS is
398 detected by the preprocessor, in the style others already exist
399
400 4. compile lib/*.c and make them into a library
401
402
403 2. Install Related Problems
404
405 2.1 configure doesn't find OpenSSL even when it is installed
406
407 This may be because of several reasons.
408
409 2.1.1 native linker doesn't find openssl
410
411 Affected platforms:
412 Solaris (native cc compiler)
413 HPUX (native cc compiler)
414 SGI IRIX (native cc compiler)
415 SCO UNIX (native cc compiler)
416
417 When configuring curl, I specify --with-ssl. OpenSSL is installed in
418 /usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
419 CRYPTO_lock in -lcrypto
420
421 Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
422 -lcrypto, so ld can't find the library. This is due to a bug in the GNU
423 autoconf tool.
424
425 Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
426 ./configure places the -L/usr/local/ssl/lib early enough in the command
427 line to make things work
428
429 2.1.2 only the libssl lib is missing
430
431 If all include files and the libcrypto lib is present, with only the
432 libssl being missing according to configure, this is most likely because
433 a few functions are left out from the libssl.
434
435 If the function names missing include RSA or RSAREF you can be certain
436 that this is because libssl requires the RSA and RSAREF libs to build.
437
438 See the INSTALL file section that explains how to add those libs to
439 configure. Make sure that you remove the config.cache file before you
440 rerun configure with the new flags.
441
442 2.2 Does curl work/build with other SSL libraries?
443
444 Curl has been written to use a generic SSL function layer internally, and
445 that SSL functionality can then be provided by one out of many different SSL
446 backends.
447
448 curl can be built to use one of the following SSL alternatives: OpenSSL,
449 libressl, BoringSSL, GnuTLS, wolfSSL, NSS, mbedTLS, MesaLink, Secure
450 Transport (native iOS/OS X), Schannel (native Windows) or GSKit (native IBM
451 i). They all have their pros and cons, and we try to maintain a comparison
452 of them here: https://curl.haxx.se/docs/ssl-compared.html
453
454 2.3 Where can I find a copy of LIBEAY32.DLL?
455
456 That is an OpenSSL binary built for Windows.
457
458 Curl can be built with OpenSSL to do the SSL stuff. The LIBEAY32.DLL is then
459 what curl needs on a windows machine to do https:// etc. Check out the curl
460 web site to find accurate and up-to-date pointers to recent OpenSSL DLLs and
461 other binary packages.
462
463 2.4 Does curl support SOCKS (RFC 1928) ?
464
465 Yes, SOCKS 4 and 5 are supported.
466
467
468 3. Usage problems
469
470 3.1 curl: (1) SSL is disabled, https: not supported
471
472 If you get this output when trying to get anything from a https:// server,
473 it means that the instance of curl/libcurl that you're using was built
474 without support for this protocol.
475
476 This could've happened if the configure script that was run at build time
477 couldn't find all libs and include files curl requires for SSL to work. If
478 the configure script fails to find them, curl is simply built without SSL
479 support.
480
481 To get the https:// support into a curl that was previously built but that
482 reports that https:// is not supported, you should dig through the document
483 and logs and check out why the configure script doesn't find the SSL libs
484 and/or include files.
485
486 Also, check out the other paragraph in this FAQ labeled "configure doesn't
487 find OpenSSL even when it is installed".
488
489 3.2 How do I tell curl to resume a transfer?
490
491 Curl supports resumed transfers both ways on both FTP and HTTP.
492 Try the -C option.
493
494 3.3 Why doesn't my posting using -F work?
495
496 You can't arbitrarily use -F or -d, the choice between -F or -d depends on the
497 HTTP operation you need curl to do and what the web server that will receive
498 your post expects.
499
500 If the form you're trying to submit uses the type 'multipart/form-data', then
501 and only then you must use the -F type. In all the most common cases, you
502 should use -d which then causes a posting with the type
503 'application/x-www-form-urlencoded'.
504
505 This is described in some detail in the MANUAL and TheArtOfHttpScripting
506 documents, and if you don't understand it the first time, read it again
507 before you post questions about this to the mailing list. Also, try reading
508 through the mailing list archives for old postings and questions regarding
509 this.
510
511 3.4 How do I tell curl to run custom FTP commands?
512
513 You can tell curl to perform optional commands both before and/or after a
514 file transfer. Study the -Q/--quote option.
515
516 Since curl is used for file transfers, you don't normally use curl to
517 perform FTP commands without transferring anything. Therefore you must
518 always specify a URL to transfer to/from even when doing custom FTP
519 commands, or use -I which implies the "no body" option sent to libcurl.
520
521 3.5 How can I disable the Accept: */* header?
522
523 You can change all internally generated headers by adding a replacement with
524 the -H/--header option. By adding a header with empty contents you safely
525 disable that one. Use -H "Accept:" to disable that specific header.
526
527 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
528
529 To curl, all contents are alike. It doesn't matter how the page was
530 generated. It may be ASP, PHP, Perl, shell-script, SSI or plain HTML
531 files. There's no difference to curl and it doesn't even know what kind of
532 language that generated the page.
533
534 See also item 3.14 regarding javascript.
535
536 3.7 Can I use curl to delete/rename a file through FTP?
537
538 Yes. You specify custom FTP commands with -Q/--quote.
539
540 One example would be to delete a file after you have downloaded it:
541
542 curl -O ftp://download.com/coolfile -Q '-DELE coolfile'
543
544 or rename a file after upload:
545
546 curl -T infile ftp://upload.com/dir/ -Q "-RNFR infile" -Q "-RNTO newname"
547
548 3.8 How do I tell curl to follow HTTP redirects?
549
550 Curl does not follow so-called redirects by default. The Location: header
551 that informs the client about this is only interpreted if you're using the
552 -L/--location option. As in:
553
554 curl -L http://redirector.com
555
556 Not all redirects are HTTP ones, see 4.14
557
558 3.9 How do I use curl in my favorite programming language?
559
560 Many programming languages have interfaces/bindings that allow you to use
561 curl without having to use the command line tool. If you are fluent in such
562 a language, you may prefer to use one of these interfaces instead.
563
564 Find out more about which languages that support curl directly, and how to
565 install and use them, in the libcurl section of the curl web site:
566 https://curl.haxx.se/libcurl/
567
568 All the various bindings to libcurl are made by other projects and people,
569 outside of the cURL project. The cURL project itself only produces libcurl
570 with its plain C API. If you don't find anywhere else to ask you can ask
571 about bindings on the curl-library list too, but be prepared that people on
572 that list may not know anything about bindings.
573
574 In February 2019, there were interfaces available for the following
575 languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Delphi, Dylan, Eiffel,
576 Euphoria, Falcon, Ferite, Gambas, glib/GTK+, Go, Guile, Harbour, Haskell,
577 Java, Julia, Lisp, Lua, Mono, .NET, node.js, Object-Pascal, OCaml, Pascal,
578 Perl, PHP, PostgreSQL, Python, R, Rexx, Ring, RPG, Ruby, Rust, Scheme,
579 Scilab, S-Lang, Smalltalk, SP-Forth, SPL, Tcl, Visual Basic, Visual FoxPro,
580 Q, wxwidgets, XBLite and Xoho. By the time you read this, additional ones
581 may have appeared!
582
583 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
584
585 Curl adheres to the HTTP spec, which basically means you can play with *any*
586 protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and
587 XML-RPC are all such ones. You can use -X to set custom requests and -H to
588 set custom headers (or replace internally generated ones).
589
590 Using libcurl is of course just as good and you'd just use the proper
591 library options to do the same.
592
593 3.11 How do I POST with a different Content-Type?
594
595 You can always replace the internally generated headers with -H/--header.
596 To make a simple HTTP POST with text/xml as content-type, do something like:
597
598 curl -d "datatopost" -H "Content-Type: text/xml" [URL]
599
600 3.12 Why do FTP-specific features over HTTP proxy fail?
601
602 Because when you use a HTTP proxy, the protocol spoken on the network will
603 be HTTP, even if you specify a FTP URL. This effectively means that you
604 normally can't use FTP-specific features such as FTP upload and FTP quote
605 etc.
606
607 There is one exception to this rule, and that is if you can "tunnel through"
608 the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
609 and is generally not available as proxy admins usually disable tunneling to
610 ports other than 443 (which is used for HTTPS access through proxies).
611
612 3.13 Why do my single/double quotes fail?
613
614 To specify a command line option that includes spaces, you might need to
615 put the entire option within quotes. Like in:
616
617 curl -d " with spaces " url.com
618
619 or perhaps
620
621 curl -d ' with spaces ' url.com
622
623 Exactly what kind of quotes and how to do this is entirely up to the shell
624 or command line interpreter that you are using. For most unix shells, you
625 can more or less pick either single (') or double (") quotes. For
626 Windows/DOS prompts I believe you're forced to use double (") quotes.
627
628 Please study the documentation for your particular environment. Examples in
629 the curl docs will use a mix of both of these as shown above. You must
630 adjust them to work in your environment.
631
632 Remember that curl works and runs on more operating systems than most single
633 individuals have ever tried.
634
635 3.14 Does curl support Javascript or PAC (automated proxy config)?
636
637 Many web pages do magic stuff using embedded Javascript. Curl and libcurl
638 have no built-in support for that, so it will be treated just like any other
639 contents.
640
641 .pac files are a netscape invention and are sometimes used by organizations
642 to allow them to differentiate which proxies to use. The .pac contents is
643 just a Javascript program that gets invoked by the browser and that returns
644 the name of the proxy to connect to. Since curl doesn't support Javascript,
645 it can't support .pac proxy configuration either.
646
647 Some workarounds usually suggested to overcome this Javascript dependency:
648
649 Depending on the Javascript complexity, write up a script that translates it
650 to another language and execute that.
651
652 Read the Javascript code and rewrite the same logic in another language.
653
654 Implement a Javascript interpreter, people have successfully used the
655 Mozilla Javascript engine in the past.
656
657 Ask your admins to stop this, for a static proxy setup or similar.
658
659 3.15 Can I do recursive fetches with curl?
660
661 No. curl itself has no code that performs recursive operations, such as
662 those performed by wget and similar tools.
663
664 There exists wrapper scripts with that functionality (for example the
665 curlmirror perl script), and you can write programs based on libcurl to do
666 it, but the command line tool curl itself cannot.
667
668 3.16 What certificates do I need when I use SSL?
669
670 There are three different kinds of "certificates" to keep track of when we
671 talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.
672
673 CLIENT CERTIFICATE
674
675 The server you communicate with may require that you can provide this in
676 order to prove that you actually are who you claim to be. If the server
677 doesn't require this, you don't need a client certificate.
678
679 A client certificate is always used together with a private key, and the
680 private key has a pass phrase that protects it.
681
682 SERVER CERTIFICATE
683
684 The server you communicate with has a server certificate. You can and should
685 verify this certificate to make sure that you are truly talking to the real
686 server and not a server impersonating it.
687
688 CERTIFICATE AUTHORITY CERTIFICATE ("CA cert")
689
690 You often have several CA certs in a CA cert bundle that can be used to
691 verify a server certificate that was signed by one of the authorities in the
692 bundle. curl does not come with a CA cert bundle but most curl installs
693 provide one. You can also override the default.
694
695 The server certificate verification process is made by using a Certificate
696 Authority certificate ("CA cert") that was used to sign the server
697 certificate. Server certificate verification is enabled by default in curl
698 and libcurl and is often the reason for problems as explained in FAQ entry
699 4.12 and the SSLCERTS document
700 (https://curl.haxx.se/docs/sslcerts.html). Server certificates that are
701 "self-signed" or otherwise signed by a CA that you do not have a CA cert
702 for, cannot be verified. If the verification during a connect fails, you are
703 refused access. You then need to explicitly disable the verification to
704 connect to the server.
705
706 3.17 How do I list the root dir of an FTP server?
707
708 There are two ways. The way defined in the RFC is to use an encoded slash
709 in the first path part. List the "/tmp" dir like this:
710
711 curl ftp://ftp.sunet.se/%2ftmp/
712
713 or the not-quite-kosher-but-more-readable way, by simply starting the path
714 section of the URL with a slash:
715
716 curl ftp://ftp.sunet.se//tmp/
717
718 3.18 Can I use curl to send a POST/PUT and not wait for a response?
719
720 No.
721
722 But you could easily write your own program using libcurl to do such stunts.
723
724 3.19 How do I get HTTP from a host using a specific IP address?
725
726 For example, you may be trying out a web site installation that isn't yet in
727 the DNS. Or you have a site using multiple IP addresses for a given host
728 name and you want to address a specific one out of the set.
729
730 Set a custom Host: header that identifies the server name you want to reach
731 but use the target IP address in the URL:
732
733 curl --header "Host: www.example.com" http://127.0.0.1/
734
735 You can also opt to add faked host name entries to curl with the --resolve
736 option. That has the added benefit that things like redirects will also work
737 properly. The above operation would instead be done as:
738
739 curl --resolve www.example.com:80:127.0.0.1 http://www.example.com/
740
741 3.20 How to SFTP from my user's home directory?
742
743 Contrary to how FTP works, SFTP and SCP URLs specify the exact directory to
744 work with. It means that if you don't specify that you want the user's home
745 directory, you get the actual root directory.
746
747 To specify a file in your user's home directory, you need to use the correct
748 URL syntax which for SFTP might look similar to:
749
750 curl -O -u user:password sftp://example.com/~/file.txt
751
752 and for SCP it is just a different protocol prefix:
753
754 curl -O -u user:password scp://example.com/~/file.txt
755
756 3.21 Protocol xxx not supported or disabled in libcurl
757
758 When passing on a URL to curl to use, it may respond that the particular
759 protocol is not supported or disabled. The particular way this error message
760 is phrased is because curl doesn't make a distinction internally of whether
761 a particular protocol is not supported (i.e. never got any code added that
762 knows how to speak that protocol) or if it was explicitly disabled. curl can
763 be built to only support a given set of protocols, and the rest would then
764 be disabled or not supported.
765
766 Note that this error will also occur if you pass a wrongly spelled protocol
767 part as in "htpt://example.com" or as in the less evident case if you prefix
768 the protocol part with a space as in " http://example.com/".
769
770 3.22 curl -X gives me HTTP problems
771
772 In normal circumstances, -X should hardly ever be used.
773
774 By default you use curl without explicitly saying which request method to
775 use when the URL identifies a HTTP transfer. If you just pass in a URL like
776 "curl http://example.com" it will use GET. If you use -d or -F curl will use
777 POST, -I will cause a HEAD and -T will make it a PUT.
778
779 If for whatever reason you're not happy with these default choices that curl
780 does for you, you can override those request methods by specifying -X
781 [WHATEVER]. This way you can for example send a DELETE by doing "curl -X
782 DELETE [URL]".
783
784 It is thus pointless to do "curl -XGET [URL]" as GET would be used
785 anyway. In the same vein it is pointless to do "curl -X POST -d data
786 [URL]"... But you can make a fun and somewhat rare request that sends a
787 request-body in a GET request with something like "curl -X GET -d data
788 [URL]"
789
790 Note that -X doesn't actually change curl's behavior as it only modifies the
791 actual string sent in the request, but that may of course trigger a
792 different set of events.
793
794 Accordingly, by using -XPOST on a command line that for example would follow
795 a 303 redirect, you will effectively prevent curl from behaving
796 correctly. Be aware.
797
798
799 4. Running Problems
800
801 4.1 Problems connecting to SSL servers.
802
803 It took a very long time before we could sort out why curl had problems to
804 connect to certain SSL servers when using SSLeay or OpenSSL v0.9+. The
805 error sometimes showed up similar to:
806
807 16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:
808
809 It turned out to be because many older SSL servers don't deal with SSLv3
810 requests properly. To correct this problem, tell curl to select SSLv2 from
811 the command line (-2/--sslv2).
812
813 There have also been examples where the remote server didn't like the SSLv2
814 request and instead you had to force curl to use SSLv3 with -3/--sslv3.
815
816 4.2 Why do I get problems when I use & or % in the URL?
817
818 In general unix shells, the & symbol is treated specially and when used, it
819 runs the specified command in the background. To safely send the & as a part
820 of a URL, you should quote the entire URL by using single (') or double (")
821 quotes around it. Similar problems can also occur on some shells with other
822 characters, including ?*!$~(){}<>\|;`. When in doubt, quote the URL.
823
824 An example that would invoke a remote CGI that uses &-symbols could be:
825
826 curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
827
828 In Windows, the standard DOS shell treats the percent sign specially and you
829 need to use TWO percent signs for each single one you want to use in the
830 URL.
831
832 If you want a literal percent sign to be part of the data you pass in a POST
833 using -d/--data you must encode it as '%25' (which then also needs the
834 percent sign doubled on Windows machines).
835
836 4.3 How can I use {, }, [ or ] to specify multiple URLs?
837
838 Because those letters have a special meaning to the shell, to be used in
839 a URL specified to curl you must quote them.
840
841 An example that downloads two URLs (sequentially) would be:
842
843 curl '{curl,www}.haxx.se'
844
845 To be able to use those characters as actual parts of the URL (without using
846 them for the curl URL "globbing" system), use the -g/--globoff option:
847
848 curl -g 'www.site.com/weirdname[].html'
849
850 4.4 Why do I get downloaded data even though the web page doesn't exist?
851
852 Curl asks remote servers for the page you specify. If the page doesn't exist
853 at the server, the HTTP protocol defines how the server should respond and
854 that means that headers and a "page" will be returned. That's simply how
855 HTTP works.
856
857 By using the --fail option you can tell curl explicitly to not get any data
858 if the HTTP return code doesn't say success.
859
860 4.5 Why do I get return code XXX from a HTTP server?
861
862 RFC2616 clearly explains the return codes. This is a short transcript. Go
863 read the RFC for exact details:
864
865 4.5.1 "400 Bad Request"
866
867 The request could not be understood by the server due to malformed
868 syntax. The client SHOULD NOT repeat the request without modifications.
869
870 4.5.2 "401 Unauthorized"
871
872 The request requires user authentication.
873
874 4.5.3 "403 Forbidden"
875
876 The server understood the request, but is refusing to fulfill it.
877 Authorization will not help and the request SHOULD NOT be repeated.
878
879 4.5.4 "404 Not Found"
880
881 The server has not found anything matching the Request-URI. No indication
882 is given of whether the condition is temporary or permanent.
883
884 4.5.5 "405 Method Not Allowed"
885
886 The method specified in the Request-Line is not allowed for the resource
887 identified by the Request-URI. The response MUST include an Allow header
888 containing a list of valid methods for the requested resource.
889
890 4.5.6 "301 Moved Permanently"
891
892 If you get this return code and an HTML output similar to this:
893
894 <H1>Moved Permanently</H1> The document has moved <A
895 HREF="http://same_url_now_with_a_trailing_slash/">here</A>.
896
897 it might be because you requested a directory URL but without the trailing
898 slash. Try the same operation again _with_ the trailing URL, or use the
899 -L/--location option to follow the redirection.
900
901 4.6 Can you tell me what error code 142 means?
902
903 All curl error codes are described at the end of the man page, in the
904 section called "EXIT CODES".
905
906 Error codes that are larger than the highest documented error code means
907 that curl has exited due to a crash. This is a serious error, and we
908 appreciate a detailed bug report from you that describes how we could go
909 ahead and repeat this!
910
911 4.7 How do I keep user names and passwords secret in Curl command lines?
912
913 This problem has two sides:
914
915 The first part is to avoid having clear-text passwords in the command line
916 so that they don't appear in 'ps' outputs and similar. That is easily
917 avoided by using the "-K" option to tell curl to read parameters from a file
918 or stdin to which you can pass the secret info. curl itself will also
919 attempt to "hide" the given password by blanking out the option - this
920 doesn't work on all platforms.
921
922 To keep the passwords in your account secret from the rest of the world is
923 not a task that curl addresses. You could of course encrypt them somehow to
924 at least hide them from being read by human eyes, but that is not what
925 anyone would call security.
926
927 Also note that regular HTTP (using Basic authentication) and FTP passwords
928 are sent as cleartext across the network. All it takes for anyone to fetch
929 them is to listen on the network. Eavesdropping is very easy. Use more secure
930 authentication methods (like Digest, Negotiate or even NTLM) or consider the
931 SSL-based alternatives HTTPS and FTPS.
932
933 4.8 I found a bug!
934
935 It is not a bug if the behavior is documented. Read the docs first.
936 Especially check out the KNOWN_BUGS file, it may be a documented bug!
937
938 If it is a problem with a binary you've downloaded or a package for your
939 particular platform, try contacting the person who built the package/archive
940 you have.
941
942 If there is a bug, read the BUGS document first. Then report it as described
943 in there.
944
945 4.9 Curl can't authenticate to the server that requires NTLM?
946
947 NTLM support requires OpenSSL, GnuTLS, mbedTLS, NSS, Secure Transport, or
948 Microsoft Windows libraries at build-time to provide this functionality.
949
950 NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You
951 should not use such ones.
952
953 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
954
955 Many web servers allow or demand that the administrator configures the
956 server properly for these requests to work on the web server.
957
958 Some servers seem to support HEAD only on certain kinds of URLs.
959
960 To fully grasp this, try the documentation for the particular server
961 software you're trying to interact with. This is not anything curl can do
962 anything about.
963
964 4.11 Why do my HTTP range requests return the full document?
965
966 Because the range may not be supported by the server, or the server may
967 choose to ignore it and return the full document anyway.
968
969 4.12 Why do I get "certificate verify failed" ?
970
971 You invoke curl 7.10 or later to communicate on a https:// URL and get an
972 error back looking something similar to this:
973
974 curl: (35) SSL: error:14090086:SSL routines:
975 SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
976
977 Then it means that curl couldn't verify that the server's certificate was
978 good. Curl verifies the certificate using the CA cert bundle that comes with
979 the curl installation.
980
981 To disable the verification (which makes it act like curl did before 7.10),
982 use -k. This does however enable man-in-the-middle attacks.
983
984 If you get this failure but are having a CA cert bundle installed and used,
985 the server's certificate is not signed by one of the CA's in the bundle. It
986 might for example be self-signed. You then correct this problem by obtaining
987 a valid CA cert for the server. Or again, decrease the security by disabling
988 this check.
989
990 Details are also in the SSLCERTS file in the release archives, found online
991 here: https://curl.haxx.se/docs/sslcerts.html
992
993 4.13 Why is curl -R on Windows one hour off?
994
995 Since curl 7.53.0 this issue should be fixed as long as curl was built with
996 any modern compiler that allows for a 64-bit curl_off_t type. For older
997 compilers or prior curl versions it may set a time that appears one hour off.
998 This happens due to a flaw in how Windows stores and uses file modification
999 times and it is not easily worked around. For more details read this:
1000 https://www.codeproject.com/Articles/1144/Beating-the-Daylight-Savings-Time-bug-and-getting
1001
1002 4.14 Redirects work in browser but not with curl!
1003
1004 curl supports HTTP redirects well (see item 3.8). Browsers generally support
1005 at least two other ways to perform redirects that curl does not:
1006
1007 Meta tags. You can write a HTML tag that will cause the browser to redirect
1008 to another given URL after a certain time.
1009
1010 Javascript. You can write a Javascript program embedded in a HTML page that
1011 redirects the browser to another given URL.
1012
1013 There is no way to make curl follow these redirects. You must either
1014 manually figure out what the page is set to do, or write a script that parses
1015 the results and fetches the new URL.
1016
1017 4.15 FTPS doesn't work
1018
1019 curl supports FTPS (sometimes known as FTP-SSL) both implicit and explicit
1020 mode.
1021
1022 When a URL is used that starts with FTPS://, curl assumes implicit SSL on
1023 the control connection and will therefore immediately connect and try to
1024 speak SSL. FTPS:// connections default to port 990.
1025
1026 To use explicit FTPS, you use a FTP:// URL and the --ftp-ssl option (or one
1027 of its related flavors). This is the most common method, and the one
1028 mandated by RFC4217. This kind of connection will then of course use the
1029 standard FTP port 21 by default.
1030
1031 4.16 My HTTP POST or PUT requests are slow!
1032
1033 libcurl makes all POST and PUT requests (except for POST requests with a
1034 very tiny request body) use the "Expect: 100-continue" header. This header
1035 allows the server to deny the operation early so that libcurl can bail out
1036 before having to send any data. This is useful in authentication
1037 cases and others.
1038
1039 However, many servers don't implement the Expect: stuff properly and if the
1040 server doesn't respond (positively) within 1 second libcurl will continue
1041 and send off the data anyway.
1042
1043 You can disable libcurl's use of the Expect: header the same way you disable
1044 any header, using -H / CURLOPT_HTTPHEADER, or by forcing it to use HTTP 1.0.
1045
1046 4.17 Non-functional connect timeouts
1047
1048 In most Windows setups having a timeout longer than 21 seconds make no
1049 difference, as it will only send 3 TCP SYN packets and no more. The second
1050 packet sent three seconds after the first and the third six seconds after
1051 the second. No more than three packets are sent, no matter how long the
1052 timeout is set.
1053
1054 See option TcpMaxConnectRetransmissions on this page:
1055 https://support.microsoft.com/en-us/kb/175523/en-us
1056
1057 Also, even on non-Windows systems there may run a firewall or anti-virus
1058 software or similar that accepts the connection but does not actually do
1059 anything else. This will make (lib)curl to consider the connection connected
1060 and thus the connect timeout won't trigger.
1061
1062 4.18 file:// URLs containing drive letters (Windows, NetWare)
1063
1064 When using curl to try to download a local file, one might use a URL
1065 in this format:
1066
1067 file://D:/blah.txt
1068
1069 You'll find that even if D:\blah.txt does exist, curl returns a 'file
1070 not found' error.
1071
1072 According to RFC 1738 (https://www.ietf.org/rfc/rfc1738.txt),
1073 file:// URLs must contain a host component, but it is ignored by
1074 most implementations. In the above example, 'D:' is treated as the
1075 host component, and is taken away. Thus, curl tries to open '/blah.txt'.
1076 If your system is installed to drive C:, that will resolve to 'C:\blah.txt',
1077 and if that doesn't exist you will get the not found error.
1078
1079 To fix this problem, use file:// URLs with *three* leading slashes:
1080
1081 file:///D:/blah.txt
1082
1083 Alternatively, if it makes more sense, specify 'localhost' as the host
1084 component:
1085
1086 file://localhost/D:/blah.txt
1087
1088 In either case, curl should now be looking for the correct file.
1089
1090 4.19 Why doesn't curl return an error when the network cable is unplugged?
1091
1092 Unplugging a cable is not an error situation. The TCP/IP protocol stack
1093 was designed to be fault tolerant, so even though there may be a physical
1094 break somewhere the connection shouldn't be affected, just possibly
1095 delayed. Eventually, the physical break will be fixed or the data will be
1096 re-routed around the physical problem through another path.
1097
1098 In such cases, the TCP/IP stack is responsible for detecting when the
1099 network connection is irrevocably lost. Since with some protocols it is
1100 perfectly legal for the client to wait indefinitely for data, the stack may
1101 never report a problem, and even when it does, it can take up to 20 minutes
1102 for it to detect an issue. The curl option --keepalive-time enables
1103 keep-alive support in the TCP/IP stack which makes it periodically probe the
1104 connection to make sure it is still available to send data. That should
1105 reliably detect any TCP/IP network failure.
1106
1107 But even that won't detect the network going down before the TCP/IP
1108 connection is established (e.g. during a DNS lookup) or using protocols that
1109 don't use TCP. To handle those situations, curl offers a number of timeouts
1110 on its own. --speed-limit/--speed-time will abort if the data transfer rate
1111 falls too low, and --connect-timeout and --max-time can be used to put an
1112 overall timeout on the connection phase or the entire transfer.
1113
1114 A libcurl-using application running in a known physical environment (e.g.
1115 an embedded device with only a single network connection) may want to act
1116 immediately if its lone network connection goes down. That can be achieved
1117 by having the application monitor the network connection on its own using an
1118 OS-specific mechanism, then signaling libcurl to abort (see also item 5.13).
1119
1120 4.20 curl doesn't return error for HTTP non-200 responses!
1121
1122 Correct. Unless you use -f (--fail).
1123
1124 When doing HTTP transfers, curl will perform exactly what you're asking it
1125 to do and if successful it will not return an error. You can use curl to
1126 test your web server's "file not found" page (that gets 404 back), you can
1127 use it to check your authentication protected web pages (that gets a 401
1128 back) and so on.
1129
1130 The specific HTTP response code does not constitute a problem or error for
1131 curl. It simply sends and delivers HTTP as you asked and if that worked,
1132 everything is fine and dandy. The response code is generally providing more
1133 higher level error information that curl doesn't care about. The error was
1134 not in the HTTP transfer.
1135
1136 If you want your command line to treat error codes in the 400 and up range
1137 as errors and thus return a non-zero value and possibly show an error
1138 message, curl has a dedicated option for that: -f (CURLOPT_FAILONERROR in
1139 libcurl speak).
1140
1141 You can also use the -w option and the variable %{response_code} to extract
1142 the exact response code that was returned in the response.
1143
1144 4.21 Why is there a HTTP/1.1 in my HTTP/2 request?
1145
1146 If you use verbose to see the HTTP request when you send off a HTTP/2
1147 request, it will still say 1.1.
1148
1149 The reason for this is that we first generate the request to send using the
1150 old 1.1 style and show that request in the verbose output, and then we
1151 convert it over to the binary header-compressed HTTP/2 style. The actual
1152 "1.1" part from that request is then not actually used in the transfer.
1153 The binary HTTP/2 headers are not human readable.
1154
1155 5. libcurl Issues
1156
1157 5.1 Is libcurl thread-safe?
1158
1159 Yes.
1160
1161 We have written the libcurl code specifically adjusted for multi-threaded
1162 programs. libcurl will use thread-safe functions instead of non-safe ones if
1163 your system has such. Note that you must never share the same handle in
1164 multiple threads.
1165
1166 There may be some exceptions to thread safety depending on how libcurl was
1167 built. Please review the guidelines for thread safety to learn more:
1168 https://curl.haxx.se/libcurl/c/threadsafe.html
1169
1170 5.2 How can I receive all data into a large memory chunk?
1171
1172 [ See also the examples/getinmemory.c source ]
1173
1174 You are in full control of the callback function that gets called every time
1175 there is data received from the remote server. You can make that callback do
1176 whatever you want. You do not have to write the received data to a file.
1177
1178 One solution to this problem could be to have a pointer to a struct that you
1179 pass to the callback function. You set the pointer using the
1180 CURLOPT_WRITEDATA option. Then that pointer will be passed to the callback
1181 instead of a FILE * to a file:
1182
1183 /* imaginary struct */
1184 struct MemoryStruct {
1185 char *memory;
1186 size_t size;
1187 };
1188
1189 /* imaginary callback function */
1190 size_t
1191 WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
1192 {
1193 size_t realsize = size * nmemb;
1194 struct MemoryStruct *mem = (struct MemoryStruct *)data;
1195
1196 mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
1197 if (mem->memory) {
1198 memcpy(&(mem->memory[mem->size]), ptr, realsize);
1199 mem->size += realsize;
1200 mem->memory[mem->size] = 0;
1201 }
1202 return realsize;
1203 }
1204
1205 5.3 How do I fetch multiple files with libcurl?
1206
1207 libcurl has excellent support for transferring multiple files. You should
1208 just repeatedly set new URLs with curl_easy_setopt() and then transfer it
1209 with curl_easy_perform(). The handle you get from curl_easy_init() is not
1210 only reusable, but you're even encouraged to reuse it if you can, as that
1211 will enable libcurl to use persistent connections.
1212
1213 5.4 Does libcurl do Winsock initialization on win32 systems?
1214
1215 Yes, if told to in the curl_global_init() call.
1216
1217 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
1218
1219 Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
1220 that DLL use the FILE * (as the DLL and the client application cannot access
1221 each others' variable memory areas). If you set CURLOPT_WRITEDATA you must
1222 also use CURLOPT_WRITEFUNCTION as well to set a function that writes the
1223 file, even if that simply writes the data to the specified FILE *.
1224 Similarly, if you use CURLOPT_READDATA you must also specify
1225 CURLOPT_READFUNCTION.
1226
1227 5.6 What about Keep-Alive or persistent connections?
1228
1229 curl and libcurl have excellent support for persistent connections when
1230 transferring several files from the same server. Curl will attempt to reuse
1231 connections for all URLs specified on the same command line/config file, and
1232 libcurl will reuse connections for all transfers that are made using the
1233 same libcurl handle.
1234
1235 When you use the easy interface the connection cache is kept within the easy
1236 handle. If you instead use the multi interface, the connection cache will be
1237 kept within the multi handle and will be shared among all the easy handles
1238 that are used within the same multi handle.
1239
1240 5.7 Link errors when building libcurl on Windows!
1241
1242 You need to make sure that your project, and all the libraries (both static
1243 and dynamic) that it links against, are compiled/linked against the same run
1244 time library.
1245
1246 This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
1247 options to the command line compiler. /MD (linking against MSVCRT dll) seems
1248 to be the most commonly used option.
1249
1250 When building an application that uses the static libcurl library, you must
1251 add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for
1252 dynamic import symbols. If you're using Visual Studio, you need to instead
1253 add CURL_STATICLIB in the "Preprocessor Definitions" section.
1254
1255 If you get linker error like "unknown symbol __imp__curl_easy_init ..." you
1256 have linked against the wrong (static) library. If you want to use the
1257 libcurl.dll and import lib, you don't need any extra CFLAGS, but use one of
1258 the import libraries below. These are the libraries produced by the various
1259 lib/Makefile.* files:
1260
1261 Target: static lib. import lib for libcurl*.dll.
1262 -----------------------------------------------------------
1263 MingW: libcurl.a libcurldll.a
1264 MSVC (release): libcurl.lib libcurl_imp.lib
1265 MSVC (debug): libcurld.lib libcurld_imp.lib
1266 Borland: libcurl.lib libcurl_imp.lib
1267
1268 5.8 libcurl.so.X: open failed: No such file or directory
1269
1270 This is an error message you might get when you try to run a program linked
1271 with a shared version of libcurl and your run-time linker (ld.so) couldn't
1272 find the shared library named libcurl.so.X. (Where X is the number of the
1273 current libcurl ABI, typically 3 or 4).
1274
1275 You need to make sure that ld.so finds libcurl.so.X. You can do that
1276 multiple ways, and it differs somewhat between different operating systems,
1277 but they are usually:
1278
1279 * Add an option to the linker command line that specify the hard-coded path
1280 the run-time linker should check for the lib (usually -R)
1281
1282 * Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
1283 should check for libs
1284
1285 * Adjust the system's config to check for libs in the directory where you've
1286 put the dir (like Linux's /etc/ld.so.conf)
1287
1288 'man ld.so' and 'man ld' will tell you more details
1289
1290 5.9 How does libcurl resolve host names?
1291
1292 libcurl supports a large a number of different name resolve functions. One
1293 of them is picked at build-time and will be used unconditionally. Thus, if
1294 you want to change name resolver function you must rebuild libcurl and tell
1295 it to use a different function.
1296
1297 - The non-IPv6 resolver that can use one of four different host name resolve
1298 calls (depending on what your system supports):
1299
1300 A - gethostbyname()
1301 B - gethostbyname_r() with 3 arguments
1302 C - gethostbyname_r() with 5 arguments
1303 D - gethostbyname_r() with 6 arguments
1304
1305 - The IPv6-resolver that uses getaddrinfo()
1306
1307 - The c-ares based name resolver that uses the c-ares library for resolves.
1308 Using this offers asynchronous name resolves.
1309
1310 - The threaded resolver (default option on Windows). It uses:
1311
1312 A - gethostbyname() on plain IPv4 hosts
1313 B - getaddrinfo() on IPv6 enabled hosts
1314
1315 Also note that libcurl never resolves or reverse-lookups addresses given as
1316 pure numbers, such as 127.0.0.1 or ::1.
1317
1318 5.10 How do I prevent libcurl from writing the response to stdout?
1319
1320 libcurl provides a default built-in write function that writes received data
1321 to stdout. Set the CURLOPT_WRITEFUNCTION to receive the data, or possibly
1322 set CURLOPT_WRITEDATA to a different FILE * handle.
1323
1324 5.11 How do I make libcurl not receive the whole HTTP response?
1325
1326 You make the write callback (or progress callback) return an error and
1327 libcurl will then abort the transfer.
1328
1329 5.12 Can I make libcurl fake or hide my real IP address?
1330
1331 No. libcurl operates on a higher level. Besides, faking IP address would
1332 imply sending IP packets with a made-up source address, and then you normally
1333 get a problem with receiving the packet sent back as they would then not be
1334 routed to you!
1335
1336 If you use a proxy to access remote sites, the sites will not see your local
1337 IP address but instead the address of the proxy.
1338
1339 Also note that on many networks NATs or other IP-munging techniques are used
1340 that makes you see and use a different IP address locally than what the
1341 remote server will see you coming from. You may also consider using
1342 https://www.torproject.org/ .
1343
1344 5.13 How do I stop an ongoing transfer?
1345
1346 With the easy interface you make sure to return the correct error code from
1347 one of the callbacks, but none of them are instant. There is no function you
1348 can call from another thread or similar that will stop it immediately.
1349 Instead, you need to make sure that one of the callbacks you use returns an
1350 appropriate value that will stop the transfer. Suitable callbacks that you
1351 can do this with include the progress callback, the read callback and the
1352 write callback.
1353
1354 If you're using the multi interface, you can also stop a transfer by
1355 removing the particular easy handle from the multi stack at any moment you
1356 think the transfer is done or when you wish to abort the transfer.
1357
1358 5.14 Using C++ non-static functions for callbacks?
1359
1360 libcurl is a C library, it doesn't know anything about C++ member functions.
1361
1362 You can overcome this "limitation" with relative ease using a static
1363 member function that is passed a pointer to the class:
1364
1365 // f is the pointer to your object.
1366 static size_t YourClass::func(void *buffer, size_t sz, size_t n, void *f)
1367 {
1368 // Call non-static member function.
1369 static_cast<YourClass*>(f)->nonStaticFunction();
1370 }
1371
1372 // This is how you pass pointer to the static function:
1373 curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass::func);
1374 curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);
1375
1376 5.15 How do I get an FTP directory listing?
1377
1378 If you end the FTP URL you request with a slash, libcurl will provide you
1379 with a directory listing of that given directory. You can also set
1380 CURLOPT_CUSTOMREQUEST to alter what exact listing command libcurl would use
1381 to list the files.
1382
1383 The follow-up question tends to be how is a program supposed to parse the
1384 directory listing. How does it know what's a file and what's a dir and what's
1385 a symlink etc. If the FTP server supports the MLSD command then it will
1386 return data in a machine-readable format that can be parsed for type. The
1387 types are specified by RFC3659 section 7.5.1. If MLSD is not supported then
1388 you have to work with what you're given. The LIST output format is entirely
1389 at the server's own liking and the NLST output doesn't reveal any types and
1390 in many cases doesn't even include all the directory entries. Also, both LIST
1391 and NLST tend to hide unix-style hidden files (those that start with a dot)
1392 by default so you need to do "LIST -a" or similar to see them.
1393
1394 Example - List only directories.
1395 ftp.funet.fi supports MLSD and ftp.kernel.org does not:
1396
1397 curl -s ftp.funet.fi/pub/ -X MLSD | \
1398 perl -lne 'print if s/(?:^|;)type=dir;[^ ]+ (.+)$/$1/'
1399
1400 curl -s ftp.kernel.org/pub/linux/kernel/ | \
1401 perl -lne 'print if s/^d[-rwx]{9}(?: +[^ ]+){7} (.+)$/$1/'
1402
1403 If you need to parse LIST output in libcurl one such existing
1404 list parser is available at https://cr.yp.to/ftpparse.html Versions of
1405 libcurl since 7.21.0 also provide the ability to specify a wildcard to
1406 download multiple files from one FTP directory.
1407
1408 5.16 I want a different time-out!
1409
1410 Time and time again users realize that CURLOPT_TIMEOUT and
1411 CURLOPT_CONNECTIMEOUT are not sufficiently advanced or flexible to cover all
1412 the various use cases and scenarios applications end up with.
1413
1414 libcurl offers many more ways to time-out operations. A common alternative
1415 is to use the CURLOPT_LOW_SPEED_LIMIT and CURLOPT_LOW_SPEED_TIME options to
1416 specify the lowest possible speed to accept before to consider the transfer
1417 timed out.
1418
1419 The most flexible way is by writing your own time-out logic and using
1420 CURLOPT_XFERINFOFUNCTION (perhaps in combination with other callbacks) and
1421 use that to figure out exactly when the right condition is met when the
1422 transfer should get stopped.
1423
1424 5.17 Can I write a server with libcurl?
1425
1426 No. libcurl offers no functions or building blocks to build any kind of
1427 internet protocol server. libcurl is only a client-side library. For server
1428 libraries, you need to continue your search elsewhere but there exist many
1429 good open source ones out there for most protocols you could possibly want a
1430 server for. And there are really good stand-alone ones that have been tested
1431 and proven for many years. There's no need for you to reinvent them!
1432
1433 5.18 Does libcurl use threads?
1434
1435 Put simply: no, libcurl will execute in the same thread you call it in. All
1436 callbacks will be called in the same thread as the one you call libcurl in.
1437
1438 If you want to avoid your thread to be blocked by the libcurl call, you make
1439 sure you use the non-blocking API which will do transfers asynchronously -
1440 but still in the same single thread.
1441
1442 libcurl will potentially internally use threads for name resolving, if it
1443 was built to work like that, but in those cases it'll create the child
1444 threads by itself and they will only be used and then killed internally by
1445 libcurl and never exposed to the outside.
1446
1447 6. License Issues
1448
1449 Curl and libcurl are released under a MIT/X derivate license. The license is
1450 very liberal and should not impose a problem for your project. This section
1451 is just a brief summary for the cases we get the most questions. (Parts of
1452 this section was much enhanced by Bjorn Reese.)
1453
1454 We are not lawyers and this is not legal advice. You should probably consult
1455 one if you want true and accurate legal insights without our prejudice. Note
1456 especially that this section concerns the libcurl license only; compiling in
1457 features of libcurl that depend on other libraries (e.g. OpenSSL) may affect
1458 the licensing obligations of your application.
1459
1460 6.1 I have a GPL program, can I use the libcurl library?
1461
1462 Yes!
1463
1464 Since libcurl may be distributed under the MIT/X derivate license, it can be
1465 used together with GPL in any software.
1466
1467 6.2 I have a closed-source program, can I use the libcurl library?
1468
1469 Yes!
1470
1471 libcurl does not put any restrictions on the program that uses the library.
1472
1473 6.3 I have a BSD licensed program, can I use the libcurl library?
1474
1475 Yes!
1476
1477 libcurl does not put any restrictions on the program that uses the library.
1478
1479 6.4 I have a program that uses LGPL libraries, can I use libcurl?
1480
1481 Yes!
1482
1483 The LGPL license doesn't clash with other licenses.
1484
1485 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
1486
1487 Yes!
1488
1489 The MIT/X derivate license practically allows you to do almost anything with
1490 the sources, on the condition that the copyright texts in the sources are
1491 left intact.
1492
1493 6.6 Can you please change the curl/libcurl license to XXXX?
1494
1495 No.
1496
1497 We have carefully picked this license after years of development and
1498 discussions and a large amount of people have contributed with source code
1499 knowing that this is the license we use. This license puts the restrictions
1500 we want on curl/libcurl and it does not spread to other programs or
1501 libraries that use it. It should be possible for everyone to use libcurl or
1502 curl in their projects, no matter what license they already have in use.
1503
1504 6.7 What are my obligations when using libcurl in my commercial apps?
1505
1506 Next to none. All you need to adhere to is the MIT-style license (stated in
1507 the COPYING file) which basically says you have to include the copyright
1508 notice in "all copies" and that you may not use the copyright holder's name
1509 when promoting your software.
1510
1511 You do not have to release any of your source code.
1512
1513 You do not have to reveal or make public any changes to the libcurl source
1514 code.
1515
1516 You do not have to broadcast to the world that you are using libcurl within
1517 your app.
1518
1519 All we ask is that you disclose "the copyright notice and this permission
1520 notice" somewhere. Most probably like in the documentation or in the section
1521 where other third party dependencies already are mentioned and acknowledged.
1522
1523 As can be seen here: https://curl.haxx.se/docs/companies.html and elsewhere,
1524 more and more companies are discovering the power of libcurl and take
1525 advantage of it even in commercial environments.
1526
1527
1528 7. PHP/CURL Issues
1529
1530 7.1 What is PHP/CURL?
1531
1532 The module for PHP that makes it possible for PHP programs to access curl-
1533 functions from within PHP.
1534
1535 In the cURL project we call this module PHP/CURL to differentiate it from
1536 curl the command line tool and libcurl the library. The PHP team however
1537 does not refer to it like this (for unknown reasons). They call it plain
1538 CURL (often using all caps) or sometimes ext/curl, but both cause much
1539 confusion to users which in turn gives us a higher question load.
1540
1541 7.2 Who wrote PHP/CURL?
1542
1543 PHP/CURL was initially written by Sterling Hughes.
1544
1545 7.3 Can I perform multiple requests using the same handle?
1546
1547 Yes - at least in PHP version 4.3.8 and later (this has been known to not
1548 work in earlier versions, but the exact version when it started to work is
1549 unknown to me).
1550
1551 After a transfer, you just set new options in the handle and make another
1552 transfer. This will make libcurl re-use the same connection if it can.
1553
1554 7.4 Does PHP/CURL have dependencies?
1555
1556 PHP/CURL is a module that comes with the regular PHP package. It depends on
1557 and uses libcurl, so you need to have libcurl installed properly before
1558 PHP/CURL can be used.