1 _ _ ____ _ 2 ___| | | | _ \| | 3 / __| | | | |_) | | 4 | (__| |_| | _ <| |___ 5 \___|\___/|_| \_\_____| 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 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 fails when using static libraries 28 2.2 Does curl work/build with other SSL libraries? 29 2.4 Does curl support SOCKS (RFC 1928) ? 30 31 3. Usage Problems 32 3.1 curl: (1) SSL is disabled, https: not supported 33 3.2 How do I tell curl to resume a transfer? 34 3.3 Why doesn't my posting using -F work? 35 3.4 How do I tell curl to run custom FTP commands? 36 3.5 How can I disable the Accept: */* header? 37 3.6 Does curl support ASP, XML, XHTML or HTML version Y? 38 3.7 Can I use curl to delete/rename a file through FTP? 39 3.8 How do I tell curl to follow HTTP redirects? 40 3.9 How do I use curl in my favorite programming language? 41 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP? 42 3.11 How do I POST with a different Content-Type? 43 3.12 Why do FTP-specific features over HTTP proxy fail? 44 3.13 Why do my single/double quotes fail? 45 3.14 Does curl support Javascript or PAC (automated proxy config)? 46 3.15 Can I do recursive fetches with curl? 47 3.16 What certificates do I need when I use SSL? 48 3.17 How do I list the root dir of an FTP server? 49 3.18 Can I use curl to send a POST/PUT and not wait for a response? 50 3.19 How do I get HTTP from a host using a specific IP address? 51 3.20 How to SFTP from my user's home directory? 52 3.21 Protocol xxx not supported or disabled in libcurl 53 3.22 curl -X gives me HTTP problems 54 55 4. Running Problems 56 4.2 Why do I get problems when I use & or % in the URL? 57 4.3 How can I use {, }, [ or ] to specify multiple URLs? 58 4.4 Why do I get downloaded data even though the web page doesn't exist? 59 4.5 Why do I get return code XXX from a HTTP server? 60 4.5.1 "400 Bad Request" 61 4.5.2 "401 Unauthorized" 62 4.5.3 "403 Forbidden" 63 4.5.4 "404 Not Found" 64 4.5.5 "405 Method Not Allowed" 65 4.5.6 "301 Moved Permanently" 66 4.6 Can you tell me what error code 142 means? 67 4.7 How do I keep user names and passwords secret in curl command lines? 68 4.8 I found a bug! 69 4.9 curl can't authenticate to the server that requires NTLM? 70 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work! 71 4.11 Why do my HTTP range requests return the full document? 72 4.12 Why do I get "certificate verify failed" ? 73 4.13 Why is curl -R on Windows one hour off? 74 4.14 Redirects work in browser but not with curl! 75 4.15 FTPS doesn't work 76 4.16 My HTTP POST or PUT requests are slow! 77 4.17 Non-functional connect timeouts on Windows 78 4.18 file:// URLs containing drive letters (Windows, NetWare) 79 4.19 Why doesn't curl return an error when the network cable is unplugged? 80 4.20 curl doesn't return error for HTTP non-200 responses! 81 82 5. libcurl Issues 83 5.1 Is libcurl thread-safe? 84 5.2 How can I receive all data into a large memory chunk? 85 5.3 How do I fetch multiple files with libcurl? 86 5.4 Does libcurl do Winsock initialization on win32 systems? 87 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ? 88 5.6 What about Keep-Alive or persistent connections? 89 5.7 Link errors when building libcurl on Windows! 90 5.8 libcurl.so.X: open failed: No such file or directory 91 5.9 How does libcurl resolve host names? 92 5.10 How do I prevent libcurl from writing the response to stdout? 93 5.11 How do I make libcurl not receive the whole HTTP response? 94 5.12 Can I make libcurl fake or hide my real IP address? 95 5.13 How do I stop an ongoing transfer? 96 5.14 Using C++ non-static functions for callbacks? 97 5.15 How do I get an FTP directory listing? 98 5.16 I want a different time-out! 99 5.17 Can I write a server with libcurl? 100 5.18 Does libcurl use threads? 101 102 6. License Issues 103 6.1 I have a GPL program, can I use the libcurl library? 104 6.2 I have a closed-source program, can I use the libcurl library? 105 6.3 I have a BSD licensed program, can I use the libcurl library? 106 6.4 I have a program that uses LGPL libraries, can I use libcurl? 107 6.5 Can I modify curl/libcurl for my program and keep the changes secret? 108 6.6 Can you please change the curl/libcurl license to XXXX? 109 6.7 What are my obligations when using libcurl in my commercial apps? 110 111 7. PHP/CURL Issues 112 7.1 What is PHP/CURL? 113 7.2 Who wrote PHP/CURL? 114 7.3 Can I perform multiple requests using the same handle? 115 7.4 Does PHP/CURL have dependencies? 116 117 8. Development 118 8.1 Why does curl use C89? 119 8.2 Will curl be rewritten? 120 121============================================================================== 122 1231. Philosophy 124 125 1.1 What is cURL? 126 127 cURL is the name of the project. The name is a play on 'Client for URLs', 128 originally with URL spelled in uppercase to make it obvious it deals with 129 URLs. The fact it can also be pronounced 'see URL' also helped, it works as 130 an abbreviation for "Client URL Request Library" or why not the recursive 131 version: "curl URL Request Library". 132 133 The cURL project produces two products: 134 135 libcurl 136 137 A free and easy-to-use client-side URL transfer library, supporting DICT, 138 FILE, FTP, FTPS, GOPHER, GOPHERS, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, 139 MQTT, POP3, POP3S, RTMP, RTMPS, RTSP, SCP, SFTP, SMB, SMBS, SMTP, SMTPS, 140 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 data 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 website mirroring program. If you want to use curl to mirror 192 something: fine, go ahead and write a script that wraps around curl or use 193 libcurl to make it reality. 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. We do this voluntarily, mostly in 250 our spare time. Companies may pay individual developers to work on curl, 251 but that's up to each company and developer. This is not controlled by nor 252 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 whom 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 risk 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 don't know how many users that knowingly have installed and use curl. 317 318 We don't know how many users that use curl without knowing that they are in 319 fact using it. 320 321 We don't 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 don't you update ca-bundle.crt 328 329 In the cURL project we've 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've not been ready to accept, and the one we can get from 332 Mozilla is perfectly fine so there's 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's a bunch of friendly people hanging out in the #curl channel on the 347 IRC network libera.chat. If you're 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 can't or won't deal with github, send your patch to 378 the curl-library mailing list. We're 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 don't 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 a very 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 very little attempts to actually figure this 415 out and you are instead encouraged to set LIBS and LDFLAGS accordingly when 416 you invoke configure, and point out the needed libraries and set the 417 necessary 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, GnuTLS, wolfSSL, NSS, mbedTLS, MesaLink, Secure 427 Transport (native iOS/OS X), Schannel (native Windows), GSKit (native IBM 428 i), BearSSL, or Rustls. They all have their pros and cons, and we try to 429 maintain a comparison of them here: https://curl.se/docs/ssl-compared.html 430 431 2.4 Does curl support SOCKS (RFC 1928) ? 432 433 Yes, SOCKS 4 and 5 are supported. 434 4353. Usage problems 436 437 3.1 curl: (1) SSL is disabled, https: not supported 438 439 If you get this output when trying to get anything from a https:// server, 440 it means that the instance of curl/libcurl that you're using was built 441 without support for this protocol. 442 443 This could've happened if the configure script that was run at build time 444 couldn't find all libs and include files curl requires for SSL to work. If 445 the configure script fails to find them, curl is simply built without SSL 446 support. 447 448 To get the https:// support into a curl that was previously built but that 449 reports that https:// is not supported, you should dig through the document 450 and logs and check out why the configure script doesn't find the SSL libs 451 and/or include files. 452 453 Also, check out the other paragraph in this FAQ labeled "configure doesn't 454 find OpenSSL even when it is installed". 455 456 3.2 How do I tell curl to resume a transfer? 457 458 curl supports resumed transfers both ways on both FTP and HTTP. 459 Try the -C option. 460 461 3.3 Why doesn't my posting using -F work? 462 463 You can't arbitrarily use -F or -d, the choice between -F or -d depends on 464 the HTTP operation you need curl to do and what the web server that will 465 receive your post expects. 466 467 If the form you're trying to submit uses the type 'multipart/form-data', 468 then and only then you must use the -F type. In all the most common cases, 469 you should use -d which then causes a posting with the type 470 'application/x-www-form-urlencoded'. 471 472 This is described in some detail in the MANUAL and TheArtOfHttpScripting 473 documents, and if you don't understand it the first time, read it again 474 before you post questions about this to the mailing list. Also, try reading 475 through the mailing list archives for old postings and questions regarding 476 this. 477 478 3.4 How do I tell curl to run custom FTP commands? 479 480 You can tell curl to perform optional commands both before and/or after a 481 file transfer. Study the -Q/--quote option. 482 483 Since curl is used for file transfers, you don't normally use curl to 484 perform FTP commands without transferring anything. Therefore you must 485 always specify a URL to transfer to/from even when doing custom FTP 486 commands, or use -I which implies the "no body" option sent to libcurl. 487 488 3.5 How can I disable the Accept: */* header? 489 490 You can change all internally generated headers by adding a replacement with 491 the -H/--header option. By adding a header with empty contents you safely 492 disable that one. Use -H "Accept:" to disable that specific header. 493 494 3.6 Does curl support ASP, XML, XHTML or HTML version Y? 495 496 To curl, all contents are alike. It doesn't matter how the page was 497 generated. It may be ASP, PHP, Perl, shell-script, SSI or plain HTML 498 files. There's no difference to curl and it doesn't even know what kind of 499 language that generated the page. 500 501 See also item 3.14 regarding javascript. 502 503 3.7 Can I use curl to delete/rename a file through FTP? 504 505 Yes. You specify custom FTP commands with -Q/--quote. 506 507 One example would be to delete a file after you have downloaded it: 508 509 curl -O ftp://download.com/coolfile -Q '-DELE coolfile' 510 511 or rename a file after upload: 512 513 curl -T infile ftp://upload.com/dir/ -Q "-RNFR infile" -Q "-RNTO newname" 514 515 3.8 How do I tell curl to follow HTTP redirects? 516 517 curl does not follow so-called redirects by default. The Location: header 518 that informs the client about this is only interpreted if you're using the 519 -L/--location option. As in: 520 521 curl -L http://redirector.com 522 523 Not all redirects are HTTP ones, see 4.14 524 525 3.9 How do I use curl in my favorite programming language? 526 527 Many programming languages have interfaces/bindings that allow you to use 528 curl without having to use the command line tool. If you are fluent in such 529 a language, you may prefer to use one of these interfaces instead. 530 531 Find out more about which languages that support curl directly, and how to 532 install and use them, in the libcurl section of the curl website: 533 https://curl.se/libcurl/ 534 535 All the various bindings to libcurl are made by other projects and people, 536 outside of the cURL project. The cURL project itself only produces libcurl 537 with its plain C API. If you don't find anywhere else to ask you can ask 538 about bindings on the curl-library list too, but be prepared that people on 539 that list may not know anything about bindings. 540 541 In February 2019, there were interfaces available for the following 542 languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Delphi, Dylan, Eiffel, 543 Euphoria, Falcon, Ferite, Gambas, glib/GTK+, Go, Guile, Harbour, Haskell, 544 Java, Julia, Lisp, Lua, Mono, .NET, node.js, Object-Pascal, OCaml, Pascal, 545 Perl, PHP, PostgreSQL, Python, R, Rexx, Ring, RPG, Ruby, Rust, Scheme, 546 Scilab, S-Lang, Smalltalk, SP-Forth, SPL, Tcl, Visual Basic, Visual FoxPro, 547 Q, wxwidgets, XBLite and Xoho. By the time you read this, additional ones 548 may have appeared! 549 550 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP? 551 552 curl adheres to the HTTP spec, which basically means you can play with *any* 553 protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and 554 XML-RPC are all such ones. You can use -X to set custom requests and -H to 555 set custom headers (or replace internally generated ones). 556 557 Using libcurl is of course just as good and you'd just use the proper 558 library options to do the same. 559 560 3.11 How do I POST with a different Content-Type? 561 562 You can always replace the internally generated headers with -H/--header. 563 To make a simple HTTP POST with text/xml as content-type, do something like: 564 565 curl -d "datatopost" -H "Content-Type: text/xml" [URL] 566 567 3.12 Why do FTP-specific features over HTTP proxy fail? 568 569 Because when you use a HTTP proxy, the protocol spoken on the network will 570 be HTTP, even if you specify a FTP URL. This effectively means that you 571 normally can't use FTP-specific features such as FTP upload and FTP quote 572 etc. 573 574 There is one exception to this rule, and that is if you can "tunnel through" 575 the given HTTP proxy. Proxy tunneling is enabled with a special option (-p) 576 and is generally not available as proxy admins usually disable tunneling to 577 ports other than 443 (which is used for HTTPS access through proxies). 578 579 3.13 Why do my single/double quotes fail? 580 581 To specify a command line option that includes spaces, you might need to 582 put the entire option within quotes. Like in: 583 584 curl -d " with spaces " url.com 585 586 or perhaps 587 588 curl -d ' with spaces ' url.com 589 590 Exactly what kind of quotes and how to do this is entirely up to the shell 591 or command line interpreter that you are using. For most unix shells, you 592 can more or less pick either single (') or double (") quotes. For 593 Windows/DOS prompts I believe you're forced to use double (") quotes. 594 595 Please study the documentation for your particular environment. Examples in 596 the curl docs will use a mix of both of these as shown above. You must 597 adjust them to work in your environment. 598 599 Remember that curl works and runs on more operating systems than most single 600 individuals have ever tried. 601 602 3.14 Does curl support Javascript or PAC (automated proxy config)? 603 604 Many web pages do magic stuff using embedded Javascript. curl and libcurl 605 have no built-in support for that, so it will be treated just like any other 606 contents. 607 608 .pac files are a netscape invention and are sometimes used by organizations 609 to allow them to differentiate which proxies to use. The .pac contents is 610 just a Javascript program that gets invoked by the browser and that returns 611 the name of the proxy to connect to. Since curl doesn't support Javascript, 612 it can't support .pac proxy configuration either. 613 614 Some workarounds usually suggested to overcome this Javascript dependency: 615 616 Depending on the Javascript complexity, write up a script that translates it 617 to another language and execute that. 618 619 Read the Javascript code and rewrite the same logic in another language. 620 621 Implement a Javascript interpreter, people have successfully used the 622 Mozilla Javascript engine in the past. 623 624 Ask your admins to stop this, for a static proxy setup or similar. 625 626 3.15 Can I do recursive fetches with curl? 627 628 No. curl itself has no code that performs recursive operations, such as 629 those performed by wget and similar tools. 630 631 There exists wrapper scripts with that functionality (for example the 632 curlmirror perl script), and you can write programs based on libcurl to do 633 it, but the command line tool curl itself cannot. 634 635 3.16 What certificates do I need when I use SSL? 636 637 There are three different kinds of "certificates" to keep track of when we 638 talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl. 639 640 CLIENT CERTIFICATE 641 642 The server you communicate with may require that you can provide this in 643 order to prove that you actually are who you claim to be. If the server 644 doesn't require this, you don't need a client certificate. 645 646 A client certificate is always used together with a private key, and the 647 private key has a pass phrase that protects it. 648 649 SERVER CERTIFICATE 650 651 The server you communicate with has a server certificate. You can and should 652 verify this certificate to make sure that you are truly talking to the real 653 server and not a server impersonating it. 654 655 CERTIFICATE AUTHORITY CERTIFICATE ("CA cert") 656 657 You often have several CA certs in a CA cert bundle that can be used to 658 verify a server certificate that was signed by one of the authorities in the 659 bundle. curl does not come with a CA cert bundle but most curl installs 660 provide one. You can also override the default. 661 662 The server certificate verification process is made by using a Certificate 663 Authority certificate ("CA cert") that was used to sign the server 664 certificate. Server certificate verification is enabled by default in curl 665 and libcurl and is often the reason for problems as explained in FAQ entry 666 4.12 and the SSLCERTS document 667 (https://curl.se/docs/sslcerts.html). Server certificates that are 668 "self-signed" or otherwise signed by a CA that you do not have a CA cert 669 for, cannot be verified. If the verification during a connect fails, you are 670 refused access. You then need to explicitly disable the verification to 671 connect to the server. 672 673 3.17 How do I list the root dir of an FTP server? 674 675 There are two ways. The way defined in the RFC is to use an encoded slash 676 in the first path part. List the "/tmp" dir like this: 677 678 curl ftp://ftp.sunet.se/%2ftmp/ 679 680 or the not-quite-kosher-but-more-readable way, by simply starting the path 681 section of the URL with a slash: 682 683 curl ftp://ftp.sunet.se//tmp/ 684 685 3.18 Can I use curl to send a POST/PUT and not wait for a response? 686 687 No. 688 689 But you could easily write your own program using libcurl to do such stunts. 690 691 3.19 How do I get HTTP from a host using a specific IP address? 692 693 For example, you may be trying out a website installation that isn't yet in 694 the DNS. Or you have a site using multiple IP addresses for a given host 695 name and you want to address a specific one out of the set. 696 697 Set a custom Host: header that identifies the server name you want to reach 698 but use the target IP address in the URL: 699 700 curl --header "Host: www.example.com" http://127.0.0.1/ 701 702 You can also opt to add faked host name entries to curl with the --resolve 703 option. That has the added benefit that things like redirects will also work 704 properly. The above operation would instead be done as: 705 706 curl --resolve www.example.com:80:127.0.0.1 http://www.example.com/ 707 708 3.20 How to SFTP from my user's home directory? 709 710 Contrary to how FTP works, SFTP and SCP URLs specify the exact directory to 711 work with. It means that if you don't specify that you want the user's home 712 directory, you get the actual root directory. 713 714 To specify a file in your user's home directory, you need to use the correct 715 URL syntax which for SFTP might look similar to: 716 717 curl -O -u user:password sftp://example.com/~/file.txt 718 719 and for SCP it is just a different protocol prefix: 720 721 curl -O -u user:password scp://example.com/~/file.txt 722 723 3.21 Protocol xxx not supported or disabled in libcurl 724 725 When passing on a URL to curl to use, it may respond that the particular 726 protocol is not supported or disabled. The particular way this error message 727 is phrased is because curl doesn't make a distinction internally of whether 728 a particular protocol is not supported (i.e. never got any code added that 729 knows how to speak that protocol) or if it was explicitly disabled. curl can 730 be built to only support a given set of protocols, and the rest would then 731 be disabled or not supported. 732 733 Note that this error will also occur if you pass a wrongly spelled protocol 734 part as in "htpt://example.com" or as in the less evident case if you prefix 735 the protocol part with a space as in " http://example.com/". 736 737 3.22 curl -X gives me HTTP problems 738 739 In normal circumstances, -X should hardly ever be used. 740 741 By default you use curl without explicitly saying which request method to 742 use when the URL identifies a HTTP transfer. If you just pass in a URL like 743 "curl http://example.com" it will use GET. If you use -d or -F curl will use 744 POST, -I will cause a HEAD and -T will make it a PUT. 745 746 If for whatever reason you're not happy with these default choices that curl 747 does for you, you can override those request methods by specifying -X 748 [WHATEVER]. This way you can for example send a DELETE by doing "curl -X 749 DELETE [URL]". 750 751 It is thus pointless to do "curl -XGET [URL]" as GET would be used 752 anyway. In the same vein it is pointless to do "curl -X POST -d data 753 [URL]"... But you can make a fun and somewhat rare request that sends a 754 request-body in a GET request with something like "curl -X GET -d data 755 [URL]" 756 757 Note that -X doesn't actually change curl's behavior as it only modifies the 758 actual string sent in the request, but that may of course trigger a 759 different set of events. 760 761 Accordingly, by using -XPOST on a command line that for example would follow 762 a 303 redirect, you will effectively prevent curl from behaving 763 correctly. Be aware. 764 765 7664. Running Problems 767 768 4.2 Why do I get problems when I use & or % in the URL? 769 770 In general unix shells, the & symbol is treated specially and when used, it 771 runs the specified command in the background. To safely send the & as a part 772 of a URL, you should quote the entire URL by using single (') or double (") 773 quotes around it. Similar problems can also occur on some shells with other 774 characters, including ?*!$~(){}<>\|;`. When in doubt, quote the URL. 775 776 An example that would invoke a remote CGI that uses &-symbols could be: 777 778 curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl' 779 780 In Windows, the standard DOS shell treats the percent sign specially and you 781 need to use TWO percent signs for each single one you want to use in the 782 URL. 783 784 If you want a literal percent sign to be part of the data you pass in a POST 785 using -d/--data you must encode it as '%25' (which then also needs the 786 percent sign doubled on Windows machines). 787 788 4.3 How can I use {, }, [ or ] to specify multiple URLs? 789 790 Because those letters have a special meaning to the shell, to be used in 791 a URL specified to curl you must quote them. 792 793 An example that downloads two URLs (sequentially) would be: 794 795 curl '{curl,www}.haxx.se' 796 797 To be able to use those characters as actual parts of the URL (without using 798 them for the curl URL "globbing" system), use the -g/--globoff option: 799 800 curl -g 'www.site.com/weirdname[].html' 801 802 4.4 Why do I get downloaded data even though the web page doesn't exist? 803 804 curl asks remote servers for the page you specify. If the page doesn't exist 805 at the server, the HTTP protocol defines how the server should respond and 806 that means that headers and a "page" will be returned. That's simply how 807 HTTP works. 808 809 By using the --fail option you can tell curl explicitly to not get any data 810 if the HTTP return code doesn't say success. 811 812 4.5 Why do I get return code XXX from a HTTP server? 813 814 RFC2616 clearly explains the return codes. This is a short transcript. Go 815 read the RFC for exact details: 816 817 4.5.1 "400 Bad Request" 818 819 The request could not be understood by the server due to malformed 820 syntax. The client SHOULD NOT repeat the request without modifications. 821 822 4.5.2 "401 Unauthorized" 823 824 The request requires user authentication. 825 826 4.5.3 "403 Forbidden" 827 828 The server understood the request, but is refusing to fulfill it. 829 Authorization will not help and the request SHOULD NOT be repeated. 830 831 4.5.4 "404 Not Found" 832 833 The server has not found anything matching the Request-URI. No indication 834 is given of whether the condition is temporary or permanent. 835 836 4.5.5 "405 Method Not Allowed" 837 838 The method specified in the Request-Line is not allowed for the resource 839 identified by the Request-URI. The response MUST include an Allow header 840 containing a list of valid methods for the requested resource. 841 842 4.5.6 "301 Moved Permanently" 843 844 If you get this return code and an HTML output similar to this: 845 846 <H1>Moved Permanently</H1> The document has moved <A 847 HREF="http://same_url_now_with_a_trailing_slash/">here</A>. 848 849 it might be because you requested a directory URL but without the trailing 850 slash. Try the same operation again _with_ the trailing URL, or use the 851 -L/--location option to follow the redirection. 852 853 4.6 Can you tell me what error code 142 means? 854 855 All curl error codes are described at the end of the man page, in the 856 section called "EXIT CODES". 857 858 Error codes that are larger than the highest documented error code means 859 that curl has exited due to a crash. This is a serious error, and we 860 appreciate a detailed bug report from you that describes how we could go 861 ahead and repeat this! 862 863 4.7 How do I keep user names and passwords secret in curl command lines? 864 865 This problem has two sides: 866 867 The first part is to avoid having clear-text passwords in the command line 868 so that they don't appear in 'ps' outputs and similar. That is easily 869 avoided by using the "-K" option to tell curl to read parameters from a file 870 or stdin to which you can pass the secret info. curl itself will also 871 attempt to "hide" the given password by blanking out the option - this 872 doesn't work on all platforms. 873 874 To keep the passwords in your account secret from the rest of the world is 875 not a task that curl addresses. You could of course encrypt them somehow to 876 at least hide them from being read by human eyes, but that is not what 877 anyone would call security. 878 879 Also note that regular HTTP (using Basic authentication) and FTP passwords 880 are sent as cleartext across the network. All it takes for anyone to fetch 881 them is to listen on the network. Eavesdropping is very easy. Use more secure 882 authentication methods (like Digest, Negotiate or even NTLM) or consider the 883 SSL-based alternatives HTTPS and FTPS. 884 885 4.8 I found a bug! 886 887 It is not a bug if the behavior is documented. Read the docs first. 888 Especially check out the KNOWN_BUGS file, it may be a documented bug! 889 890 If it is a problem with a binary you've downloaded or a package for your 891 particular platform, try contacting the person who built the package/archive 892 you have. 893 894 If there is a bug, read the BUGS document first. Then report it as described 895 in there. 896 897 4.9 curl can't authenticate to the server that requires NTLM? 898 899 NTLM support requires OpenSSL, GnuTLS, mbedTLS, NSS, Secure Transport, or 900 Microsoft Windows libraries at build-time to provide this functionality. 901 902 NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You 903 should not use such ones. 904 905 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work! 906 907 Many web servers allow or demand that the administrator configures the 908 server properly for these requests to work on the web server. 909 910 Some servers seem to support HEAD only on certain kinds of URLs. 911 912 To fully grasp this, try the documentation for the particular server 913 software you're trying to interact with. This is not anything curl can do 914 anything about. 915 916 4.11 Why do my HTTP range requests return the full document? 917 918 Because the range may not be supported by the server, or the server may 919 choose to ignore it and return the full document anyway. 920 921 4.12 Why do I get "certificate verify failed" ? 922 923 When you invoke curl and get an error 60 error back it means that curl 924 couldn't verify that the server's certificate was good. curl verifies the 925 certificate using the CA cert bundle and verifying for which names the 926 certificate has been granted. 927 928 To completely disable the certificate verification, use -k. This does 929 however enable man-in-the-middle attacks and makes the transfer INSECURE. 930 We strongly advice against doing this for more than experiments. 931 932 If you get this failure with a CA cert bundle installed and used, the 933 server's certificate might not be signed by one of the CA's in yout CA 934 store. It might for example be self-signed. You then correct this problem by 935 obtaining a valid CA cert for the server. Or again, decrease the security by 936 disabling this check. 937 938 At times, you find that the verification works in your favorite browser but 939 fails in curl. When this happens, the reason is usually that the server 940 sends an incomplete cert chain. The server is mandated to send all 941 "intermediate certificates" but doesn't. This typically works with browsers 942 anyway since they A) cache such certs and B) supports AIA which downloads 943 such missing certificates on demand. This is a server misconfiguration. A 944 good way to figure out if this is the case it to use the SSL Labs server 945 test and check the certificate chain: https://www.ssllabs.com/ssltest/ 946 947 Details are also in the SSLCERTS.md document, found online here: 948 https://curl.se/docs/sslcerts.html 949 950 4.13 Why is curl -R on Windows one hour off? 951 952 Since curl 7.53.0 this issue should be fixed as long as curl was built with 953 any modern compiler that allows for a 64-bit curl_off_t type. For older 954 compilers or prior curl versions it may set a time that appears one hour off. 955 This happens due to a flaw in how Windows stores and uses file modification 956 times and it is not easily worked around. For more details read this: 957 https://www.codeproject.com/Articles/1144/Beating-the-Daylight-Savings-Time-bug-and-getting 958 959 4.14 Redirects work in browser but not with curl! 960 961 curl supports HTTP redirects well (see item 3.8). Browsers generally support 962 at least two other ways to perform redirects that curl does not: 963 964 Meta tags. You can write a HTML tag that will cause the browser to redirect 965 to another given URL after a certain time. 966 967 Javascript. You can write a Javascript program embedded in a HTML page that 968 redirects the browser to another given URL. 969 970 There is no way to make curl follow these redirects. You must either 971 manually figure out what the page is set to do, or write a script that parses 972 the results and fetches the new URL. 973 974 4.15 FTPS doesn't work 975 976 curl supports FTPS (sometimes known as FTP-SSL) both implicit and explicit 977 mode. 978 979 When a URL is used that starts with FTPS://, curl assumes implicit SSL on 980 the control connection and will therefore immediately connect and try to 981 speak SSL. FTPS:// connections default to port 990. 982 983 To use explicit FTPS, you use a FTP:// URL and the --ftp-ssl option (or one 984 of its related flavors). This is the most common method, and the one 985 mandated by RFC4217. This kind of connection will then of course use the 986 standard FTP port 21 by default. 987 988 4.16 My HTTP POST or PUT requests are slow! 989 990 libcurl makes all POST and PUT requests (except for POST requests with a 991 very tiny request body) use the "Expect: 100-continue" header. This header 992 allows the server to deny the operation early so that libcurl can bail out 993 before having to send any data. This is useful in authentication 994 cases and others. 995 996 However, many servers don't implement the Expect: stuff properly and if the 997 server doesn't respond (positively) within 1 second libcurl will continue 998 and send off the data anyway. 999 1000 You can disable libcurl's use of the Expect: header the same way you disable 1001 any header, using -H / CURLOPT_HTTPHEADER, or by forcing it to use HTTP 1.0. 1002 1003 4.17 Non-functional connect timeouts 1004 1005 In most Windows setups having a timeout longer than 21 seconds make no 1006 difference, as it will only send 3 TCP SYN packets and no more. The second 1007 packet sent three seconds after the first and the third six seconds after 1008 the second. No more than three packets are sent, no matter how long the 1009 timeout is set. 1010 1011 See option TcpMaxConnectRetransmissions on this page: 1012 https://support.microsoft.com/en-us/kb/175523/en-us 1013 1014 Also, even on non-Windows systems there may run a firewall or anti-virus 1015 software or similar that accepts the connection but does not actually do 1016 anything else. This will make (lib)curl to consider the connection connected 1017 and thus the connect timeout won't trigger. 1018 1019 4.18 file:// URLs containing drive letters (Windows, NetWare) 1020 1021 When using curl to try to download a local file, one might use a URL 1022 in this format: 1023 1024 file://D:/blah.txt 1025 1026 You'll find that even if D:\blah.txt does exist, curl returns a 'file 1027 not found' error. 1028 1029 According to RFC 1738 (https://www.ietf.org/rfc/rfc1738.txt), 1030 file:// URLs must contain a host component, but it is ignored by 1031 most implementations. In the above example, 'D:' is treated as the 1032 host component, and is taken away. Thus, curl tries to open '/blah.txt'. 1033 If your system is installed to drive C:, that will resolve to 'C:\blah.txt', 1034 and if that doesn't exist you will get the not found error. 1035 1036 To fix this problem, use file:// URLs with *three* leading slashes: 1037 1038 file:///D:/blah.txt 1039 1040 Alternatively, if it makes more sense, specify 'localhost' as the host 1041 component: 1042 1043 file://localhost/D:/blah.txt 1044 1045 In either case, curl should now be looking for the correct file. 1046 1047 4.19 Why doesn't curl return an error when the network cable is unplugged? 1048 1049 Unplugging a cable is not an error situation. The TCP/IP protocol stack 1050 was designed to be fault tolerant, so even though there may be a physical 1051 break somewhere the connection shouldn't be affected, just possibly 1052 delayed. Eventually, the physical break will be fixed or the data will be 1053 re-routed around the physical problem through another path. 1054 1055 In such cases, the TCP/IP stack is responsible for detecting when the 1056 network connection is irrevocably lost. Since with some protocols it is 1057 perfectly legal for the client to wait indefinitely for data, the stack may 1058 never report a problem, and even when it does, it can take up to 20 minutes 1059 for it to detect an issue. The curl option --keepalive-time enables 1060 keep-alive support in the TCP/IP stack which makes it periodically probe the 1061 connection to make sure it is still available to send data. That should 1062 reliably detect any TCP/IP network failure. 1063 1064 But even that won't detect the network going down before the TCP/IP 1065 connection is established (e.g. during a DNS lookup) or using protocols that 1066 don't use TCP. To handle those situations, curl offers a number of timeouts 1067 on its own. --speed-limit/--speed-time will abort if the data transfer rate 1068 falls too low, and --connect-timeout and --max-time can be used to put an 1069 overall timeout on the connection phase or the entire transfer. 1070 1071 A libcurl-using application running in a known physical environment (e.g. 1072 an embedded device with only a single network connection) may want to act 1073 immediately if its lone network connection goes down. That can be achieved 1074 by having the application monitor the network connection on its own using an 1075 OS-specific mechanism, then signaling libcurl to abort (see also item 5.13). 1076 1077 4.20 curl doesn't return error for HTTP non-200 responses! 1078 1079 Correct. Unless you use -f (--fail). 1080 1081 When doing HTTP transfers, curl will perform exactly what you're asking it 1082 to do and if successful it will not return an error. You can use curl to 1083 test your web server's "file not found" page (that gets 404 back), you can 1084 use it to check your authentication protected web pages (that gets a 401 1085 back) and so on. 1086 1087 The specific HTTP response code does not constitute a problem or error for 1088 curl. It simply sends and delivers HTTP as you asked and if that worked, 1089 everything is fine and dandy. The response code is generally providing more 1090 higher level error information that curl doesn't care about. The error was 1091 not in the HTTP transfer. 1092 1093 If you want your command line to treat error codes in the 400 and up range 1094 as errors and thus return a non-zero value and possibly show an error 1095 message, curl has a dedicated option for that: -f (CURLOPT_FAILONERROR in 1096 libcurl speak). 1097 1098 You can also use the -w option and the variable %{response_code} to extract 1099 the exact response code that was returned in the response. 1100 11015. libcurl Issues 1102 1103 5.1 Is libcurl thread-safe? 1104 1105 Yes. 1106 1107 We have written the libcurl code specifically adjusted for multi-threaded 1108 programs. libcurl will use thread-safe functions instead of non-safe ones if 1109 your system has such. Note that you must never share the same handle in 1110 multiple threads. 1111 1112 There may be some exceptions to thread safety depending on how libcurl was 1113 built. Please review the guidelines for thread safety to learn more: 1114 https://curl.se/libcurl/c/threadsafe.html 1115 1116 5.2 How can I receive all data into a large memory chunk? 1117 1118 [ See also the examples/getinmemory.c source ] 1119 1120 You are in full control of the callback function that gets called every time 1121 there is data received from the remote server. You can make that callback do 1122 whatever you want. You do not have to write the received data to a file. 1123 1124 One solution to this problem could be to have a pointer to a struct that you 1125 pass to the callback function. You set the pointer using the 1126 CURLOPT_WRITEDATA option. Then that pointer will be passed to the callback 1127 instead of a FILE * to a file: 1128 1129 /* imaginary struct */ 1130 struct MemoryStruct { 1131 char *memory; 1132 size_t size; 1133 }; 1134 1135 /* imaginary callback function */ 1136 size_t 1137 WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data) 1138 { 1139 size_t realsize = size * nmemb; 1140 struct MemoryStruct *mem = (struct MemoryStruct *)data; 1141 1142 mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1); 1143 if (mem->memory) { 1144 memcpy(&(mem->memory[mem->size]), ptr, realsize); 1145 mem->size += realsize; 1146 mem->memory[mem->size] = 0; 1147 } 1148 return realsize; 1149 } 1150 1151 5.3 How do I fetch multiple files with libcurl? 1152 1153 libcurl has excellent support for transferring multiple files. You should 1154 just repeatedly set new URLs with curl_easy_setopt() and then transfer it 1155 with curl_easy_perform(). The handle you get from curl_easy_init() is not 1156 only reusable, but you're even encouraged to reuse it if you can, as that 1157 will enable libcurl to use persistent connections. 1158 1159 5.4 Does libcurl do Winsock initialization on win32 systems? 1160 1161 Yes, if told to in the curl_global_init() call. 1162 1163 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ? 1164 1165 Yes, but you cannot open a FILE * and pass the pointer to a DLL and have 1166 that DLL use the FILE * (as the DLL and the client application cannot access 1167 each others' variable memory areas). If you set CURLOPT_WRITEDATA you must 1168 also use CURLOPT_WRITEFUNCTION as well to set a function that writes the 1169 file, even if that simply writes the data to the specified FILE *. 1170 Similarly, if you use CURLOPT_READDATA you must also specify 1171 CURLOPT_READFUNCTION. 1172 1173 5.6 What about Keep-Alive or persistent connections? 1174 1175 curl and libcurl have excellent support for persistent connections when 1176 transferring several files from the same server. curl will attempt to reuse 1177 connections for all URLs specified on the same command line/config file, and 1178 libcurl will reuse connections for all transfers that are made using the 1179 same libcurl handle. 1180 1181 When you use the easy interface the connection cache is kept within the easy 1182 handle. If you instead use the multi interface, the connection cache will be 1183 kept within the multi handle and will be shared among all the easy handles 1184 that are used within the same multi handle. 1185 1186 5.7 Link errors when building libcurl on Windows! 1187 1188 You need to make sure that your project, and all the libraries (both static 1189 and dynamic) that it links against, are compiled/linked against the same run 1190 time library. 1191 1192 This is determined by the /MD, /ML, /MT (and their corresponding /M?d) 1193 options to the command line compiler. /MD (linking against MSVCRT dll) seems 1194 to be the most commonly used option. 1195 1196 When building an application that uses the static libcurl library, you must 1197 add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for 1198 dynamic import symbols. If you're using Visual Studio, you need to instead 1199 add CURL_STATICLIB in the "Preprocessor Definitions" section. 1200 1201 If you get linker error like "unknown symbol __imp__curl_easy_init ..." you 1202 have linked against the wrong (static) library. If you want to use the 1203 libcurl.dll and import lib, you don't need any extra CFLAGS, but use one of 1204 the import libraries below. These are the libraries produced by the various 1205 lib/Makefile.* files: 1206 1207 Target: static lib. import lib for libcurl*.dll. 1208 ----------------------------------------------------------- 1209 MingW: libcurl.a libcurldll.a 1210 MSVC (release): libcurl.lib libcurl_imp.lib 1211 MSVC (debug): libcurld.lib libcurld_imp.lib 1212 Borland: libcurl.lib libcurl_imp.lib 1213 1214 5.8 libcurl.so.X: open failed: No such file or directory 1215 1216 This is an error message you might get when you try to run a program linked 1217 with a shared version of libcurl and your run-time linker (ld.so) couldn't 1218 find the shared library named libcurl.so.X. (Where X is the number of the 1219 current libcurl ABI, typically 3 or 4). 1220 1221 You need to make sure that ld.so finds libcurl.so.X. You can do that 1222 multiple ways, and it differs somewhat between different operating systems, 1223 but they are usually: 1224 1225 * Add an option to the linker command line that specify the hard-coded path 1226 the run-time linker should check for the lib (usually -R) 1227 1228 * Set an environment variable (LD_LIBRARY_PATH for example) where ld.so 1229 should check for libs 1230 1231 * Adjust the system's config to check for libs in the directory where you've 1232 put the dir (like Linux's /etc/ld.so.conf) 1233 1234 'man ld.so' and 'man ld' will tell you more details 1235 1236 5.9 How does libcurl resolve host names? 1237 1238 libcurl supports a large a number of different name resolve functions. One 1239 of them is picked at build-time and will be used unconditionally. Thus, if 1240 you want to change name resolver function you must rebuild libcurl and tell 1241 it to use a different function. 1242 1243 - The non-IPv6 resolver that can use one of four different host name resolve 1244 calls (depending on what your system supports): 1245 1246 A - gethostbyname() 1247 B - gethostbyname_r() with 3 arguments 1248 C - gethostbyname_r() with 5 arguments 1249 D - gethostbyname_r() with 6 arguments 1250 1251 - The IPv6-resolver that uses getaddrinfo() 1252 1253 - The c-ares based name resolver that uses the c-ares library for resolves. 1254 Using this offers asynchronous name resolves. 1255 1256 - The threaded resolver (default option on Windows). It uses: 1257 1258 A - gethostbyname() on plain IPv4 hosts 1259 B - getaddrinfo() on IPv6 enabled hosts 1260 1261 Also note that libcurl never resolves or reverse-lookups addresses given as 1262 pure numbers, such as 127.0.0.1 or ::1. 1263 1264 5.10 How do I prevent libcurl from writing the response to stdout? 1265 1266 libcurl provides a default built-in write function that writes received data 1267 to stdout. Set the CURLOPT_WRITEFUNCTION to receive the data, or possibly 1268 set CURLOPT_WRITEDATA to a different FILE * handle. 1269 1270 5.11 How do I make libcurl not receive the whole HTTP response? 1271 1272 You make the write callback (or progress callback) return an error and 1273 libcurl will then abort the transfer. 1274 1275 5.12 Can I make libcurl fake or hide my real IP address? 1276 1277 No. libcurl operates on a higher level. Besides, faking IP address would 1278 imply sending IP packets with a made-up source address, and then you normally 1279 get a problem with receiving the packet sent back as they would then not be 1280 routed to you! 1281 1282 If you use a proxy to access remote sites, the sites will not see your local 1283 IP address but instead the address of the proxy. 1284 1285 Also note that on many networks NATs or other IP-munging techniques are used 1286 that makes you see and use a different IP address locally than what the 1287 remote server will see you coming from. You may also consider using 1288 https://www.torproject.org/ . 1289 1290 5.13 How do I stop an ongoing transfer? 1291 1292 With the easy interface you make sure to return the correct error code from 1293 one of the callbacks, but none of them are instant. There is no function you 1294 can call from another thread or similar that will stop it immediately. 1295 Instead, you need to make sure that one of the callbacks you use returns an 1296 appropriate value that will stop the transfer. Suitable callbacks that you 1297 can do this with include the progress callback, the read callback and the 1298 write callback. 1299 1300 If you're using the multi interface, you can also stop a transfer by 1301 removing the particular easy handle from the multi stack at any moment you 1302 think the transfer is done or when you wish to abort the transfer. 1303 1304 5.14 Using C++ non-static functions for callbacks? 1305 1306 libcurl is a C library, it doesn't know anything about C++ member functions. 1307 1308 You can overcome this "limitation" with relative ease using a static 1309 member function that is passed a pointer to the class: 1310 1311 // f is the pointer to your object. 1312 static size_t YourClass::func(void *buffer, size_t sz, size_t n, void *f) 1313 { 1314 // Call non-static member function. 1315 static_cast<YourClass*>(f)->nonStaticFunction(); 1316 } 1317 1318 // This is how you pass pointer to the static function: 1319 curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass::func); 1320 curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this); 1321 1322 5.15 How do I get an FTP directory listing? 1323 1324 If you end the FTP URL you request with a slash, libcurl will provide you 1325 with a directory listing of that given directory. You can also set 1326 CURLOPT_CUSTOMREQUEST to alter what exact listing command libcurl would use 1327 to list the files. 1328 1329 The follow-up question tends to be how is a program supposed to parse the 1330 directory listing. How does it know what's a file and what's a dir and what's 1331 a symlink etc. If the FTP server supports the MLSD command then it will 1332 return data in a machine-readable format that can be parsed for type. The 1333 types are specified by RFC3659 section 7.5.1. If MLSD is not supported then 1334 you have to work with what you're given. The LIST output format is entirely 1335 at the server's own liking and the NLST output doesn't reveal any types and 1336 in many cases doesn't even include all the directory entries. Also, both LIST 1337 and NLST tend to hide unix-style hidden files (those that start with a dot) 1338 by default so you need to do "LIST -a" or similar to see them. 1339 1340 Example - List only directories. 1341 ftp.funet.fi supports MLSD and ftp.kernel.org does not: 1342 1343 curl -s ftp.funet.fi/pub/ -X MLSD | \ 1344 perl -lne 'print if s/(?:^|;)type=dir;[^ ]+ (.+)$/$1/' 1345 1346 curl -s ftp.kernel.org/pub/linux/kernel/ | \ 1347 perl -lne 'print if s/^d[-rwx]{9}(?: +[^ ]+){7} (.+)$/$1/' 1348 1349 If you need to parse LIST output in libcurl one such existing 1350 list parser is available at https://cr.yp.to/ftpparse.html Versions of 1351 libcurl since 7.21.0 also provide the ability to specify a wildcard to 1352 download multiple files from one FTP directory. 1353 1354 5.16 I want a different time-out! 1355 1356 Time and time again users realize that CURLOPT_TIMEOUT and 1357 CURLOPT_CONNECTIMEOUT are not sufficiently advanced or flexible to cover all 1358 the various use cases and scenarios applications end up with. 1359 1360 libcurl offers many more ways to time-out operations. A common alternative 1361 is to use the CURLOPT_LOW_SPEED_LIMIT and CURLOPT_LOW_SPEED_TIME options to 1362 specify the lowest possible speed to accept before to consider the transfer 1363 timed out. 1364 1365 The most flexible way is by writing your own time-out logic and using 1366 CURLOPT_XFERINFOFUNCTION (perhaps in combination with other callbacks) and 1367 use that to figure out exactly when the right condition is met when the 1368 transfer should get stopped. 1369 1370 5.17 Can I write a server with libcurl? 1371 1372 No. libcurl offers no functions or building blocks to build any kind of 1373 internet protocol server. libcurl is only a client-side library. For server 1374 libraries, you need to continue your search elsewhere but there exist many 1375 good open source ones out there for most protocols you could possibly want a 1376 server for. And there are really good stand-alone ones that have been tested 1377 and proven for many years. There's no need for you to reinvent them! 1378 1379 5.18 Does libcurl use threads? 1380 1381 Put simply: no, libcurl will execute in the same thread you call it in. All 1382 callbacks will be called in the same thread as the one you call libcurl in. 1383 1384 If you want to avoid your thread to be blocked by the libcurl call, you make 1385 sure you use the non-blocking API which will do transfers asynchronously - 1386 but still in the same single thread. 1387 1388 libcurl will potentially internally use threads for name resolving, if it 1389 was built to work like that, but in those cases it'll create the child 1390 threads by itself and they will only be used and then killed internally by 1391 libcurl and never exposed to the outside. 1392 13936. License Issues 1394 1395 curl and libcurl are released under a MIT/X derivative license. The license is 1396 very liberal and should not impose a problem for your project. This section 1397 is just a brief summary for the cases we get the most questions. (Parts of 1398 this section was much enhanced by Bjorn Reese.) 1399 1400 We are not lawyers and this is not legal advice. You should probably consult 1401 one if you want true and accurate legal insights without our prejudice. Note 1402 especially that this section concerns the libcurl license only; compiling in 1403 features of libcurl that depend on other libraries (e.g. OpenSSL) may affect 1404 the licensing obligations of your application. 1405 1406 6.1 I have a GPL program, can I use the libcurl library? 1407 1408 Yes! 1409 1410 Since libcurl may be distributed under the MIT/X derivative license, it can be 1411 used together with GPL in any software. 1412 1413 6.2 I have a closed-source program, can I use the libcurl library? 1414 1415 Yes! 1416 1417 libcurl does not put any restrictions on the program that uses the library. 1418 1419 6.3 I have a BSD licensed program, can I use the libcurl library? 1420 1421 Yes! 1422 1423 libcurl does not put any restrictions on the program that uses the library. 1424 1425 6.4 I have a program that uses LGPL libraries, can I use libcurl? 1426 1427 Yes! 1428 1429 The LGPL license doesn't clash with other licenses. 1430 1431 6.5 Can I modify curl/libcurl for my program and keep the changes secret? 1432 1433 Yes! 1434 1435 The MIT/X derivative license practically allows you to do almost anything with 1436 the sources, on the condition that the copyright texts in the sources are 1437 left intact. 1438 1439 6.6 Can you please change the curl/libcurl license to XXXX? 1440 1441 No. 1442 1443 We have carefully picked this license after years of development and 1444 discussions and a large amount of people have contributed with source code 1445 knowing that this is the license we use. This license puts the restrictions 1446 we want on curl/libcurl and it does not spread to other programs or 1447 libraries that use it. It should be possible for everyone to use libcurl or 1448 curl in their projects, no matter what license they already have in use. 1449 1450 6.7 What are my obligations when using libcurl in my commercial apps? 1451 1452 Next to none. All you need to adhere to is the MIT-style license (stated in 1453 the COPYING file) which basically says you have to include the copyright 1454 notice in "all copies" and that you may not use the copyright holder's name 1455 when promoting your software. 1456 1457 You do not have to release any of your source code. 1458 1459 You do not have to reveal or make public any changes to the libcurl source 1460 code. 1461 1462 You do not have to broadcast to the world that you are using libcurl within 1463 your app. 1464 1465 All we ask is that you disclose "the copyright notice and this permission 1466 notice" somewhere. Most probably like in the documentation or in the section 1467 where other third party dependencies already are mentioned and acknowledged. 1468 1469 As can be seen here: https://curl.se/docs/companies.html and elsewhere, 1470 more and more companies are discovering the power of libcurl and take 1471 advantage of it even in commercial environments. 1472 1473 14747. PHP/CURL Issues 1475 1476 7.1 What is PHP/CURL? 1477 1478 The module for PHP that makes it possible for PHP programs to access curl- 1479 functions from within PHP. 1480 1481 In the cURL project we call this module PHP/CURL to differentiate it from 1482 curl the command line tool and libcurl the library. The PHP team however 1483 does not refer to it like this (for unknown reasons). They call it plain 1484 CURL (often using all caps) or sometimes ext/curl, but both cause much 1485 confusion to users which in turn gives us a higher question load. 1486 1487 7.2 Who wrote PHP/CURL? 1488 1489 PHP/CURL was initially written by Sterling Hughes. 1490 1491 7.3 Can I perform multiple requests using the same handle? 1492 1493 Yes - at least in PHP version 4.3.8 and later (this has been known to not 1494 work in earlier versions, but the exact version when it started to work is 1495 unknown to me). 1496 1497 After a transfer, you just set new options in the handle and make another 1498 transfer. This will make libcurl re-use the same connection if it can. 1499 1500 7.4 Does PHP/CURL have dependencies? 1501 1502 PHP/CURL is a module that comes with the regular PHP package. It depends on 1503 and uses libcurl, so you need to have libcurl installed properly before 1504 PHP/CURL can be used. 1505 15068. Development 1507 1508 8.1 Why does curl use C89? 1509 1510 As with everything in curl, there's a history and we keep using what we've 1511 used before until someone brings up the subject and argues for and works on 1512 changing it. 1513 1514 We started out using C89 in the 1990s because that was the only way to write 1515 a truly portable C program and have it run as widely as possible. C89 was for 1516 a long time even necessary to make things work on otherwise considered modern 1517 platforms such as Windows. Today, we don't really know how many users that 1518 still require the use of a C89 compiler. 1519 1520 We will continue to use C89 for as long as nobody brings up a strong enough 1521 reason for us to change our minds. The core developers of the project don't 1522 feel restricted by this and we are not convinced that going C99 will offer us 1523 enough of a benefit to warrant the risk of cutting off a share of users. 1524 1525 8.2 Will curl be rewritten? 1526 1527 In one go: no. Little by little over time? Maybe. 1528 1529 Over the years, new languages and clever operating environments come and go. 1530 Every now and then the urge apparently arises to request that we rewrite curl 1531 in another language. 1532 1533 Some the most important properties in curl are maintaining the API and ABI 1534 for libcurl and keeping the behavior for the command line tool. As long as we 1535 can do that, everything else is up for discussion. To maintain the ABI, we 1536 probably have to maintain a certain amount of code in C, and to remain rock 1537 stable, we will never risk anything by rewriting a lot of things in one go. 1538 That said, we can certainly offer more and more optional backends written in 1539 other languages, as long as those backends can be plugged in at build-time. 1540 Back-ends can be written in any language, but should probably provide APIs 1541 usable from C to ease integration and transition. 1542