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6
7FAQ
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 do you not 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 fails when using static libraries
28  2.2 Does curl work/build with other SSL libraries?
29  2.3 How do I upgrade curl.exe in Windows?
30  2.4 Does curl support SOCKS (RFC 1928) ?
31
32 3. Usage Problems
33  3.1 curl: (1) SSL is disabled, https: not supported
34  3.2 How do I tell curl to resume a transfer?
35  3.3 Why does my posting using -F not work?
36  3.4 How do I tell curl to run custom FTP commands?
37  3.5 How can I disable the Accept: */* header?
38  3.6 Does curl support ASP, XML, XHTML or HTML version Y?
39  3.7 Can I use curl to delete/rename a file through FTP?
40  3.8 How do I tell curl to follow HTTP redirects?
41  3.9 How do I use curl in my favorite programming language?
42  3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
43  3.11 How do I POST with a different Content-Type?
44  3.12 Why do FTP-specific features over HTTP proxy fail?
45  3.13 Why do my single/double quotes fail?
46  3.14 Does curl support JavaScript or PAC (automated proxy config)?
47  3.15 Can I do recursive fetches with curl?
48  3.16 What certificates do I need when I use SSL?
49  3.17 How do I list the root directory of an FTP server?
50  3.18 Can I use curl to send a POST/PUT and not wait for a response?
51  3.19 How do I get HTTP from a host using a specific IP address?
52  3.20 How to SFTP from my user's home directory?
53  3.21 Protocol xxx not supported or disabled in libcurl
54  3.22 curl -X gives me HTTP problems
55
56 4. Running Problems
57  4.2 Why do I get problems when I use & or % in the URL?
58  4.3 How can I use {, }, [ or ] to specify multiple URLs?
59  4.4 Why do I get downloaded data even though the webpage does not exist?
60  4.5 Why do I get return code XXX from an HTTP server?
61   4.5.1 "400 Bad Request"
62   4.5.2 "401 Unauthorized"
63   4.5.3 "403 Forbidden"
64   4.5.4 "404 Not Found"
65   4.5.5 "405 Method Not Allowed"
66   4.5.6 "301 Moved Permanently"
67  4.6 Can you tell me what error code 142 means?
68  4.7 How do I keep usernames and passwords secret in curl command lines?
69  4.8 I found a bug
70  4.9 curl cannot authenticate to a server that requires NTLM?
71  4.10 My HTTP request using HEAD, PUT or DELETE does not work
72  4.11 Why do my HTTP range requests return the full document?
73  4.12 Why do I get "certificate verify failed" ?
74  4.13 Why is curl -R on Windows one hour off?
75  4.14 Redirects work in browser but not with curl
76  4.15 FTPS does not work
77  4.16 My HTTP POST or PUT requests are slow
78  4.17 Non-functional connect timeouts on Windows
79  4.18 file:// URLs containing drive letters (Windows, NetWare)
80  4.19 Why does not curl return an error when the network cable is unplugged?
81  4.20 curl does not return error for HTTP non-200 responses
82
83 5. libcurl Issues
84  5.1 Is libcurl thread-safe?
85  5.2 How can I receive all data into a large memory chunk?
86  5.3 How do I fetch multiple files with libcurl?
87  5.4 Does libcurl do Winsock initialization on Win32 systems?
88  5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on Win32 ?
89  5.6 What about Keep-Alive or persistent connections?
90  5.7 Link errors when building libcurl on Windows
91  5.8 libcurl.so.X: open failed: No such file or directory
92  5.9 How does libcurl resolve hostnames?
93  5.10 How do I prevent libcurl from writing the response to stdout?
94  5.11 How do I make libcurl not receive the whole HTTP response?
95  5.12 Can I make libcurl fake or hide my real IP address?
96  5.13 How do I stop an ongoing transfer?
97  5.14 Using C++ non-static functions for callbacks?
98  5.15 How do I get an FTP directory listing?
99  5.16 I want a different time-out
100  5.17 Can I write a server with libcurl?
101  5.18 Does libcurl use threads?
102
103 6. License Issues
104  6.1 I have a GPL program, can I use the libcurl library?
105  6.2 I have a closed-source program, can I use the libcurl library?
106  6.3 I have a BSD licensed program, can I use the libcurl library?
107  6.4 I have a program that uses LGPL libraries, can I use libcurl?
108  6.5 Can I modify curl/libcurl for my program and keep the changes secret?
109  6.6 Can you please change the curl/libcurl license to XXXX?
110  6.7 What are my obligations when using libcurl in my commercial apps?
111
112 7. PHP/CURL Issues
113  7.1 What is PHP/CURL?
114  7.2 Who wrote PHP/CURL?
115  7.3 Can I perform multiple requests using the same handle?
116  7.4 Does PHP/CURL have dependencies?
117
118 8. Development
119  8.1 Why does curl use C89?
120  8.2 Will curl be rewritten?
121
122==============================================================================
123
1241. 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 read as '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 client-side URL transfer library, supporting DICT, FILE, FTP, FTPS,
139    GOPHER, GOPHERS, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, MQTT, POP3, POP3S,
140    RTMP, RTMPS, RTSP, SCP, SFTP, SMB, SMBS, SMTP, SMTPS, TELNET, TFTP, WS
141    and WSS.
142
143    libcurl supports HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading,
144    Kerberos, SPNEGO, HTTP form based upload, proxies, cookies, user+password
145    authentication, file transfer resume, http proxy tunneling and more.
146
147    libcurl is highly portable, it builds and works identically on numerous
148    platforms, including Solaris, NetBSD, FreeBSD, OpenBSD, Darwin, HP-UX,
149    IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, macOS,
150    Ultrix, QNX, OpenVMS, RISC OS, Novell NetWare, DOS, Symbian, OSF, Android,
151    Minix, IBM TPF and more...
152
153    libcurl is free, thread-safe, IPv6 compatible, feature rich, well
154    supported and fast.
155
156  curl
157
158    A command line tool for getting or sending data using URL syntax.
159
160    Since curl uses libcurl, curl supports the same wide range of common
161    Internet protocols that libcurl does.
162
163  We pronounce curl with an initial k sound. It rhymes with words like girl
164  and earl. This is a short WAV file to help you:
165
166     https://media.merriam-webster.com/soundc11/c/curl0001.wav
167
168  There are numerous sub-projects and related projects that also use the word
169  curl in the project names in various combinations, but you should take
170  notice that this FAQ is directed at the command-line tool named curl (and
171  libcurl the library), and may therefore not be valid for other curl-related
172  projects. (There is however a small section for the PHP/CURL in this FAQ.)
173
174  1.2 What is libcurl?
175
176  libcurl is a reliable and portable library for doing Internet data transfers
177  using one or more of its supported Internet protocols.
178
179  You can use libcurl freely in your application, be it open source,
180  commercial or closed-source.
181
182  libcurl is most probably the most portable, most powerful and most often
183  used C-based multi-platform file transfer library on this planet - be it
184  open source or commercial.
185
186  1.3 What is curl not?
187
188  curl is not a wget clone. That is a common misconception. Never, during
189  curl's development, have we intended curl to replace wget or compete on its
190  market. curl is targeted at single-shot file transfers.
191
192  curl is not a website mirroring program. If you want to use curl to mirror
193  something: fine, go ahead and write a script that wraps around curl or use
194  libcurl to make it reality.
195
196  curl is not an FTP site mirroring program. Sure, get and send FTP with curl
197  but if you want systematic and sequential behavior you should write a
198  script (or write a new program that interfaces libcurl) and do it.
199
200  curl is not a PHP tool, even though it works perfectly well when used from
201  or with PHP (when using the PHP/CURL module).
202
203  curl is not a program for a single operating system. curl exists, compiles,
204  builds and runs under a wide range of operating systems, including all
205  modern Unixes (and a bunch of older ones too), Windows, Amiga, OS/2, macOS,
206  QNX etc.
207
208  1.4 When will you make curl do XXXX ?
209
210  We love suggestions of what to change in order to make curl and libcurl
211  better. We do however believe in a few rules when it comes to the future of
212  curl:
213
214  curl -- the command line tool -- is to remain a non-graphical command line
215  tool. If you want GUIs or fancy scripting capabilities, you should look for
216  another tool that uses libcurl.
217
218  We do not add things to curl that other small and available tools already do
219  well at the side. curl's output can be piped into another program or
220  redirected to another file for the next program to interpret.
221
222  We focus on protocol related issues and improvements. If you want to do more
223  magic with the supported protocols than curl currently does, chances are
224  good we will agree. If you want to add more protocols, we may agree.
225
226  If you want someone else to do all the work while you wait for us to
227  implement it for you, that is not a friendly attitude. We spend a
228  considerable time already on maintaining and developing curl. In order to
229  get more out of us, you should consider trading in some of your time and
230  effort in return. Simply go to the GitHub repository which resides at
231  https://github.com/curl/curl, fork the project, and create pull requests
232  with your proposed changes.
233
234  If you write the code, chances are better that it will get into curl faster.
235
236  1.5 Who makes curl?
237
238  curl and libcurl are not made by any single individual. Daniel Stenberg is
239  project leader and main developer, but other persons' submissions are
240  important and crucial. Anyone can contribute and post their changes and
241  improvements and have them inserted in the main sources (of course on the
242  condition that developers agree that the fixes are good).
243
244  The full list of all contributors is found in the docs/THANKS file.
245
246  curl is developed by a community, with Daniel at the wheel.
247
248  1.6 What do you get for making curl?
249
250  Project cURL is entirely free and open. We do this voluntarily, mostly in
251  our spare time. Companies may pay individual developers to work on curl.
252  This is not controlled by nor supervised in any way by the curl project.
253
254  We get help from companies. Haxx provides website, bandwidth, mailing lists
255  etc, GitHub hosts the primary git repository and other services like the bug
256  tracker at https://github.com/curl/curl. Also again, some companies have
257  sponsored certain parts of the development in the past and I hope some will
258  continue to do so in the future.
259
260  If you want to support our project, consider a donation or a banner-program
261  or even better: by helping us with coding, documenting or testing etc.
262
263  See also: https://curl.se/sponsors.html
264
265  1.7 What about CURL from curl.com?
266
267  During the summer of 2001, curl.com was busy advertising their client-side
268  programming language for the web, named CURL.
269
270  We are in no way associated with curl.com or their CURL programming
271  language.
272
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
275  rights to the name.
276
277  We recognize that we will be living in parallel with curl.com and wish them
278  every success.
279
280  1.8 I have a problem, who do I mail?
281
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.se/mail/
286
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.
293
294  If you have found or simply suspect a security problem in curl or libcurl,
295  submit all the details at https://hackerone.one/curl. On there we keep the
296  issue private while we investigate, confirm it, work and validate a fix and
297  agree on a time schedule for publication etc. That way we produce a fix in a
298  timely manner before the flaw is announced to the world, reducing the impact
299  the problem risks having on existing users.
300
301  Security issues can also be taking to the curl security team by emailing
302  security at curl.se (closed list of receivers, mails are not disclosed).
303
304  1.9 Where do I buy commercial support for curl?
305
306  curl is fully open source. It means you can hire any skilled engineer to fix
307  your curl-related problems.
308
309  We list available alternatives on the curl website:
310  https://curl.se/support.html
311
312  1.10 How many are using curl?
313
314  It is impossible to tell.
315
316  We do not know how many users that knowingly have installed and use curl.
317
318  We do not know how many users that use curl without knowing that they are in
319  fact using it.
320
321  We do not know how many users that downloaded or installed curl and then
322  never use it.
323
324  In 2020, we estimate that curl runs in roughly ten billion installations
325  world wide.
326
327  1.11 Why do you not update ca-bundle.crt
328
329  In the cURL project we have decided not to attempt to keep this file updated
330  (or even present) since deciding what to add to a ca cert bundle is an
331  undertaking we have not been ready to accept, and the one we can get from
332  Mozilla is perfectly fine so there is no need to duplicate that work.
333
334  Today, with many services performed over HTTPS, every operating system
335  should come with a default ca cert bundle that can be deemed somewhat
336  trustworthy and that collection (if reasonably updated) should be deemed to
337  be a lot better than a private curl version.
338
339  If you want the most recent collection of ca certs that Mozilla Firefox
340  uses, we recommend that you extract the collection yourself from Mozilla
341  Firefox (by running 'make ca-bundle), or by using our online service setup
342  for this purpose: https://curl.se/docs/caextract.html
343
344  1.12 I have a problem who, can I chat with?
345
346  There is a bunch of friendly people hanging out in the #curl channel on the
347  IRC network libera.chat. If you are polite and nice, chances are good that
348  you can get -- or provide -- help instantly.
349
350  1.13 curl's ECCN number?
351
352  The US government restricts exports of software that contains or uses
353  cryptography. When doing so, the Export Control Classification Number (ECCN)
354  is used to identify the level of export control etc.
355
356  Apache Software Foundation gives a good explanation of ECCNs at
357  https://www.apache.org/dev/crypto.html
358
359  We believe curl's number might be ECCN 5D002, another possibility is
360  5D992. It seems necessary to write them (the authority that administers ECCN
361  numbers), asking to confirm.
362
363  Comprehensible explanations of the meaning of such numbers and how to obtain
364  them (resp.) are here
365
366  https://www.bis.doc.gov/licensing/exportingbasics.htm
367  https://www.bis.doc.gov/licensing/do_i_needaneccn.html
368
369  An incomprehensible description of the two numbers above is here
370  https://www.bis.doc.gov/index.php/documents/new-encryption/1653-ccl5-pt2-3
371
372  1.14 How do I submit my patch?
373
374  We strongly encourage you to submit changes and improvements directly as
375  "pull requests" on GitHub: https://github.com/curl/curl/pulls
376
377  If you for any reason cannot or will not deal with GitHub, send your patch to
378  the curl-library mailing list. We are many subscribers there and there are
379  lots of people who can review patches, comment on them and "receive" them
380  properly.
381
382  Lots of more details are found in the CONTRIBUTE.md and INTERNALS.md
383  documents.
384
385  1.15 How do I port libcurl to my OS?
386
387  Here's a rough step-by-step:
388
389  1. copy a suitable lib/config-*.h file as a start to lib/config-[youros].h
390
391  2. edit lib/config-[youros].h to match your OS and setup
392
393  3. edit lib/curl_setup.h to include config-[youros].h when your OS is
394     detected by the preprocessor, in the style others already exist
395
396  4. compile lib/*.c and make them into a library
397
398
3992. Install Related Problems
400
401  2.1 configure fails when using static libraries
402
403  You may find that configure fails to properly detect the entire dependency
404  chain of libraries when you provide static versions of the libraries that
405  configure checks for.
406
407  The reason why static libraries is much harder to deal with is that for them
408  we do not get any help but the script itself must know or check what more
409  libraries that are needed (with shared libraries, that dependency "chain" is
410  handled automatically). This is an error-prone process and one that also
411  tends to vary over time depending on the release versions of the involved
412  components and may also differ between operating systems.
413
414  For that reason, configure does few attempts to actually figure this out and
415  you are instead encouraged to set LIBS and LDFLAGS accordingly when you
416  invoke configure, and point out the needed libraries and set the necessary
417  flags yourself.
418
419  2.2 Does curl work with other SSL libraries?
420
421  curl has been written to use a generic SSL function layer internally, and
422  that SSL functionality can then be provided by one out of many different SSL
423  backends.
424
425  curl can be built to use one of the following SSL alternatives: OpenSSL,
426  LibreSSL, BoringSSL, AWS-LC, GnuTLS, wolfSSL, mbedTLS, Secure Transport
427  (native iOS/macOS), Schannel (native Windows), BearSSL or Rustls. They all
428  have their pros and cons, and we try to maintain a comparison of them here:
429  https://curl.se/docs/ssl-compared.html
430
431  2.3 How do I upgrade curl.exe in Windows?
432
433  The curl tool that is shipped as an integrated component of Windows 10 and
434  Windows 11 is managed by Microsoft. If you were to delete the file or
435  replace it with a newer version downloaded from https://curl.se/windows,
436  then Windows Update will cease to work on your system.
437
438  There is no way to independently force an upgrade of the curl.exe that is
439  part of Windows other than through the regular Windows update process. There
440  is also nothing the curl project itself can do about this, since this is
441  managed and controlled entirely by Microsoft as owners of the operating
442  system.
443
444  You can always download and install the latest version of curl for Windows
445  from https://curl.se/windows into a separate location.
446
447  2.4 Does curl support SOCKS (RFC 1928) ?
448
449  Yes, SOCKS 4 and 5 are supported.
450
4513. Usage problems
452
453  3.1 curl: (1) SSL is disabled, https: not supported
454
455  If you get this output when trying to get anything from an HTTPS server, it
456  means that the instance of curl/libcurl that you are using was built without
457  support for this protocol.
458
459  This could have happened if the configure script that was run at build time
460  could not find all libs and include files curl requires for SSL to work. If
461  the configure script fails to find them, curl is simply built without SSL
462  support.
463
464  To get HTTPS support into a curl that was previously built but that reports
465  that HTTPS is not supported, you should dig through the document and logs
466  and check out why the configure script does not find the SSL libs and/or
467  include files.
468
469  Also, check out the other paragraph in this FAQ labeled "configure does not
470  find OpenSSL even when it is installed".
471
472  3.2 How do I tell curl to resume a transfer?
473
474  curl supports resumed transfers both ways on both FTP and HTTP.
475  Try the -C option.
476
477  3.3 Why does my posting using -F not work?
478
479  You cannot arbitrarily use -F or -d, the choice between -F or -d depends on
480  the HTTP operation you need curl to do and what the web server that will
481  receive your post expects.
482
483  If the form you are trying to submit uses the type 'multipart/form-data',
484  then and only then you must use the -F type. In all the most common cases,
485  you should use -d which then causes a posting with the type
486  'application/x-www-form-urlencoded'.
487
488  This is described in some detail in the MANUAL and TheArtOfHttpScripting
489  documents, and if you do not understand it the first time, read it again
490  before you post questions about this to the mailing list. Also, try reading
491  through the mailing list archives for old postings and questions regarding
492  this.
493
494  3.4 How do I tell curl to run custom FTP commands?
495
496  You can tell curl to perform optional commands both before and/or after a
497  file transfer. Study the -Q/--quote option.
498
499  Since curl is used for file transfers, you do not normally use curl to
500  perform FTP commands without transferring anything. Therefore you must
501  always specify a URL to transfer to/from even when doing custom FTP
502  commands, or use -I which implies the "no body" option sent to libcurl.
503
504  3.5 How can I disable the Accept: */* header?
505
506  You can change all internally generated headers by adding a replacement with
507  the -H/--header option. By adding a header with empty contents you safely
508  disable that one. Use -H "Accept:" to disable that specific header.
509
510  3.6 Does curl support ASP, XML, XHTML or HTML version Y?
511
512  To curl, all contents are alike. It does not matter how the page was
513  generated. It may be ASP, PHP, Perl, shell-script, SSI or plain HTML
514  files. There is no difference to curl and it does not even know what kind of
515  language that generated the page.
516
517  See also item 3.14 regarding JavaScript.
518
519  3.7 Can I use curl to delete/rename a file through FTP?
520
521  Yes. You specify custom FTP commands with -Q/--quote.
522
523  One example would be to delete a file after you have downloaded it:
524
525     curl -O ftp://example.com/coolfile -Q '-DELE coolfile'
526
527  or rename a file after upload:
528
529     curl -T infile ftp://example.com/dir/ -Q "-RNFR infile" -Q "-RNTO newname"
530
531  3.8 How do I tell curl to follow HTTP redirects?
532
533  curl does not follow so-called redirects by default. The Location: header
534  that informs the client about this is only interpreted if you are using the
535  -L/--location option. As in:
536
537     curl -L http://example.com
538
539  Not all redirects are HTTP ones, see 4.14
540
541  3.9 How do I use curl in my favorite programming language?
542
543  Many programming languages have interfaces/bindings that allow you to use
544  curl without having to use the command line tool. If you are fluent in such
545  a language, you may prefer to use one of these interfaces instead.
546
547  Find out more about which languages that support curl directly, and how to
548  install and use them, in the libcurl section of the curl website:
549  https://curl.se/libcurl/
550
551  All the various bindings to libcurl are made by other projects and people,
552  outside of the cURL project. The cURL project itself only produces libcurl
553  with its plain C API. If you do not find anywhere else to ask you can ask
554  about bindings on the curl-library list too, but be prepared that people on
555  that list may not know anything about bindings.
556
557  In December 2021, there were interfaces available for the following
558  languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Delphi, Dylan, Eiffel,
559  Euphoria, Falcon, Ferite, Gambas, glib/GTK+, Go, Guile, Harbour, Haskell,
560  Java, Julia, Lisp, Lua, Mono, .NET, node.js, Object-Pascal, OCaml, Pascal,
561  Perl, PHP, PostgreSQL, Python, R, Rexx, Ring, RPG, Ruby, Rust, Scheme,
562  Scilab, S-Lang, Smalltalk, SP-Forth, SPL, Tcl, Visual Basic, Visual FoxPro,
563  Q, wxwidgets, XBLite and Xoho. By the time you read this, additional ones
564  may have appeared.
565
566  3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
567
568  curl adheres to the HTTP spec, which basically means you can play with *any*
569  protocol that is built on top of HTTP. Protocols such as SOAP, WebDAV and
570  XML-RPC are all such ones. You can use -X to set custom requests and -H to
571  set custom headers (or replace internally generated ones).
572
573  Using libcurl is of course just as good and you would just use the proper
574  library options to do the same.
575
576  3.11 How do I POST with a different Content-Type?
577
578  You can always replace the internally generated headers with -H/--header.
579  To make a simple HTTP POST with text/xml as content-type, do something like:
580
581        curl -d "datatopost" -H "Content-Type: text/xml" [URL]
582
583  3.12 Why do FTP-specific features over HTTP proxy fail?
584
585  Because when you use an HTTP proxy, the protocol spoken on the network will
586  be HTTP, even if you specify an FTP URL. This effectively means that you
587  normally cannot use FTP-specific features such as FTP upload and FTP quote
588  etc.
589
590  There is one exception to this rule, and that is if you can "tunnel through"
591  the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
592  and is generally not available as proxy admins usually disable tunneling to
593  ports other than 443 (which is used for HTTPS access through proxies).
594
595  3.13 Why do my single/double quotes fail?
596
597  To specify a command line option that includes spaces, you might need to
598  put the entire option within quotes. Like in:
599
600   curl -d " with spaces " example.com
601
602  or perhaps
603
604   curl -d ' with spaces ' example.com
605
606  Exactly what kind of quotes and how to do this is entirely up to the shell
607  or command line interpreter that you are using. For most Unix shells, you
608  can more or less pick either single (') or double (") quotes. For
609  Windows/DOS command prompts you must use double (") quotes, and if the
610  option string contains inner double quotes you can escape them with a
611  backslash.
612
613  For Windows powershell the arguments are not always passed on as expected
614  because curl is not a powershell script. You may or may not be able to use
615  single quotes. To escape inner double quotes seems to require a
616  backslash-backtick escape sequence and the outer quotes as double quotes.
617
618  Please study the documentation for your particular environment. Examples in
619  the curl docs will use a mix of both of these as shown above. You must
620  adjust them to work in your environment.
621
622  Remember that curl works and runs on more operating systems than most single
623  individuals have ever tried.
624
625  3.14 Does curl support JavaScript or PAC (automated proxy config)?
626
627  Many webpages do magic stuff using embedded JavaScript. curl and libcurl
628  have no built-in support for that, so it will be treated just like any other
629  contents.
630
631  .pac files are a Netscape invention and are sometimes used by organizations
632  to allow them to differentiate which proxies to use. The .pac contents is
633  just a JavaScript program that gets invoked by the browser and that returns
634  the name of the proxy to connect to. Since curl does not support JavaScript,
635  it cannot support .pac proxy configuration either.
636
637  Some workarounds usually suggested to overcome this JavaScript dependency:
638
639  Depending on the JavaScript complexity, write up a script that translates it
640  to another language and execute that.
641
642  Read the JavaScript code and rewrite the same logic in another language.
643
644  Implement a JavaScript interpreter, people have successfully used the
645  Mozilla JavaScript engine in the past.
646
647  Ask your admins to stop this, for a static proxy setup or similar.
648
649  3.15 Can I do recursive fetches with curl?
650
651  No. curl itself has no code that performs recursive operations, such as
652  those performed by wget and similar tools.
653
654  There exists wrapper scripts with that functionality (for example the
655  curlmirror perl script), and you can write programs based on libcurl to do
656  it, but the command line tool curl itself cannot.
657
658  3.16 What certificates do I need when I use SSL?
659
660  There are three different kinds of "certificates" to keep track of when we
661  talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.
662
663  CLIENT CERTIFICATE
664
665  The server you communicate with may require that you can provide this in
666  order to prove that you actually are who you claim to be. If the server
667  does not require this, you do not need a client certificate.
668
669  A client certificate is always used together with a private key, and the
670  private key has a pass phrase that protects it.
671
672  SERVER CERTIFICATE
673
674  The server you communicate with has a server certificate. You can and should
675  verify this certificate to make sure that you are truly talking to the real
676  server and not a server impersonating it.
677
678  CERTIFICATE AUTHORITY CERTIFICATE ("CA cert")
679
680  You often have several CA certs in a CA cert bundle that can be used to
681  verify a server certificate that was signed by one of the authorities in the
682  bundle. curl does not come with a CA cert bundle but most curl installs
683  provide one. You can also override the default.
684
685  The server certificate verification process is made by using a Certificate
686  Authority certificate ("CA cert") that was used to sign the server
687  certificate. Server certificate verification is enabled by default in curl
688  and libcurl and is often the reason for problems as explained in FAQ entry
689  4.12 and the SSLCERTS document
690  (https://curl.se/docs/sslcerts.html). Server certificates that are
691  "self-signed" or otherwise signed by a CA that you do not have a CA cert
692  for, cannot be verified. If the verification during a connect fails, you are
693  refused access. You then need to explicitly disable the verification to
694  connect to the server.
695
696  3.17 How do I list the root directory of an FTP server?
697
698  There are two ways. The way defined in the RFC is to use an encoded slash
699  in the first path part. List the "/tmp" directory like this:
700
701     curl ftp://ftp.example.com/%2ftmp/
702
703  or the not-quite-kosher-but-more-readable way, by simply starting the path
704  section of the URL with a slash:
705
706     curl ftp://ftp.example.com//tmp/
707
708  3.18 Can I use curl to send a POST/PUT and not wait for a response?
709
710  No.
711
712  You can easily write your own program using libcurl to do such stunts.
713
714  3.19 How do I get HTTP from a host using a specific IP address?
715
716  For example, you may be trying out a website installation that is not yet in
717  the DNS. Or you have a site using multiple IP addresses for a given host
718  name and you want to address a specific one out of the set.
719
720  Set a custom Host: header that identifies the server name you want to reach
721  but use the target IP address in the URL:
722
723    curl --header "Host: www.example.com" http://127.0.0.1/
724
725  You can also opt to add faked hostname entries to curl with the --resolve
726  option. That has the added benefit that things like redirects will also work
727  properly. The above operation would instead be done as:
728
729    curl --resolve www.example.com:80:127.0.0.1 http://www.example.com/
730
731  3.20 How to SFTP from my user's home directory?
732
733  Contrary to how FTP works, SFTP and SCP URLs specify the exact directory to
734  work with. It means that if you do not specify that you want the user's home
735  directory, you get the actual root directory.
736
737  To specify a file in your user's home directory, you need to use the correct
738  URL syntax which for SFTP might look similar to:
739
740    curl -O -u user:password sftp://example.com/~/file.txt
741
742  and for SCP it is just a different protocol prefix:
743
744    curl -O -u user:password scp://example.com/~/file.txt
745
746  3.21 Protocol xxx not supported or disabled in libcurl
747
748  When passing on a URL to curl to use, it may respond that the particular
749  protocol is not supported or disabled. The particular way this error message
750  is phrased is because curl does not make a distinction internally of whether
751  a particular protocol is not supported (i.e. never got any code added that
752  knows how to speak that protocol) or if it was explicitly disabled. curl can
753  be built to only support a given set of protocols, and the rest would then
754  be disabled or not supported.
755
756  Note that this error will also occur if you pass a wrongly spelled protocol
757  part as in "htpt://example.com" or as in the less evident case if you prefix
758  the protocol part with a space as in " http://example.com/".
759
760  3.22 curl -X gives me HTTP problems
761
762  In normal circumstances, -X should hardly ever be used.
763
764  By default you use curl without explicitly saying which request method to
765  use when the URL identifies an HTTP transfer. If you just pass in a URL like
766  "curl http://example.com" it will use GET. If you use -d or -F curl will use
767  POST, -I will cause a HEAD and -T will make it a PUT.
768
769  If for whatever reason you are not happy with these default choices that curl
770  does for you, you can override those request methods by specifying -X
771  [WHATEVER]. This way you can for example send a DELETE by doing "curl -X
772  DELETE [URL]".
773
774  It is thus pointless to do "curl -XGET [URL]" as GET would be used anyway.
775  In the same vein it is pointless to do "curl -X POST -d data [URL]". You can
776  make a fun and somewhat rare request that sends a request-body in a GET
777  request with something like "curl -X GET -d data [URL]"
778
779  Note that -X does not actually change curl's behavior as it only modifies the
780  actual string sent in the request, but that may of course trigger a
781  different set of events.
782
783  Accordingly, by using -XPOST on a command line that for example would follow
784  a 303 redirect, you will effectively prevent curl from behaving
785  correctly. Be aware.
786
787
7884. Running Problems
789
790  4.2 Why do I get problems when I use & or % in the URL?
791
792  In general Unix shells, the & symbol is treated specially and when used, it
793  runs the specified command in the background. To safely send the & as a part
794  of a URL, you should quote the entire URL by using single (') or double (")
795  quotes around it. Similar problems can also occur on some shells with other
796  characters, including ?*!$~(){}<>\|;`. When in doubt, quote the URL.
797
798  An example that would invoke a remote CGI that uses &-symbols could be:
799
800     curl 'http://www.example.com/cgi-bin/query?text=yes&q=curl'
801
802  In Windows, the standard DOS shell treats the percent sign specially and you
803  need to use TWO percent signs for each single one you want to use in the
804  URL.
805
806  If you want a literal percent sign to be part of the data you pass in a POST
807  using -d/--data you must encode it as '%25' (which then also needs the
808  percent sign doubled on Windows machines).
809
810  4.3 How can I use {, }, [ or ] to specify multiple URLs?
811
812  Because those letters have a special meaning to the shell, to be used in
813  a URL specified to curl you must quote them.
814
815  An example that downloads two URLs (sequentially) would be:
816
817    curl '{curl,www}.haxx.se'
818
819  To be able to use those characters as actual parts of the URL (without using
820  them for the curl URL "globbing" system), use the -g/--globoff option:
821
822    curl -g 'www.example.com/weirdname[].html'
823
824  4.4 Why do I get downloaded data even though the webpage does not exist?
825
826  curl asks remote servers for the page you specify. If the page does not exist
827  at the server, the HTTP protocol defines how the server should respond and
828  that means that headers and a "page" will be returned. That is simply how
829  HTTP works.
830
831  By using the --fail option you can tell curl explicitly to not get any data
832  if the HTTP return code does not say success.
833
834  4.5 Why do I get return code XXX from an HTTP server?
835
836  RFC 2616 clearly explains the return codes. This is a short transcript. Go
837  read the RFC for exact details:
838
839    4.5.1 "400 Bad Request"
840
841    The request could not be understood by the server due to malformed
842    syntax. The client SHOULD NOT repeat the request without modifications.
843
844    4.5.2 "401 Unauthorized"
845
846    The request requires user authentication.
847
848    4.5.3 "403 Forbidden"
849
850    The server understood the request, but is refusing to fulfill it.
851    Authorization will not help and the request SHOULD NOT be repeated.
852
853    4.5.4 "404 Not Found"
854
855    The server has not found anything matching the Request-URI. No indication
856    is given as to whether the condition is temporary or permanent.
857
858    4.5.5 "405 Method Not Allowed"
859
860    The method specified in the Request-Line is not allowed for the resource
861    identified by the Request-URI. The response MUST include an Allow header
862    containing a list of valid methods for the requested resource.
863
864    4.5.6 "301 Moved Permanently"
865
866    If you get this return code and an HTML output similar to this:
867
868       <H1>Moved Permanently</H1> The document has moved <A
869       HREF="http://same_url_now_with_a_trailing_slash/">here</A>.
870
871    it might be because you requested a directory URL but without the trailing
872    slash. Try the same operation again _with_ the trailing URL, or use the
873    -L/--location option to follow the redirection.
874
875  4.6 Can you tell me what error code 142 means?
876
877  All curl error codes are described at the end of the man page, in the
878  section called "EXIT CODES".
879
880  Error codes that are larger than the highest documented error code means
881  that curl has exited due to a crash. This is a serious error, and we
882  appreciate a detailed bug report from you that describes how we could go
883  ahead and repeat this.
884
885  4.7 How do I keep usernames and passwords secret in curl command lines?
886
887  This problem has two sides:
888
889  The first part is to avoid having clear-text passwords in the command line
890  so that they do not appear in 'ps' outputs and similar. That is easily
891  avoided by using the "-K" option to tell curl to read parameters from a file
892  or stdin to which you can pass the secret info. curl itself will also
893  attempt to "hide" the given password by blanking out the option - this
894  does not work on all platforms.
895
896  To keep the passwords in your account secret from the rest of the world is
897  not a task that curl addresses. You could of course encrypt them somehow to
898  at least hide them from being read by human eyes, but that is not what
899  anyone would call security.
900
901  Also note that regular HTTP (using Basic authentication) and FTP passwords
902  are sent as cleartext across the network. All it takes for anyone to fetch
903  them is to listen on the network. Eavesdropping is easy. Use more secure
904  authentication methods (like Digest, Negotiate or even NTLM) or consider the
905  SSL-based alternatives HTTPS and FTPS.
906
907  4.8 I found a bug
908
909  It is not a bug if the behavior is documented. Read the docs first.
910  Especially check out the KNOWN_BUGS file, it may be a documented bug.
911
912  If it is a problem with a binary you have downloaded or a package for your
913  particular platform, try contacting the person who built the package/archive
914  you have.
915
916  If there is a bug, read the BUGS document first. Then report it as described
917  in there.
918
919  4.9 curl cannot authenticate to a server that requires NTLM?
920
921  NTLM support requires OpenSSL, GnuTLS, mbedTLS, Secure Transport, or
922  Microsoft Windows libraries at build-time to provide this functionality.
923
924  4.10 My HTTP request using HEAD, PUT or DELETE does not work
925
926  Many web servers allow or demand that the administrator configures the
927  server properly for these requests to work on the web server.
928
929  Some servers seem to support HEAD only on certain kinds of URLs.
930
931  To fully grasp this, try the documentation for the particular server
932  software you are trying to interact with. This is not anything curl can do
933  anything about.
934
935  4.11 Why do my HTTP range requests return the full document?
936
937  Because the range may not be supported by the server, or the server may
938  choose to ignore it and return the full document anyway.
939
940  4.12 Why do I get "certificate verify failed" ?
941
942  When you invoke curl and get an error 60 error back it means that curl
943  could not verify that the server's certificate was good. curl verifies the
944  certificate using the CA cert bundle and verifying for which names the
945  certificate has been granted.
946
947  To completely disable the certificate verification, use -k. This does
948  however enable man-in-the-middle attacks and makes the transfer INSECURE.
949  We strongly advise against doing this for more than experiments.
950
951  If you get this failure with a CA cert bundle installed and used, the
952  server's certificate might not be signed by one of the CA's in your CA
953  store. It might for example be self-signed. You then correct this problem by
954  obtaining a valid CA cert for the server. Or again, decrease the security by
955  disabling this check.
956
957  At times, you find that the verification works in your favorite browser but
958  fails in curl. When this happens, the reason is usually that the server
959  sends an incomplete cert chain. The server is mandated to send all
960  "intermediate certificates" but does not. This typically works with browsers
961  anyway since they A) cache such certs and B) supports AIA which downloads
962  such missing certificates on demand. This is a server misconfiguration. A
963  good way to figure out if this is the case it to use the SSL Labs server
964  test and check the certificate chain: https://www.ssllabs.com/ssltest/
965
966  Details are also in the SSLCERTS.md document, found online here:
967  https://curl.se/docs/sslcerts.html
968
969  4.13 Why is curl -R on Windows one hour off?
970
971  Since curl 7.53.0 this issue should be fixed as long as curl was built with
972  any modern compiler that allows for a 64-bit curl_off_t type. For older
973  compilers or prior curl versions it may set a time that appears one hour off.
974  This happens due to a flaw in how Windows stores and uses file modification
975  times and it is not easily worked around. For more details read this:
976  https://www.codeproject.com/Articles/1144/Beating-the-Daylight-Savings-Time-bug-and-getting
977
978  4.14 Redirects work in browser but not with curl
979
980  curl supports HTTP redirects well (see item 3.8). Browsers generally support
981  at least two other ways to perform redirects that curl does not:
982
983  Meta tags. You can write an HTML tag that will cause the browser to redirect
984  to another given URL after a certain time.
985
986  JavaScript. You can write a JavaScript program embedded in an HTML page that
987  redirects the browser to another given URL.
988
989  There is no way to make curl follow these redirects. You must either
990  manually figure out what the page is set to do, or write a script that parses
991  the results and fetches the new URL.
992
993  4.15 FTPS does not work
994
995  curl supports FTPS (sometimes known as FTP-SSL) both implicit and explicit
996  mode.
997
998  When a URL is used that starts with FTPS://, curl assumes implicit SSL on
999  the control connection and will therefore immediately connect and try to
1000  speak SSL. FTPS:// connections default to port 990.
1001
1002  To use explicit FTPS, you use an FTP:// URL and the --ftp-ssl option (or one
1003  of its related flavors). This is the most common method, and the one
1004  mandated by RFC 4217. This kind of connection will then of course use the
1005  standard FTP port 21 by default.
1006
1007  4.16 My HTTP POST or PUT requests are slow
1008
1009  libcurl makes all POST and PUT requests (except for requests with a small
1010  request body) use the "Expect: 100-continue" header. This header allows the
1011  server to deny the operation early so that libcurl can bail out before having
1012  to send any data. This is useful in authentication cases and others.
1013
1014  However, many servers do not implement the Expect: stuff properly and if the
1015  server does not respond (positively) within 1 second libcurl will continue
1016  and send off the data anyway.
1017
1018  You can disable libcurl's use of the Expect: header the same way you disable
1019  any header, using -H / CURLOPT_HTTPHEADER, or by forcing it to use HTTP 1.0.
1020
1021  4.17 Non-functional connect timeouts
1022
1023  In most Windows setups having a timeout longer than 21 seconds make no
1024  difference, as it will only send 3 TCP SYN packets and no more. The second
1025  packet sent three seconds after the first and the third six seconds after
1026  the second. No more than three packets are sent, no matter how long the
1027  timeout is set.
1028
1029  See option TcpMaxConnectRetransmissions on this page:
1030  https://support.microsoft.com/en-us/kb/175523/en-us
1031
1032  Also, even on non-Windows systems there may run a firewall or anti-virus
1033  software or similar that accepts the connection but does not actually do
1034  anything else. This will make (lib)curl to consider the connection connected
1035  and thus the connect timeout will not trigger.
1036
1037  4.18 file:// URLs containing drive letters (Windows, NetWare)
1038
1039  When using curl to try to download a local file, one might use a URL
1040  in this format:
1041
1042  file://D:/blah.txt
1043
1044  you will find that even if D:\blah.txt does exist, curl returns a 'file
1045  not found' error.
1046
1047  According to RFC 1738 (https://www.ietf.org/rfc/rfc1738.txt),
1048  file:// URLs must contain a host component, but it is ignored by
1049  most implementations. In the above example, 'D:' is treated as the
1050  host component, and is taken away. Thus, curl tries to open '/blah.txt'.
1051  If your system is installed to drive C:, that will resolve to 'C:\blah.txt',
1052  and if that does not exist you will get the not found error.
1053
1054  To fix this problem, use file:// URLs with *three* leading slashes:
1055
1056  file:///D:/blah.txt
1057
1058  Alternatively, if it makes more sense, specify 'localhost' as the host
1059  component:
1060
1061  file://localhost/D:/blah.txt
1062
1063  In either case, curl should now be looking for the correct file.
1064
1065  4.19 Why does not curl return an error when the network cable is unplugged?
1066
1067  Unplugging a cable is not an error situation. The TCP/IP protocol stack
1068  was designed to be fault tolerant, so even though there may be a physical
1069  break somewhere the connection should not be affected, just possibly
1070  delayed. Eventually, the physical break will be fixed or the data will be
1071  re-routed around the physical problem through another path.
1072
1073  In such cases, the TCP/IP stack is responsible for detecting when the
1074  network connection is irrevocably lost. Since with some protocols it is
1075  perfectly legal for the client to wait indefinitely for data, the stack may
1076  never report a problem, and even when it does, it can take up to 20 minutes
1077  for it to detect an issue. The curl option --keepalive-time enables
1078  keep-alive support in the TCP/IP stack which makes it periodically probe the
1079  connection to make sure it is still available to send data. That should
1080  reliably detect any TCP/IP network failure.
1081
1082  TCP keep alive will not detect the network going down before the TCP/IP
1083  connection is established (e.g. during a DNS lookup) or using protocols that
1084  do not use TCP. To handle those situations, curl offers a number of timeouts
1085  on its own. --speed-limit/--speed-time will abort if the data transfer rate
1086  falls too low, and --connect-timeout and --max-time can be used to put an
1087  overall timeout on the connection phase or the entire transfer.
1088
1089  A libcurl-using application running in a known physical environment (e.g.
1090  an embedded device with only a single network connection) may want to act
1091  immediately if its lone network connection goes down. That can be achieved
1092  by having the application monitor the network connection on its own using an
1093  OS-specific mechanism, then signaling libcurl to abort (see also item 5.13).
1094
1095  4.20 curl does not return error for HTTP non-200 responses
1096
1097  Correct. Unless you use -f (--fail).
1098
1099  When doing HTTP transfers, curl will perform exactly what you are asking it
1100  to do and if successful it will not return an error. You can use curl to
1101  test your web server's "file not found" page (that gets 404 back), you can
1102  use it to check your authentication protected webpages (that gets a 401
1103  back) and so on.
1104
1105  The specific HTTP response code does not constitute a problem or error for
1106  curl. It simply sends and delivers HTTP as you asked and if that worked,
1107  everything is fine and dandy. The response code is generally providing more
1108  higher level error information that curl does not care about. The error was
1109  not in the HTTP transfer.
1110
1111  If you want your command line to treat error codes in the 400 and up range
1112  as errors and thus return a non-zero value and possibly show an error
1113  message, curl has a dedicated option for that: -f (CURLOPT_FAILONERROR in
1114  libcurl speak).
1115
1116  You can also use the -w option and the variable %{response_code} to extract
1117  the exact response code that was returned in the response.
1118
11195. libcurl Issues
1120
1121  5.1 Is libcurl thread-safe?
1122
1123  Yes.
1124
1125  We have written the libcurl code specifically adjusted for multi-threaded
1126  programs. libcurl will use thread-safe functions instead of non-safe ones if
1127  your system has such. Note that you must never share the same handle in
1128  multiple threads.
1129
1130  There may be some exceptions to thread safety depending on how libcurl was
1131  built. Please review the guidelines for thread safety to learn more:
1132  https://curl.se/libcurl/c/threadsafe.html
1133
1134  5.2 How can I receive all data into a large memory chunk?
1135
1136  [ See also the examples/getinmemory.c source ]
1137
1138  You are in full control of the callback function that gets called every time
1139  there is data received from the remote server. You can make that callback do
1140  whatever you want. You do not have to write the received data to a file.
1141
1142  One solution to this problem could be to have a pointer to a struct that you
1143  pass to the callback function. You set the pointer using the
1144  CURLOPT_WRITEDATA option. Then that pointer will be passed to the callback
1145  instead of a FILE * to a file:
1146
1147        /* imaginary struct */
1148        struct MemoryStruct {
1149          char *memory;
1150          size_t size;
1151        };
1152
1153        /* imaginary callback function */
1154        size_t
1155        WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
1156        {
1157          size_t realsize = size * nmemb;
1158          struct MemoryStruct *mem = (struct MemoryStruct *)data;
1159
1160          mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
1161          if (mem->memory) {
1162            memcpy(&(mem->memory[mem->size]), ptr, realsize);
1163            mem->size += realsize;
1164            mem->memory[mem->size] = 0;
1165          }
1166          return realsize;
1167        }
1168
1169  5.3 How do I fetch multiple files with libcurl?
1170
1171  libcurl has excellent support for transferring multiple files. You should
1172  just repeatedly set new URLs with curl_easy_setopt() and then transfer it
1173  with curl_easy_perform(). The handle you get from curl_easy_init() is not
1174  only reusable, but you are even encouraged to reuse it if you can, as that
1175  will enable libcurl to use persistent connections.
1176
1177  5.4 Does libcurl do Winsock initialization on Win32 systems?
1178
1179  Yes, if told to in the curl_global_init() call.
1180
1181  5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on Win32 ?
1182
1183  Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
1184  that DLL use the FILE * (as the DLL and the client application cannot access
1185  each others' variable memory areas). If you set CURLOPT_WRITEDATA you must
1186  also use CURLOPT_WRITEFUNCTION as well to set a function that writes the
1187  file, even if that simply writes the data to the specified FILE *.
1188  Similarly, if you use CURLOPT_READDATA you must also specify
1189  CURLOPT_READFUNCTION.
1190
1191  5.6 What about Keep-Alive or persistent connections?
1192
1193  curl and libcurl have excellent support for persistent connections when
1194  transferring several files from the same server. curl will attempt to reuse
1195  connections for all URLs specified on the same command line/config file, and
1196  libcurl will reuse connections for all transfers that are made using the
1197  same libcurl handle.
1198
1199  When you use the easy interface the connection cache is kept within the easy
1200  handle. If you instead use the multi interface, the connection cache will be
1201  kept within the multi handle and will be shared among all the easy handles
1202  that are used within the same multi handle.
1203
1204  5.7 Link errors when building libcurl on Windows
1205
1206  You need to make sure that your project, and all the libraries (both static
1207  and dynamic) that it links against, are compiled/linked against the same run
1208  time library.
1209
1210  This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
1211  options to the command line compiler. /MD (linking against MSVCRT dll) seems
1212  to be the most commonly used option.
1213
1214  When building an application that uses the static libcurl library, you must
1215  add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for
1216  dynamic import symbols. If you are using Visual Studio, you need to instead
1217  add CURL_STATICLIB in the "Preprocessor Definitions" section.
1218
1219  If you get a linker error like "unknown symbol __imp__curl_easy_init ..." you
1220  have linked against the wrong (static) library. If you want to use the
1221  libcurl.dll and import lib, you do not need any extra CFLAGS, but use one of
1222  the import libraries below. These are the libraries produced by the various
1223  lib/Makefile.* files:
1224
1225       Target:          static lib.   import lib for libcurl*.dll.
1226       -----------------------------------------------------------
1227       MinGW:           libcurl.a     libcurldll.a
1228       MSVC (release):  libcurl.lib   libcurl_imp.lib
1229       MSVC (debug):    libcurld.lib  libcurld_imp.lib
1230       Borland:         libcurl.lib   libcurl_imp.lib
1231
1232  5.8 libcurl.so.X: open failed: No such file or directory
1233
1234  This is an error message you might get when you try to run a program linked
1235  with a shared version of libcurl and your runtime linker (ld.so) could not
1236  find the shared library named libcurl.so.X. (Where X is the number of the
1237  current libcurl ABI, typically 3 or 4).
1238
1239  You need to make sure that ld.so finds libcurl.so.X. You can do that
1240  multiple ways, and it differs somewhat between different operating systems.
1241  They are usually:
1242
1243  * Add an option to the linker command line that specify the hard-coded path
1244    the runtime linker should check for the lib (usually -R)
1245
1246  * Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
1247    should check for libs
1248
1249  * Adjust the system's config to check for libs in the directory where you have
1250    put the library (like Linux's /etc/ld.so.conf)
1251
1252  'man ld.so' and 'man ld' will tell you more details
1253
1254  5.9 How does libcurl resolve hostnames?
1255
1256  libcurl supports a large number of name resolve functions. One of them is
1257  picked at build-time and will be used unconditionally. Thus, if you want to
1258  change name resolver function you must rebuild libcurl and tell it to use a
1259  different function.
1260
1261  - The non-IPv6 resolver that can use one of four different hostname resolve
1262  calls (depending on what your system supports):
1263
1264      A - gethostbyname()
1265      B - gethostbyname_r() with 3 arguments
1266      C - gethostbyname_r() with 5 arguments
1267      D - gethostbyname_r() with 6 arguments
1268
1269  - The IPv6-resolver that uses getaddrinfo()
1270
1271  - The c-ares based name resolver that uses the c-ares library for resolves.
1272    Using this offers asynchronous name resolves.
1273
1274  - The threaded resolver (default option on Windows). It uses:
1275
1276      A - gethostbyname() on plain IPv4 hosts
1277      B - getaddrinfo() on IPv6 enabled hosts
1278
1279  Also note that libcurl never resolves or reverse-lookups addresses given as
1280  pure numbers, such as 127.0.0.1 or ::1.
1281
1282  5.10 How do I prevent libcurl from writing the response to stdout?
1283
1284  libcurl provides a default built-in write function that writes received data
1285  to stdout. Set the CURLOPT_WRITEFUNCTION to receive the data, or possibly
1286  set CURLOPT_WRITEDATA to a different FILE * handle.
1287
1288  5.11 How do I make libcurl not receive the whole HTTP response?
1289
1290  You make the write callback (or progress callback) return an error and
1291  libcurl will then abort the transfer.
1292
1293  5.12 Can I make libcurl fake or hide my real IP address?
1294
1295  No. libcurl operates on a higher level. Besides, faking IP address would
1296  imply sending IP packets with a made-up source address, and then you normally
1297  get a problem with receiving the packet sent back as they would then not be
1298  routed to you.
1299
1300  If you use a proxy to access remote sites, the sites will not see your local
1301  IP address but instead the address of the proxy.
1302
1303  Also note that on many networks NATs or other IP-munging techniques are used
1304  that makes you see and use a different IP address locally than what the
1305  remote server will see you coming from. You may also consider using
1306  https://www.torproject.org/ .
1307
1308  5.13 How do I stop an ongoing transfer?
1309
1310  With the easy interface you make sure to return the correct error code from
1311  one of the callbacks, but none of them are instant. There is no function you
1312  can call from another thread or similar that will stop it immediately.
1313  Instead, you need to make sure that one of the callbacks you use returns an
1314  appropriate value that will stop the transfer. Suitable callbacks that you
1315  can do this with include the progress callback, the read callback and the
1316  write callback.
1317
1318  If you are using the multi interface, you can also stop a transfer by
1319  removing the particular easy handle from the multi stack at any moment you
1320  think the transfer is done or when you wish to abort the transfer.
1321
1322  5.14 Using C++ non-static functions for callbacks?
1323
1324  libcurl is a C library, it does not know anything about C++ member functions.
1325
1326  You can overcome this "limitation" with relative ease using a static
1327  member function that is passed a pointer to the class:
1328
1329     // f is the pointer to your object.
1330     static size_t YourClass::func(void *buffer, size_t sz, size_t n, void *f)
1331     {
1332       // Call non-static member function.
1333       static_cast<YourClass*>(f)->nonStaticFunction();
1334     }
1335
1336     // This is how you pass pointer to the static function:
1337     curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass::func);
1338     curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);
1339
1340  5.15 How do I get an FTP directory listing?
1341
1342  If you end the FTP URL you request with a slash, libcurl will provide you
1343  with a directory listing of that given directory. You can also set
1344  CURLOPT_CUSTOMREQUEST to alter what exact listing command libcurl would use
1345  to list the files.
1346
1347  The follow-up question tends to be how is a program supposed to parse the
1348  directory listing. How does it know what's a file and what's a directory and
1349  what's a symlink etc. If the FTP server supports the MLSD command then it
1350  will return data in a machine-readable format that can be parsed for type.
1351  The types are specified by RFC 3659 section 7.5.1. If MLSD is not supported
1352  then you have to work with what you are given. The LIST output format is
1353  entirely at the server's own liking and the NLST output does not reveal any
1354  types and in many cases does not even include all the directory entries.
1355  Also, both LIST and NLST tend to hide Unix-style hidden files (those that
1356  start with a dot) by default so you need to do "LIST -a" or similar to see
1357  them.
1358
1359  Example - List only directories.
1360  ftp.funet.fi supports MLSD and ftp.kernel.org does not:
1361
1362     curl -s ftp.funet.fi/pub/ -X MLSD | \
1363       perl -lne 'print if s/(?:^|;)type=dir;[^ ]+ (.+)$/$1/'
1364
1365     curl -s ftp.kernel.org/pub/linux/kernel/ | \
1366       perl -lne 'print if s/^d[-rwx]{9}(?: +[^ ]+){7} (.+)$/$1/'
1367
1368  If you need to parse LIST output in libcurl one such existing
1369  list parser is available at https://cr.yp.to/ftpparse.html  Versions of
1370  libcurl since 7.21.0 also provide the ability to specify a wildcard to
1371  download multiple files from one FTP directory.
1372
1373  5.16 I want a different time-out
1374
1375  Sometimes users realize that CURLOPT_TIMEOUT and CURLOPT_CONNECTIMEOUT are
1376  not sufficiently advanced or flexible to cover all the various use cases and
1377  scenarios applications end up with.
1378
1379  libcurl offers many more ways to time-out operations. A common alternative
1380  is to use the CURLOPT_LOW_SPEED_LIMIT and CURLOPT_LOW_SPEED_TIME options to
1381  specify the lowest possible speed to accept before to consider the transfer
1382  timed out.
1383
1384  The most flexible way is by writing your own time-out logic and using
1385  CURLOPT_XFERINFOFUNCTION (perhaps in combination with other callbacks) and
1386  use that to figure out exactly when the right condition is met when the
1387  transfer should get stopped.
1388
1389  5.17 Can I write a server with libcurl?
1390
1391  No. libcurl offers no functions or building blocks to build any kind of
1392  Internet protocol server. libcurl is only a client-side library. For server
1393  libraries, you need to continue your search elsewhere but there exist many
1394  good open source ones out there for most protocols you could want a server
1395  for. There are also really good stand-alone servers that have been tested
1396  and proven for many years. There is no need for you to reinvent them.
1397
1398  5.18 Does libcurl use threads?
1399
1400  Put simply: no, libcurl will execute in the same thread you call it in. All
1401  callbacks will be called in the same thread as the one you call libcurl in.
1402
1403  If you want to avoid your thread to be blocked by the libcurl call, you make
1404  sure you use the non-blocking multi API which will do transfers
1405  asynchronously - still in the same single thread.
1406
1407  libcurl will potentially internally use threads for name resolving, if it
1408  was built to work like that, but in those cases it will create the child
1409  threads by itself and they will only be used and then killed internally by
1410  libcurl and never exposed to the outside.
1411
14126. License Issues
1413
1414  curl and libcurl are released under a MIT/X derivative license. The license
1415  is liberal and should not impose a problem for your project. This section is
1416  just a brief summary for the cases we get the most questions. (Parts of this
1417  section was much enhanced by Bjorn Reese.)
1418
1419  We are not lawyers and this is not legal advice. You should probably consult
1420  one if you want true and accurate legal insights without our prejudice. Note
1421  especially that this section concerns the libcurl license only; compiling in
1422  features of libcurl that depend on other libraries (e.g. OpenSSL) may affect
1423  the licensing obligations of your application.
1424
1425  6.1 I have a GPL program, can I use the libcurl library?
1426
1427  Yes
1428
1429  Since libcurl may be distributed under the MIT/X derivative license, it can
1430  be used together with GPL in any software.
1431
1432  6.2 I have a closed-source program, can I use the libcurl library?
1433
1434  Yes
1435
1436  libcurl does not put any restrictions on the program that uses the library.
1437
1438  6.3 I have a BSD licensed program, can I use the libcurl library?
1439
1440  Yes
1441
1442  libcurl does not put any restrictions on the program that uses the library.
1443
1444  6.4 I have a program that uses LGPL libraries, can I use libcurl?
1445
1446  Yes
1447
1448  The LGPL license does not clash with other licenses.
1449
1450  6.5 Can I modify curl/libcurl for my program and keep the changes secret?
1451
1452  Yes
1453
1454  The MIT/X derivative license practically allows you to do almost anything
1455  with the sources, on the condition that the copyright texts in the sources
1456  are left intact.
1457
1458  6.6 Can you please change the curl/libcurl license to XXXX?
1459
1460  No.
1461
1462  We have carefully picked this license after years of development and
1463  discussions and a large amount of people have contributed with source code
1464  knowing that this is the license we use. This license puts the restrictions
1465  we want on curl/libcurl and it does not spread to other programs or
1466  libraries that use it. It should be possible for everyone to use libcurl or
1467  curl in their projects, no matter what license they already have in use.
1468
1469  6.7 What are my obligations when using libcurl in my commercial apps?
1470
1471  Next to none. All you need to adhere to is the MIT-style license (stated in
1472  the COPYING file) which basically says you have to include the copyright
1473  notice in "all copies" and that you may not use the copyright holder's name
1474  when promoting your software.
1475
1476  You do not have to release any of your source code.
1477
1478  You do not have to reveal or make public any changes to the libcurl source
1479  code.
1480
1481  You do not have to broadcast to the world that you are using libcurl within
1482  your app.
1483
1484  All we ask is that you disclose "the copyright notice and this permission
1485  notice" somewhere. Most probably like in the documentation or in the section
1486  where other third party dependencies already are mentioned and acknowledged.
1487
1488  As can be seen here: https://curl.se/docs/companies.html and elsewhere,
1489  more and more companies are discovering the power of libcurl and take
1490  advantage of it even in commercial environments.
1491
1492
14937. PHP/CURL Issues
1494
1495  7.1 What is PHP/CURL?
1496
1497  The module for PHP that makes it possible for PHP programs to access curl-
1498  functions from within PHP.
1499
1500  In the cURL project we call this module PHP/CURL to differentiate it from
1501  curl the command line tool and libcurl the library. The PHP team however
1502  does not refer to it like this (for unknown reasons). They call it plain
1503  CURL (often using all caps) or sometimes ext/curl, but both cause much
1504  confusion to users which in turn gives us a higher question load.
1505
1506  7.2 Who wrote PHP/CURL?
1507
1508  PHP/CURL was initially written by Sterling Hughes.
1509
1510  7.3 Can I perform multiple requests using the same handle?
1511
1512  Yes - at least in PHP version 4.3.8 and later (this has been known to not
1513  work in earlier versions, but the exact version when it started to work is
1514  unknown to me).
1515
1516  After a transfer, you just set new options in the handle and make another
1517  transfer. This will make libcurl reuse the same connection if it can.
1518
1519  7.4 Does PHP/CURL have dependencies?
1520
1521  PHP/CURL is a module that comes with the regular PHP package. It depends on
1522  and uses libcurl, so you need to have libcurl installed properly before
1523  PHP/CURL can be used.
1524
15258. Development
1526
1527 8.1 Why does curl use C89?
1528
1529 As with everything in curl, there is a history and we keep using what we have
1530 used before until someone brings up the subject and argues for and works on
1531 changing it.
1532
1533 We started out using C89 in the 1990s because that was the only way to write
1534 a truly portable C program and have it run as widely as possible. C89 was for
1535 a long time even necessary to make things work on otherwise considered modern
1536 platforms such as Windows. Today, we do not really know how many users that
1537 still require the use of a C89 compiler.
1538
1539 We will continue to use C89 for as long as nobody brings up a strong enough
1540 reason for us to change our minds. The core developers of the project do not
1541 feel restricted by this and we are not convinced that going C99 will offer us
1542 enough of a benefit to warrant the risk of cutting off a share of users.
1543
1544 8.2 Will curl be rewritten?
1545
1546 In one go: no. Little by little over time? Maybe.
1547
1548 Over the years, new languages and clever operating environments come and go.
1549 Every now and then the urge apparently arises to request that we rewrite curl
1550 in another language.
1551
1552 Some the most important properties in curl are maintaining the API and ABI
1553 for libcurl and keeping the behavior for the command line tool. As long as we
1554 can do that, everything else is up for discussion. To maintain the ABI, we
1555 probably have to maintain a certain amount of code in C, and to remain rock
1556 stable, we will never risk anything by rewriting a lot of things in one go.
1557 That said, we can certainly offer more and more optional backends written in
1558 other languages, as long as those backends can be plugged in at build-time.
1559 Backends can be written in any language, but should probably provide APIs
1560 usable from C to ease integration and transition.
1561