1libcurl-tutorial(3) libcurl programming libcurl-tutorial(3)
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6 libcurl-tutorial - libcurl programming tutorial
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9 This document attempts to describe the general principles and some ba‐
10 sic approaches to consider when programming with libcurl. The text will
11 focus mainly on the C interface but might apply fairly well on other
12 interfaces as well as they usually follow the C one pretty closely.
13 This document will refer to 'the user' as the person writing the source
15 code that uses libcurl. That would probably be you or someone in your
16 position. What will be generally referred to as 'the program' will be
17 the collected source code that you write that is using libcurl for
18 transfers. The program is outside libcurl and libcurl is outside of the
19 program.
20 To get more details on all options and functions described herein,
22 please refer to their respective man pages.
23
26 There are many different ways to build C programs. This chapter will
27 assume a Unix style build process. If you use a different build system,
28 you can still read this to get general information that may apply to
29 your environment as well.
30 Compiling the Program
32 Your compiler needs to know where the libcurl headers are lo‐
33 cated. Therefore you must set your compiler's include path to
34 point to the directory where you installed them. The 'curl-con‐
35 fig'[3] tool can be used to get this information:
36 $ curl-config --cflags
38 Linking the Program with libcurl
41 When having compiled the program, you need to link your object
42 files to create a single executable. For that to succeed, you
43 need to link with libcurl and possibly also with other libraries
44 that libcurl itself depends on. Like the OpenSSL libraries, but
45 even some standard OS libraries may be needed on the command
46 line. To figure out which flags to use, once again the 'curl-
47 config' tool comes to the rescue:
48 $ curl-config --libs
50 SSL or Not
53 libcurl can be built and customized in many ways. One of the
54 things that varies from different libraries and builds is the
55 support for SSL-based transfers, like HTTPS and FTPS. If a sup‐
56 ported SSL library was detected properly at build-time, libcurl
57 will be built with SSL support. To figure out if an installed
58 libcurl has been built with SSL support enabled, use 'curl-con‐
59 fig' like this:
60 $ curl-config --feature
62 And if SSL is supported, the keyword 'SSL' will be written to
64 stdout, possibly together with a few other features that could
65 be either on or off on for different libcurls.
66 See also the "Features libcurl Provides" further down.
68 autoconf macro
70 When you write your configure script to detect libcurl and setup
71 variables accordingly, we offer a prewritten macro that probably
72 does everything you need in this area. See
73 docs/libcurl/libcurl.m4 file - it includes docs on how to use
74 it.
75
78 The people behind libcurl have put a considerable effort to make
79 libcurl work on a large amount of different operating systems and envi‐
80 ronments.
81 You program libcurl the same way on all platforms that libcurl runs on.
83 There are only a few minor details that differ. If you just make sure
84 to write your code portable enough, you can create a portable program.
85 libcurl should not stop you from that.
86
89 The program must initialize some of the libcurl functionality globally.
90 That means it should be done exactly once, no matter how many times you
91 intend to use the library. Once for your program's entire life time.
92 This is done using
93 curl_global_init()
95 and it takes one parameter which is a bit pattern that tells libcurl
97 what to initialize. Using CURL_GLOBAL_ALL will make it initialize all
98 known internal sub modules, and might be a good default option. The
99 current two bits that are specified are:
100 CURL_GLOBAL_WIN32
102 which only does anything on Windows machines. When used
103 on a Windows machine, it will make libcurl initialize the
104 win32 socket stuff. Without having that initialized prop‐
105 erly, your program cannot use sockets properly. You
106 should only do this once for each application, so if your
107 program already does this or of another library in use
108 does it, you should not tell libcurl to do this as well.
109 CURL_GLOBAL_SSL
111 which only does anything on libcurls compiled and built
112 SSL-enabled. On these systems, this will make libcurl
113 initialize the SSL library properly for this application.
114 This only needs to be done once for each application so
115 if your program or another library already does this,
116 this bit should not be needed.
117 libcurl has a default protection mechanism that detects if
119 curl_global_init(3) has not been called by the time curl_easy_per‐
120 form(3) is called and if that is the case, libcurl runs the function
121 itself with a guessed bit pattern. Please note that depending solely on
122 this is not considered nice nor good.
123 When the program no longer uses libcurl, it should call
125 curl_global_cleanup(3), which is the opposite of the init call. It will
126 then do the reversed operations to cleanup the resources the
127 curl_global_init(3) call initialized.
128 Repeated calls to curl_global_init(3) and curl_global_cleanup(3) should
130 be avoided. They should only be called once each.
131
134 It is considered best-practice to determine libcurl features at run-
135 time rather than at build-time (if possible of course). By calling
136 curl_version_info(3) and checking out the details of the returned
137 struct, your program can figure out exactly what the currently running
138 libcurl supports.
139
142 libcurl first introduced the so called easy interface. All operations
143 in the easy interface are prefixed with 'curl_easy'. The easy interface
144 lets you do single transfers with a synchronous and blocking function
145 call.
146 libcurl also offers another interface that allows multiple simultaneous
148 transfers in a single thread, the so called multi interface. More about
149 that interface is detailed in a separate chapter further down. You
150 still need to understand the easy interface first, so please continue
151 reading for better understanding.
152
154 To use the easy interface, you must first create yourself an easy han‐
155 dle. You need one handle for each easy session you want to perform. Ba‐
156 sically, you should use one handle for every thread you plan to use for
157 transferring. You must never share the same handle in multiple threads.
158 Get an easy handle with
160 easyhandle = curl_easy_init();
162 It returns an easy handle. Using that you proceed to the next step:
164 setting up your preferred actions. A handle is just a logic entity for
165 the upcoming transfer or series of transfers.
166 You set properties and options for this handle using curl_easy_se‐
168 topt(3). They control how the subsequent transfer or transfers will be
169 made. Options remain set in the handle until set again to something
170 different. They are sticky. Multiple requests using the same handle
171 will use the same options.
172 If you at any point would like to blank all previously set options for
174 a single easy handle, you can call curl_easy_reset(3) and you can also
175 make a clone of an easy handle (with all its set options) using
176 curl_easy_duphandle(3).
177 Many of the options you set in libcurl are "strings", pointers to data
179 terminated with a zero byte. When you set strings with curl_easy_se‐
180 topt(3), libcurl makes its own copy so that they do not need to be kept
181 around in your application after being set[4].
182 One of the most basic properties to set in the handle is the URL. You
184 set your preferred URL to transfer with CURLOPT_URL(3) in a manner sim‐
185 ilar to:
186 curl_easy_setopt(handle, CURLOPT_URL, "http://domain.com/");
188 Let's assume for a while that you want to receive data as the URL iden‐
190 tifies a remote resource you want to get here. Since you write a sort
191 of application that needs this transfer, I assume that you would like
192 to get the data passed to you directly instead of simply getting it
193 passed to stdout. So, you write your own function that matches this
194 prototype:
195 size_t write_data(void *buffer, size_t size, size_t nmemb, void
197 *userp);
198 You tell libcurl to pass all data to this function by issuing a func‐
200 tion similar to this:
201 curl_easy_setopt(easyhandle, CURLOPT_WRITEFUNCTION, write_data);
203 You can control what data your callback function gets in the fourth ar‐
205 gument by setting another property:
206 curl_easy_setopt(easyhandle, CURLOPT_WRITEDATA, &internal_struct);
208 Using that property, you can easily pass local data between your appli‐
210 cation and the function that gets invoked by libcurl. libcurl itself
211 will not touch the data you pass with CURLOPT_WRITEDATA(3).
212 libcurl offers its own default internal callback that will take care of
214 the data if you do not set the callback with CURLOPT_WRITEFUNCTION(3).
215 It will then simply output the received data to stdout. You can have
216 the default callback write the data to a different file handle by pass‐
217 ing a 'FILE *' to a file opened for writing with the CURLOPT_WRITE‐
218 DATA(3) option.
219 Now, we need to take a step back and have a deep breath. Here's one of
221 those rare platform-dependent nitpicks. Did you spot it? On some plat‐
222 forms[2], libcurl will not be able to operate on files opened by the
223 program. Thus, if you use the default callback and pass in an open file
224 with CURLOPT_WRITEDATA(3), it will crash. You should therefore avoid
225 this to make your program run fine virtually everywhere.
226 (CURLOPT_WRITEDATA(3) was formerly known as CURLOPT_FILE. Both names
228 still work and do the same thing).
229 If you are using libcurl as a win32 DLL, you MUST use the CUR‐
231 LOPT_WRITEFUNCTION(3) if you set CURLOPT_WRITEDATA(3) - or you will ex‐
232 perience crashes.
233 There are of course many more options you can set, and we will get back
235 to a few of them later. Let's instead continue to the actual transfer:
236 success = curl_easy_perform(easyhandle);
238 curl_easy_perform(3) will connect to the remote site, do the necessary
240 commands and receive the transfer. Whenever it receives data, it calls
241 the callback function we previously set. The function may get one byte
242 at a time, or it may get many kilobytes at once. libcurl delivers as
243 much as possible as often as possible. Your callback function should
244 return the number of bytes it "took care of". If that is not the same
245 amount of bytes that was passed to it, libcurl will abort the operation
246 and return with an error code.
247 When the transfer is complete, the function returns a return code that
249 informs you if it succeeded in its mission or not. If a return code is
250 not enough for you, you can use the CURLOPT_ERRORBUFFER(3) to point
251 libcurl to a buffer of yours where it will store a human readable error
252 message as well.
253 If you then want to transfer another file, the handle is ready to be
255 used again. Mind you, it is even preferred that you re-use an existing
256 handle if you intend to make another transfer. libcurl will then at‐
257 tempt to re-use the previous connection.
258 For some protocols, downloading a file can involve a complicated
260 process of logging in, setting the transfer mode, changing the current
261 directory and finally transferring the file data. libcurl takes care of
262 all that complication for you. Given simply the URL to a file, libcurl
263 will take care of all the details needed to get the file moved from one
264 machine to another.
265
268 libcurl is thread safe but there are a few exceptions. Refer to
269 libcurl-thread(3) for more information.
270
273 There will always be times when the transfer fails for some reason. You
274 might have set the wrong libcurl option or misunderstood what the
275 libcurl option actually does, or the remote server might return non-
276 standard replies that confuse the library which then confuses your pro‐
277 gram.
278 There's one golden rule when these things occur: set the CURLOPT_VER‐
280 BOSE(3) option to 1. it will cause the library to spew out the entire
281 protocol details it sends, some internal info and some received proto‐
282 col data as well (especially when using FTP). If you are using HTTP,
283 adding the headers in the received output to study is also a clever way
284 to get a better understanding why the server behaves the way it does.
285 Include headers in the normal body output with CURLOPT_HEADER(3) set 1.
286 Of course, there are bugs left. We need to know about them to be able
288 to fix them, so we are quite dependent on your bug reports. When you do
289 report suspected bugs in libcurl, please include as many details as you
290 possibly can: a protocol dump that CURLOPT_VERBOSE(3) produces, library
291 version, as much as possible of your code that uses libcurl, operating
292 system name and version, compiler name and version etc.
293 If CURLOPT_VERBOSE(3) is not enough, you increase the level of debug
295 data your application receive by using the CURLOPT_DEBUGFUNCTION(3).
296 Getting some in-depth knowledge about the protocols involved is never
298 wrong, and if you are trying to do funny things, you might understand
299 libcurl and how to use it better if you study the appropriate RFC docu‐
300 ments at least briefly.
301
304 libcurl tries to keep a protocol independent approach to most trans‐
305 fers, thus uploading to a remote FTP site is similar to uploading data
306 to an HTTP server with a PUT request.
307 Of course, first you either create an easy handle or you re-use one ex‐
309 isting one. Then you set the URL to operate on just like before. This
310 is the remote URL, that we now will upload.
311 Since we write an application, we most likely want libcurl to get the
313 upload data by asking us for it. To make it do that, we set the read
314 callback and the custom pointer libcurl will pass to our read callback.
315 The read callback should have a prototype similar to:
316 size_t function(char *bufptr, size_t size, size_t nitems, void
318 *userp);
319 Where bufptr is the pointer to a buffer we fill in with data to upload
321 and size*nitems is the size of the buffer and therefore also the maxi‐
322 mum amount of data we can return to libcurl in this call. The 'userp'
323 pointer is the custom pointer we set to point to a struct of ours to
324 pass private data between the application and the callback.
325 curl_easy_setopt(easyhandle, CURLOPT_READFUNCTION, read_function);
327 curl_easy_setopt(easyhandle, CURLOPT_READDATA, &filedata);
329 Tell libcurl that we want to upload:
331 curl_easy_setopt(easyhandle, CURLOPT_UPLOAD, 1L);
333 A few protocols will not behave properly when uploads are done without
335 any prior knowledge of the expected file size. So, set the upload file
336 size using the CURLOPT_INFILESIZE_LARGE(3) for all known file sizes
337 like this[1]:
338 /* in this example, file_size must be an curl_off_t variable */
340 curl_easy_setopt(easyhandle, CURLOPT_INFILESIZE_LARGE, file_size);
341 When you call curl_easy_perform(3) this time, it will perform all the
343 necessary operations and when it has invoked the upload it will call
344 your supplied callback to get the data to upload. The program should
345 return as much data as possible in every invoke, as that is likely to
346 make the upload perform as fast as possible. The callback should return
347 the number of bytes it wrote in the buffer. Returning 0 will signal the
348 end of the upload.
349
352 Many protocols use or even require that user name and password are pro‐
353 vided to be able to download or upload the data of your choice. libcurl
354 offers several ways to specify them.
355 Most protocols support that you specify the name and password in the
357 URL itself. libcurl will detect this and use them accordingly. This is
358 written like this:
359 protocol://user:password@example.com/path/
361 If you need any odd letters in your user name or password, you should
363 enter them URL encoded, as %XX where XX is a two-digit hexadecimal num‐
364 ber.
365 libcurl also provides options to set various passwords. The user name
367 and password as shown embedded in the URL can instead get set with the
368 CURLOPT_USERPWD(3) option. The argument passed to libcurl should be a
369 char * to a string in the format "user:password". In a manner like
370 this:
371 curl_easy_setopt(easyhandle, CURLOPT_USERPWD, "myname:thesecret");
373 Another case where name and password might be needed at times, is for
375 those users who need to authenticate themselves to a proxy they use.
376 libcurl offers another option for this, the CURLOPT_PROXYUSERPWD(3). It
377 is used quite similar to the CURLOPT_USERPWD(3) option like this:
378 curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "myname:these‐
380 cret");
381 There's a long time Unix "standard" way of storing FTP user names and
383 passwords, namely in the $HOME/.netrc file. The file should be made
384 private so that only the user may read it (see also the "Security Con‐
385 siderations" chapter), as it might contain the password in plain text.
386 libcurl has the ability to use this file to figure out what set of user
387 name and password to use for a particular host. As an extension to the
388 normal functionality, libcurl also supports this file for non-FTP pro‐
389 tocols such as HTTP. To make curl use this file, use the CUR‐
390 LOPT_NETRC(3) option:
391 curl_easy_setopt(easyhandle, CURLOPT_NETRC, 1L);
393 And a basic example of how such a .netrc file may look like:
395 machine myhost.mydomain.com
397 login userlogin
398 password secretword
399 All these examples have been cases where the password has been op‐
401 tional, or at least you could leave it out and have libcurl attempt to
402 do its job without it. There are times when the password is not op‐
403 tional, like when you are using an SSL private key for secure trans‐
404 fers.
405 To pass the known private key password to libcurl:
407 curl_easy_setopt(easyhandle, CURLOPT_KEYPASSWD, "keypassword");
409
412 The previous chapter showed how to set user name and password for get‐
413 ting URLs that require authentication. When using the HTTP protocol,
414 there are many different ways a client can provide those credentials to
415 the server and you can control which way libcurl will (attempt to) use
416 them. The default HTTP authentication method is called 'Basic', which
417 is sending the name and password in clear-text in the HTTP request,
418 base64-encoded. This is insecure.
419 At the time of this writing, libcurl can be built to use: Basic, Di‐
421 gest, NTLM, Negotiate (SPNEGO). You can tell libcurl which one to use
422 with CURLOPT_HTTPAUTH(3) as in:
423 curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH, CURLAUTH_DIGEST);
425 And when you send authentication to a proxy, you can also set authenti‐
427 cation type the same way but instead with CURLOPT_PROXYAUTH(3):
428 curl_easy_setopt(easyhandle, CURLOPT_PROXYAUTH, CURLAUTH_NTLM);
430 Both these options allow you to set multiple types (by ORing them to‐
432 gether), to make libcurl pick the most secure one out of the types the
433 server/proxy claims to support. This method does however add a round-
434 trip since libcurl must first ask the server what it supports:
435 curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH,
437 CURLAUTH_DIGEST|CURLAUTH_BASIC);
438 For convenience, you can use the 'CURLAUTH_ANY' define (instead of a
440 list with specific types) which allows libcurl to use whatever method
441 it wants.
442 When asking for multiple types, libcurl will pick the available one it
444 considers "best" in its own internal order of preference.
445
448 We get many questions regarding how to issue HTTP POSTs with libcurl
449 the proper way. This chapter will thus include examples using both dif‐
450 ferent versions of HTTP POST that libcurl supports.
451 The first version is the simple POST, the most common version, that
453 most HTML pages using the <form> tag uses. We provide a pointer to the
454 data and tell libcurl to post it all to the remote site:
455 char *data="name=daniel&project=curl";
457 curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, data);
458 curl_easy_setopt(easyhandle, CURLOPT_URL, "http://posthere.com/");
459 curl_easy_perform(easyhandle); /* post away! */
461 Simple enough, huh? Since you set the POST options with the CUR‐
463 LOPT_POSTFIELDS(3), this automatically switches the handle to use POST
464 in the upcoming request.
465 Ok, so what if you want to post binary data that also requires you to
467 set the Content-Type: header of the post? Well, binary posts prevent
468 libcurl from being able to do strlen() on the data to figure out the
469 size, so therefore we must tell libcurl the size of the post data. Set‐
470 ting headers in libcurl requests are done in a generic way, by building
471 a list of our own headers and then passing that list to libcurl.
472 struct curl_slist *headers=NULL;
474 headers = curl_slist_append(headers, "Content-Type: text/xml");
475 /* post binary data */
477 curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, binaryptr);
478 /* set the size of the postfields data */
480 curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDSIZE, 23L);
481 /* pass our list of custom made headers */
483 curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers);
484 curl_easy_perform(easyhandle); /* post away! */
486 curl_slist_free_all(headers); /* free the header list */
488 While the simple examples above cover the majority of all cases where
490 HTTP POST operations are required, they do not do multi-part formposts.
491 Multi-part formposts were introduced as a better way to post (possibly
492 large) binary data and were first documented in the RFC1867 (updated in
493 RFC2388). they are called multi-part because they are built by a chain
494 of parts, each part being a single unit of data. Each part has its own
495 name and contents. You can in fact create and post a multi-part form‐
496 post with the regular libcurl POST support described above, but that
497 would require that you build a formpost yourself and provide to
498 libcurl. To make that easier, libcurl provides a MIME API consisting in
499 several functions: using those, you can create and fill a multi-part
500 form. Function curl_mime_init(3) creates a multi-part body; you can
501 then append new parts to a multi-part body using curl_mime_addpart(3).
502 There are three possible data sources for a part: memory using
503 curl_mime_data(3), file using curl_mime_filedata(3) and user-defined
504 data read callback using curl_mime_data_cb(3). curl_mime_name(3) sets
505 a part's (i.e.: form field) name, while curl_mime_filename(3) fills in
506 the remote file name. With curl_mime_type(3), you can tell the MIME
507 type of a part, curl_mime_headers(3) allows defining the part's head‐
508 ers. When a multi-part body is no longer needed, you can destroy it us‐
509 ing curl_mime_free(3).
510 The following example sets two simple text parts with plain textual
512 contents, and then a file with binary contents and uploads the whole
513 thing.
514 curl_mime *multipart = curl_mime_init(easyhandle);
516 curl_mimepart *part = curl_mime_addpart(multipart);
517 curl_mime_name(part, "name");
518 curl_mime_data(part, "daniel", CURL_ZERO_TERMINATED);
519 part = curl_mime_addpart(multipart);
520 curl_mime_name(part, "project");
521 curl_mime_data(part, "curl", CURL_ZERO_TERMINATED);
522 part = curl_mime_addpart(multipart);
523 curl_mime_name(part, "logotype-image");
524 curl_mime_filedata(part, "curl.png");
525 /* Set the form info */
527 curl_easy_setopt(easyhandle, CURLOPT_MIMEPOST, multipart);
528 curl_easy_perform(easyhandle); /* post away! */
530 /* free the post data again */
532 curl_mime_free(multipart);
533 To post multiple files for a single form field, you must supply each
535 file in a separate part, all with the same field name. Although func‐
536 tion curl_mime_subparts(3) implements nested multi-parts, this way of
537 multiple files posting is deprecated by RFC 7578, chapter 4.3.
538 To set the data source from an already opened FILE pointer, use:
540 curl_mime_data_cb(part, filesize, (curl_read_callback) fread,
542 (curl_seek_callback) fseek, NULL, filepointer);
543 A deprecated curl_formadd(3) function is still supported in libcurl.
545 It should however not be used anymore for new designs and programs us‐
546 ing it ought to be converted to the MIME API. It is however described
547 here as an aid to conversion.
548 Using curl_formadd, you add parts to the form. When you are done adding
550 parts, you post the whole form.
551 The MIME API example above is expressed as follows using this function:
553 struct curl_httppost *post=NULL;
555 struct curl_httppost *last=NULL;
556 curl_formadd(&post, &last,
557 CURLFORM_COPYNAME, "name",
558 CURLFORM_COPYCONTENTS, "daniel", CURLFORM_END);
559 curl_formadd(&post, &last,
560 CURLFORM_COPYNAME, "project",
561 CURLFORM_COPYCONTENTS, "curl", CURLFORM_END);
562 curl_formadd(&post, &last,
563 CURLFORM_COPYNAME, "logotype-image",
564 CURLFORM_FILECONTENT, "curl.png", CURLFORM_END);
565 /* Set the form info */
567 curl_easy_setopt(easyhandle, CURLOPT_HTTPPOST, post);
568 curl_easy_perform(easyhandle); /* post away! */
570 /* free the post data again */
572 curl_formfree(post);
573 Multipart formposts are chains of parts using MIME-style separators and
575 headers. It means that each one of these separate parts get a few head‐
576 ers set that describe the individual content-type, size etc. To enable
577 your application to handicraft this formpost even more, libcurl allows
578 you to supply your own set of custom headers to such an individual form
579 part. You can of course supply headers to as many parts as you like,
580 but this little example will show how you set headers to one specific
581 part when you add that to the post handle:
582 struct curl_slist *headers=NULL;
584 headers = curl_slist_append(headers, "Content-Type: text/xml");
585 curl_formadd(&post, &last,
587 CURLFORM_COPYNAME, "logotype-image",
588 CURLFORM_FILECONTENT, "curl.xml",
589 CURLFORM_CONTENTHEADER, headers,
590 CURLFORM_END);
591 curl_easy_perform(easyhandle); /* post away! */
593 curl_formfree(post); /* free post */
595 curl_slist_free_all(headers); /* free custom header list */
596 Since all options on an easyhandle are "sticky", they remain the same
598 until changed even if you do call curl_easy_perform(3), you may need to
599 tell curl to go back to a plain GET request if you intend to do one as
600 your next request. You force an easyhandle to go back to GET by using
601 the CURLOPT_HTTPGET(3) option:
602 curl_easy_setopt(easyhandle, CURLOPT_HTTPGET, 1L);
604 Just setting CURLOPT_POSTFIELDS(3) to "" or NULL will *not* stop
606 libcurl from doing a POST. It will just make it POST without any data
607 to send!
608
611 Four rules have to be respected in building the multi-part:
612 - The easy handle must be created before building the multi-part.
613 - The multi-part is always created by a call to curl_mime_init(easyhan‐
614 dle).
615 - Each part is created by a call to curl_mime_addpart(multipart).
616 - When complete, the multi-part must be bound to the easy handle using
617 CURLOPT_MIMEPOST(3) instead of CURLOPT_HTTPPOST(3).
618 Here are some example of curl_formadd calls to MIME API sequences:
620 curl_formadd(&post, &last,
622 CURLFORM_COPYNAME, "id",
623 CURLFORM_COPYCONTENTS, "daniel", CURLFORM_END);
624 CURLFORM_CONTENTHEADER, headers,
625 CURLFORM_END);
626 becomes:
627 part = curl_mime_addpart(multipart);
628 curl_mime_name(part, "id");
629 curl_mime_data(part, "daniel", CURL_ZERO_TERMINATED);
630 curl_mime_headers(part, headers, FALSE);
631 Setting the last curl_mime_headers argument to TRUE would have caused
633 the headers to be automatically released upon destroyed the multi-part,
634 thus saving a clean-up call to curl_slist_free_all(3).
635 curl_formadd(&post, &last,
637 CURLFORM_PTRNAME, "logotype-image",
638 CURLFORM_FILECONTENT, "-",
639 CURLFORM_END);
640 becomes:
641 part = curl_mime_addpart(multipart);
642 curl_mime_name(part, "logotype-image");
643 curl_mime_data_cb(part, (curl_off_t) -1, fread, fseek, NULL, stdin);
644 curl_mime_name always copies the field name. The special file name "-"
646 is not supported by curl_mime_file: to read an open file, use a call‐
647 back source using fread(). The transfer will be chunked since the data
648 size is unknown.
649 curl_formadd(&post, &last,
651 CURLFORM_COPYNAME, "datafile[]",
652 CURLFORM_FILE, "file1",
653 CURLFORM_FILE, "file2",
654 CURLFORM_END);
655 becomes:
656 part = curl_mime_addpart(multipart);
657 curl_mime_name(part, "datafile[]");
658 curl_mime_filedata(part, "file1");
659 part = curl_mime_addpart(multipart);
660 curl_mime_name(part, "datafile[]");
661 curl_mime_filedata(part, "file2");
662 The deprecated multipart/mixed implementation of multiple files field
664 is translated to two distinct parts with the same name.
665 curl_easy_setopt(easyhandle, CURLOPT_READFUNCTION, myreadfunc);
667 curl_formadd(&post, &last,
668 CURLFORM_COPYNAME, "stream",
669 CURLFORM_STREAM, arg,
670 CURLFORM_CONTENTLEN, (curl_off_t) datasize,
671 CURLFORM_FILENAME, "archive.zip",
672 CURLFORM_CONTENTTYPE, "application/zip",
673 CURLFORM_END);
674 becomes:
675 part = curl_mime_addpart(multipart);
676 curl_mime_name(part, "stream");
677 curl_mime_data_cb(part, (curl_off_t) datasize,
678 myreadfunc, NULL, NULL, arg);
679 curl_mime_filename(part, "archive.zip");
680 curl_mime_type(part, "application/zip");
681 CURLOPT_READFUNCTION callback is not used: it is replace by directly
683 setting the part source data from the callback read function.
684 curl_formadd(&post, &last,
686 CURLFORM_COPYNAME, "memfile",
687 CURLFORM_BUFFER, "memfile.bin",
688 CURLFORM_BUFFERPTR, databuffer,
689 CURLFORM_BUFFERLENGTH, (long) sizeof databuffer,
690 CURLFORM_END);
691 becomes:
692 part = curl_mime_addpart(multipart);
693 curl_mime_name(part, "memfile");
694 curl_mime_data(part, databuffer, (curl_off_t) sizeof databuffer);
695 curl_mime_filename(part, "memfile.bin");
696 curl_mime_data always copies the initial data: data buffer is thus free
698 for immediate reuse.
699 curl_formadd(&post, &last,
701 CURLFORM_COPYNAME, "message",
702 CURLFORM_FILECONTENT, "msg.txt",
703 CURLFORM_END);
704 becomes:
705 part = curl_mime_addpart(multipart);
706 curl_mime_name(part, "message");
707 curl_mime_filedata(part, "msg.txt");
708 curl_mime_filename(part, NULL);
709 Use of curl_mime_filedata sets the remote file name as a side effect:
711 it is therefore necessary to clear it for CURLFORM_FILECONTENT emula‐
712 tion.
713
716 For historical and traditional reasons, libcurl has a built-in progress
717 meter that can be switched on and then makes it present a progress me‐
718 ter in your terminal.
719 Switch on the progress meter by, oddly enough, setting CURLOPT_NO‐
721 PROGRESS(3) to zero. This option is set to 1 by default.
722 For most applications however, the built-in progress meter is useless
724 and what instead is interesting is the ability to specify a progress
725 callback. The function pointer you pass to libcurl will then be called
726 on irregular intervals with information about the current transfer.
727 Set the progress callback by using CURLOPT_PROGRESSFUNCTION(3). And
729 pass a pointer to a function that matches this prototype:
730 int progress_callback(void *clientp,
732 double dltotal,
733 double dlnow,
734 double ultotal,
735 double ulnow);
736 If any of the input arguments is unknown, a 0 will be passed. The first
738 argument, the 'clientp' is the pointer you pass to libcurl with CUR‐
739 LOPT_PROGRESSDATA(3). libcurl will not touch it.
740
743 There's basically only one thing to keep in mind when using C++ instead
744 of C when interfacing libcurl:
745 The callbacks CANNOT be non-static class member functions
747 Example C++ code:
749 class AClass {
751 static size_t write_data(void *ptr, size_t size, size_t nmemb,
752 void *ourpointer)
753 {
754 /* do what you want with the data */
755 }
756 }
757
760 What "proxy" means according to Merriam-Webster: "a person authorized
761 to act for another" but also "the agency, function, or office of a
762 deputy who acts as a substitute for another".
763 Proxies are exceedingly common these days. Companies often only offer
765 Internet access to employees through their proxies. Network clients or
766 user-agents ask the proxy for documents, the proxy does the actual re‐
767 quest and then it returns them.
768 libcurl supports SOCKS and HTTP proxies. When a given URL is wanted,
770 libcurl will ask the proxy for it instead of trying to connect to the
771 actual host identified in the URL.
772 If you are using a SOCKS proxy, you may find that libcurl does not
774 quite support all operations through it.
775 For HTTP proxies: the fact that the proxy is an HTTP proxy puts certain
777 restrictions on what can actually happen. A requested URL that might
778 not be a HTTP URL will be still be passed to the HTTP proxy to deliver
779 back to libcurl. This happens transparently, and an application may not
780 need to know. I say "may", because at times it is important to under‐
781 stand that all operations over an HTTP proxy use the HTTP protocol. For
782 example, you cannot invoke your own custom FTP commands or even proper
783 FTP directory listings.
784 Proxy Options
787 To tell libcurl to use a proxy at a given port number:
789 curl_easy_setopt(easyhandle, CURLOPT_PROXY, "proxy-
791 host.com:8080");
792 Some proxies require user authentication before allowing a re‐
794 quest, and you pass that information similar to this:
795 curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "user:pass‐
797 word");
798 If you want to, you can specify the host name only in the CUR‐
800 LOPT_PROXY(3) option, and set the port number separately with
801 CURLOPT_PROXYPORT(3).
802 Tell libcurl what kind of proxy it is with CURLOPT_PROXYTYPE(3)
804 (if not, it will default to assume an HTTP proxy):
805 curl_easy_setopt(easyhandle, CURLOPT_PROXYTYPE, CURL‐
807 PROXY_SOCKS4);
808 Environment Variables
811 libcurl automatically checks and uses a set of environment vari‐
813 ables to know what proxies to use for certain protocols. The
814 names of the variables are following an ancient de facto stan‐
815 dard and are built up as "[protocol]_proxy" (note the lower cas‐
816 ing). Which makes the variable 'http_proxy' checked for a name
817 of a proxy to use when the input URL is HTTP. Following the same
818 rule, the variable named 'ftp_proxy' is checked for FTP URLs.
819 Again, the proxies are always HTTP proxies, the different names
820 of the variables simply allows different HTTP proxies to be
821 used.
822 The proxy environment variable contents should be in the format
824 "[protocol://][user:password@]machine[:port]". Where the proto‐
825 col:// part is simply ignored if present (so http://proxy and
826 bluerk://proxy will do the same) and the optional port number
827 specifies on which port the proxy operates on the host. If not
828 specified, the internal default port number will be used and
829 that is most likely *not* the one you would like it to be.
830 There are two special environment variables. 'all_proxy' is what
832 sets proxy for any URL in case the protocol specific variable
833 was not set, and 'no_proxy' defines a list of hosts that should
834 not use a proxy even though a variable may say so. If 'no_proxy'
835 is a plain asterisk ("*") it matches all hosts.
836 To explicitly disable libcurl's checking for and using the proxy
838 environment variables, set the proxy name to "" - an empty
839 string - with CURLOPT_PROXY(3).
840 SSL and Proxies
842 SSL is for secure point-to-point connections. This involves
844 strong encryption and similar things, which effectively makes it
845 impossible for a proxy to operate as a "man in between" which
846 the proxy's task is, as previously discussed. Instead, the only
847 way to have SSL work over an HTTP proxy is to ask the proxy to
848 tunnel trough everything without being able to check or fiddle
849 with the traffic.
850 Opening an SSL connection over an HTTP proxy is therefore a mat‐
852 ter of asking the proxy for a straight connection to the target
853 host on a specified port. This is made with the HTTP request
854 CONNECT. ("please mr proxy, connect me to that remote host").
855 Because of the nature of this operation, where the proxy has no
857 idea what kind of data that is passed in and out through this
858 tunnel, this breaks some of the few advantages that come from
859 using a proxy, such as caching. Many organizations prevent this
860 kind of tunneling to other destination port numbers than 443
861 (which is the default HTTPS port number).
862 Tunneling Through Proxy
865 As explained above, tunneling is required for SSL to work and
866 often even restricted to the operation intended for SSL; HTTPS.
867 This is however not the only time proxy-tunneling might offer
869 benefits to you or your application.
870 As tunneling opens a direct connection from your application to
872 the remote machine, it suddenly also re-introduces the ability
873 to do non-HTTP operations over an HTTP proxy. You can in fact
874 use things such as FTP upload or FTP custom commands this way.
875 Again, this is often prevented by the administrators of proxies
877 and is rarely allowed.
878 Tell libcurl to use proxy tunneling like this:
880 curl_easy_setopt(easyhandle, CURLOPT_HTTPPROXYTUNNEL, 1L);
882 In fact, there might even be times when you want to do plain
884 HTTP operations using a tunnel like this, as it then enables you
885 to operate on the remote server instead of asking the proxy to
886 do so. libcurl will not stand in the way for such innovative ac‐
887 tions either!
888 Proxy Auto-Config
891 Netscape first came up with this. It is basically a web page
893 (usually using a .pac extension) with a Javascript that when ex‐
894 ecuted by the browser with the requested URL as input, returns
895 information to the browser on how to connect to the URL. The re‐
896 turned information might be "DIRECT" (which means no proxy
897 should be used), "PROXY host:port" (to tell the browser where
898 the proxy for this particular URL is) or "SOCKS host:port" (to
899 direct the browser to a SOCKS proxy).
900 libcurl has no means to interpret or evaluate Javascript and
902 thus it does not support this. If you get yourself in a position
903 where you face this nasty invention, the following advice have
904 been mentioned and used in the past:
905 - Depending on the Javascript complexity, write up a script that
907 translates it to another language and execute that.
908 - Read the Javascript code and rewrite the same logic in another
910 language.
911 - Implement a Javascript interpreter; people have successfully
913 used the Mozilla Javascript engine in the past.
914 - Ask your admins to stop this, for a static proxy setup or sim‐
916 ilar.
917
920 Re-cycling the same easy handle several times when doing multiple re‐
921 quests is the way to go.
922 After each single curl_easy_perform(3) operation, libcurl will keep the
924 connection alive and open. A subsequent request using the same easy
925 handle to the same host might just be able to use the already open con‐
926 nection! This reduces network impact a lot.
927 Even if the connection is dropped, all connections involving SSL to the
929 same host again, will benefit from libcurl's session ID cache that
930 drastically reduces re-connection time.
931 FTP connections that are kept alive save a lot of time, as the command-
933 response round-trips are skipped, and also you do not risk getting
934 blocked without permission to login again like on many FTP servers only
935 allowing N persons to be logged in at the same time.
936 libcurl caches DNS name resolving results, to make lookups of a previ‐
938 ously looked up name a lot faster.
939 Other interesting details that improve performance for subsequent re‐
941 quests may also be added in the future.
942 Each easy handle will attempt to keep the last few connections alive
944 for a while in case they are to be used again. You can set the size of
945 this "cache" with the CURLOPT_MAXCONNECTS(3) option. Default is 5.
946 There is rarely any point in changing this value, and if you think of
947 changing this it is often just a matter of thinking again.
948 To force your upcoming request to not use an already existing connec‐
950 tion (it will even close one first if there happens to be one alive to
951 the same host you are about to operate on), you can do that by setting
952 CURLOPT_FRESH_CONNECT(3) to 1. In a similar spirit, you can also forbid
953 the upcoming request to be "lying" around and possibly get re-used af‐
954 ter the request by setting CURLOPT_FORBID_REUSE(3) to 1.
955
958 When you use libcurl to do HTTP requests, it will pass along a series
959 of headers automatically. It might be good for you to know and under‐
960 stand these. You can replace or remove them by using the CURLOPT_HTTP‐
961 HEADER(3) option.
962 Host This header is required by HTTP 1.1 and even many 1.0 servers
965 and should be the name of the server we want to talk to. This
966 includes the port number if anything but default.
967 Accept "*/*".
970 Expect When doing POST requests, libcurl sets this header to "100-con‐
973 tinue" to ask the server for an "OK" message before it proceeds
974 with sending the data part of the post. If the POSTed data
975 amount is deemed "small", libcurl will not use this header.
976
979 There is an ongoing development today where more and more protocols are
980 built upon HTTP for transport. This has obvious benefits as HTTP is a
981 tested and reliable protocol that is widely deployed and has excellent
982 proxy-support.
983 When you use one of these protocols, and even when doing other kinds of
985 programming you may need to change the traditional HTTP (or FTP or...)
986 manners. You may need to change words, headers or various data.
987 libcurl is your friend here too.
989 CUSTOMREQUEST
992 If just changing the actual HTTP request keyword is what you
993 want, like when GET, HEAD or POST is not good enough for you,
994 CURLOPT_CUSTOMREQUEST(3) is there for you. It is simple to use:
995 curl_easy_setopt(easyhandle, CURLOPT_CUSTOMREQUEST, "MYOWNRE‐
997 QUEST");
998 When using the custom request, you change the request keyword of
1000 the actual request you are performing. Thus, by default you make
1001 a GET request but you can also make a POST operation (as de‐
1002 scribed before) and then replace the POST keyword if you want
1003 to. you are the boss.
1004 Modify Headers
1007 HTTP-like protocols pass a series of headers to the server when
1008 doing the request, and you are free to pass any amount of extra
1009 headers that you think fit. Adding headers is this easy:
1010 struct curl_slist *headers=NULL; /* init to NULL is important */
1012 headers = curl_slist_append(headers, "Hey-server-hey: how are you?");
1014 headers = curl_slist_append(headers, "X-silly-content: yes");
1015 /* pass our list of custom made headers */
1017 curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers);
1018 curl_easy_perform(easyhandle); /* transfer http */
1020 curl_slist_free_all(headers); /* free the header list */
1022 ... and if you think some of the internally generated headers,
1024 such as Accept: or Host: do not contain the data you want them
1025 to contain, you can replace them by simply setting them too:
1026 headers = curl_slist_append(headers, "Accept: Agent-007");
1028 headers = curl_slist_append(headers, "Host: munged.host.line");
1029 Delete Headers
1032 If you replace an existing header with one with no contents, you
1033 will prevent the header from being sent. For instance, if you
1034 want to completely prevent the "Accept:" header from being sent,
1035 you can disable it with code similar to this:
1036 headers = curl_slist_append(headers, "Accept:");
1038 Both replacing and canceling internal headers should be done
1040 with careful consideration and you should be aware that you may
1041 violate the HTTP protocol when doing so.
1042 Enforcing chunked transfer-encoding
1045 By making sure a request uses the custom header "Transfer-Encod‐
1047 ing: chunked" when doing a non-GET HTTP operation, libcurl will
1048 switch over to "chunked" upload, even though the size of the
1049 data to upload might be known. By default, libcurl usually
1050 switches over to chunked upload automatically if the upload data
1051 size is unknown.
1052 HTTP Version
1055 All HTTP requests includes the version number to tell the server
1057 which version we support. libcurl speaks HTTP 1.1 by default.
1058 Some old servers do not like getting 1.1-requests and when deal‐
1059 ing with stubborn old things like that, you can tell libcurl to
1060 use 1.0 instead by doing something like this:
1061 curl_easy_setopt(easyhandle, CURLOPT_HTTP_VERSION,
1063 CURL_HTTP_VERSION_1_0);
1064 FTP Custom Commands
1067 Not all protocols are HTTP-like, and thus the above may not help
1069 you when you want to make, for example, your FTP transfers to
1070 behave differently.
1071 Sending custom commands to an FTP server means that you need to
1073 send the commands exactly as the FTP server expects them (RFC959
1074 is a good guide here), and you can only use commands that work
1075 on the control-connection alone. All kinds of commands that re‐
1076 quire data interchange and thus need a data-connection must be
1077 left to libcurl's own judgement. Also be aware that libcurl will
1078 do its best to change directory to the target directory before
1079 doing any transfer, so if you change directory (with CWD or sim‐
1080 ilar) you might confuse libcurl and then it might not attempt to
1081 transfer the file in the correct remote directory.
1082 A little example that deletes a given file before an operation:
1084 headers = curl_slist_append(headers, "DELE file-to-remove");
1086 /* pass the list of custom commands to the handle */
1088 curl_easy_setopt(easyhandle, CURLOPT_QUOTE, headers);
1089 curl_easy_perform(easyhandle); /* transfer ftp data! */
1091 curl_slist_free_all(headers); /* free the header list */
1093 If you would instead want this operation (or chain of opera‐
1095 tions) to happen _after_ the data transfer took place the option
1096 to curl_easy_setopt(3) would instead be called CUR‐
1097 LOPT_POSTQUOTE(3) and used the exact same way.
1098 The custom FTP command will be issued to the server in the same
1100 order they are added to the list, and if a command gets an error
1101 code returned back from the server, no more commands will be is‐
1102 sued and libcurl will bail out with an error code
1103 (CURLE_QUOTE_ERROR). Note that if you use CURLOPT_QUOTE(3) to
1104 send commands before a transfer, no transfer will actually take
1105 place when a quote command has failed.
1106 If you set the CURLOPT_HEADER(3) to 1, you will tell libcurl to
1108 get information about the target file and output "headers" about
1109 it. The headers will be in "HTTP-style", looking like they do in
1110 HTTP.
1111 The option to enable headers or to run custom FTP commands may
1113 be useful to combine with CURLOPT_NOBODY(3). If this option is
1114 set, no actual file content transfer will be performed.
1115 FTP Custom CUSTOMREQUEST
1118 If you do want to list the contents of an FTP directory using
1119 your own defined FTP command, CURLOPT_CUSTOMREQUEST(3) will do
1120 just that. "NLST" is the default one for listing directories but
1121 you are free to pass in your idea of a good alternative.
1122
1125 In the HTTP sense, a cookie is a name with an associated value. A
1126 server sends the name and value to the client, and expects it to get
1127 sent back on every subsequent request to the server that matches the
1128 particular conditions set. The conditions include that the domain name
1129 and path match and that the cookie has not become too old.
1130 In real-world cases, servers send new cookies to replace existing ones
1132 to update them. Server use cookies to "track" users and to keep "ses‐
1133 sions".
1134 Cookies are sent from server to clients with the header Set-Cookie: and
1136 they are sent from clients to servers with the Cookie: header.
1137 To just send whatever cookie you want to a server, you can use CUR‐
1139 LOPT_COOKIE(3) to set a cookie string like this:
1140 curl_easy_setopt(easyhandle, CURLOPT_COOKIE, "name1=var1;
1142 name2=var2;");
1143 In many cases, that is not enough. You might want to dynamically save
1145 whatever cookies the remote server passes to you, and make sure those
1146 cookies are then used accordingly on later requests.
1147 One way to do this, is to save all headers you receive in a plain file
1149 and when you make a request, you tell libcurl to read the previous
1150 headers to figure out which cookies to use. Set the header file to read
1151 cookies from with CURLOPT_COOKIEFILE(3).
1152 The CURLOPT_COOKIEFILE(3) option also automatically enables the cookie
1154 parser in libcurl. Until the cookie parser is enabled, libcurl will not
1155 parse or understand incoming cookies and they will just be ignored.
1156 However, when the parser is enabled the cookies will be understood and
1157 the cookies will be kept in memory and used properly in subsequent re‐
1158 quests when the same handle is used. Many times this is enough, and you
1159 may not have to save the cookies to disk at all. Note that the file you
1160 specify to CURLOPT_COOKIEFILE(3) does not have to exist to enable the
1161 parser, so a common way to just enable the parser and not read any
1162 cookies is to use the name of a file you know does not exist.
1163 If you would rather use existing cookies that you have previously re‐
1165 ceived with your Netscape or Mozilla browsers, you can make libcurl use
1166 that cookie file as input. The CURLOPT_COOKIEFILE(3) is used for that
1167 too, as libcurl will automatically find out what kind of file it is and
1168 act accordingly.
1169 Perhaps the most advanced cookie operation libcurl offers, is saving
1171 the entire internal cookie state back into a Netscape/Mozilla formatted
1172 cookie file. We call that the cookie-jar. When you set a file name with
1173 CURLOPT_COOKIEJAR(3), that file name will be created and all received
1174 cookies will be stored in it when curl_easy_cleanup(3) is called. This
1175 enables cookies to get passed on properly between multiple handles
1176 without any information getting lost.
1177
1180 FTP transfers use a second TCP/IP connection for the data transfer.
1181 This is usually a fact you can forget and ignore but at times this fact
1182 will come back to haunt you. libcurl offers several different ways to
1183 customize how the second connection is being made.
1184 libcurl can either connect to the server a second time or tell the
1186 server to connect back to it. The first option is the default and it is
1187 also what works best for all the people behind firewalls, NATs or IP-
1188 masquerading setups. libcurl then tells the server to open up a new
1189 port and wait for a second connection. This is by default attempted
1190 with EPSV first, and if that does not work it tries PASV instead. (EPSV
1191 is an extension to the original FTP spec and does not exist nor work on
1192 all FTP servers.)
1193 You can prevent libcurl from first trying the EPSV command by setting
1195 CURLOPT_FTP_USE_EPSV(3) to zero.
1196 In some cases, you will prefer to have the server connect back to you
1198 for the second connection. This might be when the server is perhaps be‐
1199 hind a firewall or something and only allows connections on a single
1200 port. libcurl then informs the remote server which IP address and port
1201 number to connect to. This is made with the CURLOPT_FTPPORT(3) option.
1202 If you set it to "-", libcurl will use your system's "default IP ad‐
1203 dress". If you want to use a particular IP, you can set the full IP ad‐
1204 dress, a host name to resolve to an IP address or even a local network
1205 interface name that libcurl will get the IP address from.
1206 When doing the "PORT" approach, libcurl will attempt to use the EPRT
1208 and the LPRT before trying PORT, as they work with more protocols. You
1209 can disable this behavior by setting CURLOPT_FTP_USE_EPRT(3) to zero.
1210
1213 In addition to support HTTP multi-part form fields, the MIME API can be
1214 used to build structured email messages and send them via SMTP or ap‐
1215 pend such messages to IMAP directories.
1216 A structured email message may contain several parts: some are dis‐
1218 played inline by the MUA, some are attachments. Parts can also be
1219 structured as multi-part, for example to include another email message
1220 or to offer several text formats alternatives. This can be nested to
1221 any level.
1222 To build such a message, you prepare the nth-level multi-part and then
1224 include it as a source to the parent multi-part using function
1225 curl_mime_subparts(3). Once it has been bound to its parent multi-part,
1226 a nth-level multi-part belongs to it and should not be freed explic‐
1227 itly.
1228 Email messages data is not supposed to be non-ascii and line length is
1230 limited: fortunately, some transfer encodings are defined by the stan‐
1231 dards to support the transmission of such incompatible data. Function
1232 curl_mime_encoder(3) tells a part that its source data must be encoded
1233 before being sent. It also generates the corresponding header for that
1234 part. If the part data you want to send is already encoded in such a
1235 scheme, do not use this function (this would over-encode it), but ex‐
1236 plicitly set the corresponding part header.
1237 Upon sending such a message, libcurl prepends it with the header list
1239 set with CURLOPT_HTTPHEADER(3), as 0th-level mime part headers.
1240 Here is an example building an email message with an inline plain/html
1242 text alternative and a file attachment encoded in base64:
1243 curl_mime *message = curl_mime_init(easyhandle);
1245 /* The inline part is an alternative proposing the html and the text
1247 versions of the email. */
1248 curl_mime *alt = curl_mime_init(easyhandle);
1249 /* HTML message. */
1251 curl_mimepart *part = curl_mime_addpart(alt);
1252 curl_mime_data(part, "<html><body><p>This is HTML</p></body></html>",
1253 CURL_ZERO_TERMINATED);
1254 curl_mime_type(part, "text/html");
1255 /* Text message. */
1257 part = curl_mime_addpart(alt);
1258 curl_mime_data(part, "This is plain text message",
1259 CURL_ZERO_TERMINATED);
1260 /* Create the inline part. */
1262 part = curl_mime_addpart(message);
1263 curl_mime_subparts(part, alt);
1264 curl_mime_type(part, "multipart/alternative");
1265 struct curl_slist *headers = curl_slist_append(NULL,
1266 "Content-Disposition: inline");
1267 curl_mime_headers(part, headers, TRUE);
1268 /* Add the attachment. */
1270 part = curl_mime_addpart(message);
1271 curl_mime_filedata(part, "manual.pdf");
1272 curl_mime_encoder(part, "base64");
1273 /* Build the mail headers. */
1275 headers = curl_slist_append(NULL, "From: me@example.com");
1276 headers = curl_slist_append(headers, "To: you@example.com");
1277 /* Set these into the easy handle. */
1279 curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers);
1280 curl_easy_setopt(easyhandle, CURLOPT_MIMEPOST, mime);
1281 It should be noted that appending a message to an IMAP directory re‐
1283 quires the message size to be known prior upload. It is therefore not
1284 possible to include parts with unknown data size in this context.
1285
1288 Some protocols provide "headers", meta-data separated from the normal
1289 data. These headers are by default not included in the normal data
1290 stream, but you can make them appear in the data stream by setting CUR‐
1291 LOPT_HEADER(3) to 1.
1292 What might be even more useful, is libcurl's ability to separate the
1294 headers from the data and thus make the callbacks differ. You can for
1295 example set a different pointer to pass to the ordinary write callback
1296 by setting CURLOPT_HEADERDATA(3).
1297 Or, you can set an entirely separate function to receive the headers,
1299 by using CURLOPT_HEADERFUNCTION(3).
1300 The headers are passed to the callback function one by one, and you can
1302 depend on that fact. It makes it easier for you to add custom header
1303 parsers etc.
1304 "Headers" for FTP transfers equal all the FTP server responses. They
1306 are not actually true headers, but in this case we pretend they are!
1307 ;-)
1308
1311 See curl_easy_getinfo(3).
1312
1314 The easy interface as described in detail in this document is a syn‐
1315 chronous interface that transfers one file at a time and does not re‐
1316 turn until it is done.
1317 The multi interface, on the other hand, allows your program to transfer
1319 multiple files in both directions at the same time, without forcing you
1320 to use multiple threads. The name might make it seem that the multi in‐
1321 terface is for multi-threaded programs, but the truth is almost the re‐
1322 verse. The multi interface allows a single-threaded application to per‐
1323 form the same kinds of multiple, simultaneous transfers that multi-
1324 threaded programs can perform. It allows many of the benefits of multi-
1325 threaded transfers without the complexity of managing and synchronizing
1326 many threads.
1327 To complicate matters somewhat more, there are even two versions of the
1329 multi interface. The event based one, also called multi_socket and the
1330 "normal one" designed for using with select(). See the libcurl-multi.3
1331 man page for details on the multi_socket event based API, this descrip‐
1332 tion here is for the select() oriented one.
1333 To use this interface, you are better off if you first understand the
1335 basics of how to use the easy interface. The multi interface is simply
1336 a way to make multiple transfers at the same time by adding up multiple
1337 easy handles into a "multi stack".
1338 You create the easy handles you want, one for each concurrent transfer,
1340 and you set all the options just like you learned above, and then you
1341 create a multi handle with curl_multi_init(3) and add all those easy
1342 handles to that multi handle with curl_multi_add_handle(3).
1343 When you have added the handles you have for the moment (you can still
1345 add new ones at any time), you start the transfers by calling
1346 curl_multi_perform(3).
1347 curl_multi_perform(3) is asynchronous. It will only perform what can be
1349 done now and then return control to your program. It is designed to
1350 never block. You need to keep calling the function until all transfers
1351 are completed.
1352 The best usage of this interface is when you do a select() on all pos‐
1354 sible file descriptors or sockets to know when to call libcurl again.
1355 This also makes it easy for you to wait and respond to actions on your
1356 own application's sockets/handles. You figure out what to select() for
1357 by using curl_multi_fdset(3), that fills in a set of fd_set variables
1358 for you with the particular file descriptors libcurl uses for the mo‐
1359 ment.
1360 When you then call select(), it will return when one of the file han‐
1362 dles signal action and you then call curl_multi_perform(3) to allow
1363 libcurl to do what it wants to do. Take note that libcurl does also
1364 feature some time-out code so we advise you to never use long timeouts
1365 on select() before you call curl_multi_perform(3) again.
1366 curl_multi_timeout(3) is provided to help you get a suitable timeout
1367 period.
1368 Another precaution you should use: always call curl_multi_fdset(3) im‐
1370 mediately before the select() call since the current set of file de‐
1371 scriptors may change in any curl function invoke.
1372 If you want to stop the transfer of one of the easy handles in the
1374 stack, you can use curl_multi_remove_handle(3) to remove individual
1375 easy handles. Remember that easy handles should be
1376 curl_easy_cleanup(3)ed.
1377 When a transfer within the multi stack has finished, the counter of
1379 running transfers (as filled in by curl_multi_perform(3)) will de‐
1380 crease. When the number reaches zero, all transfers are done.
1381 curl_multi_info_read(3) can be used to get information about completed
1383 transfers. It then returns the CURLcode for each easy transfer, to al‐
1384 low you to figure out success on each individual transfer.
1385
1388 [ seeding, passwords, keys, certificates, ENGINE, ca certs ]
1389
1392 You can share some data between easy handles when the easy interface is
1393 used, and some data is share automatically when you use the multi in‐
1394 terface.
1395 When you add easy handles to a multi handle, these easy handles will
1397 automatically share a lot of the data that otherwise would be kept on a
1398 per-easy handle basis when the easy interface is used.
1399 The DNS cache is shared between handles within a multi handle, making
1401 subsequent name resolving faster, and the connection pool that is kept
1402 to better allow persistent connections and connection re-use is also
1403 shared. If you are using the easy interface, you can still share these
1404 between specific easy handles by using the share interface, see
1405 libcurl-share(3).
1406 Some things are never shared automatically, not within multi handles,
1408 like for example cookies so the only way to share that is with the
1409 share interface.
1410
1412 [1] libcurl 7.10.3 and later have the ability to switch over to
1413 chunked Transfer-Encoding in cases where HTTP uploads are done
1414 with data of an unknown size.
1415 [2] This happens on Windows machines when libcurl is built and used
1417 as a DLL. However, you can still do this on Windows if you link
1418 with a static library.
1419 [3] The curl-config tool is generated at build-time (on Unix-like
1421 systems) and should be installed with the 'make install' or sim‐
1422 ilar instruction that installs the library, header files, man
1423 pages etc.
1424 [4] This behavior was different in versions before 7.17.0, where
1426 strings had to remain valid past the end of the curl_easy_se‐
1427 topt(3) call.
1428
1430 libcurl-errors(3), libcurl-multi(3), libcurl-easy(3)
1431libcurl 7.82.0 December 20, 2021 libcurl-tutorial(3)