1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
|
$Id$
_ _ ____ _
___| | | | _ \| |
/ __| | | | |_) | |
| (__| |_| | _ <| |___
\___|\___/|_| \_\_____|
PROGRAMMING WITH LIBCURL
About this Document
This document will attempt to describe the general principle and some basic
approaches to consider when programming with libcurl. The text will focus
mainly on the C/C++ interface but might apply fairly well on other interfaces
as well as they usually follow the C one pretty closely.
This document will refer to 'the user' as the person writing the source code
that uses libcurl. That would probably be you or someone in your position.
What will be generally refered to as 'the program' will be the collected
source code that you write that is using libcurl for transfers. The program
is outside libcurl and libcurl is outside of the program.
Building
There are many different ways to build C programs. This chapter will assume
a unix-style build process. If you use a different build system, you can
still read this to get general information that may apply to your
environment as well.
Compiling the Program
Your compiler needs to know where the libcurl headers are
located. Therefore you must set your compiler's include path to point to
the directory where you installed them. The 'curl-config'[3] tool can be
used to get this information:
$ curl-config --cflags
Linking the Program with libcurl
When having compiled the program, you need to link your object files to
create a single executable. For that to succeed, you need to link with
libcurl and possibly also with other libraries that libcurl itself depends
on. Like OpenSSL librararies, but even some standard OS libraries may be
needed on the command line. To figure out which flags to use, once again
the 'curl-config' tool comes to the rescue:
$ curl-config --libs
SSL or Not
libcurl can be built and customized in many ways. One of the things that
varies from different libraries and builds is the support for SSL-based
transfers, like HTTPS and FTPS. If OpenSSL was detected properly at
build-time, libcurl will be built with SSL support. To figure out if an
installed libcurl has been built with SSL support enabled, use
'curl-config' like this:
$ curl-config --feature
And if SSL is supported, the keyword 'SSL' will be written to stdout,
possibly together with a few other features that can be on and off on
different libcurls.
Portable Code in a Portable World
The people behind libcurl have put a considerable effort to make libcurl work
on a large amount of different operating systems and environments.
You program libcurl the same way on all platforms that libcurl runs on. There
are only very few minor considerations that differs. If you just make sure to
write your code portable enough, you may very well create yourself a very
portable program. libcurl shouldn't stop you from that.
Global Preparation
The program must initialize some of the libcurl functionality globally. That
means it should be done exactly once, no matter how many times you intend to
use the library. Once for your program's entire life time. This is done using
curl_global_init()
and it takes one parameter which is a bit pattern that tells libcurl what to
intialize. Using CURL_GLOBAL_ALL will make it initialize all known internal
sub modules, and might be a good default option. The current two bits that
are specified are:
CURL_GLOBAL_WIN32 which only does anything on Windows machines. When used on
a Windows machine, it'll make libcurl intialize the win32 socket
stuff. Without having that initialized properly, your program cannot use
sockets properly. You should only do this once for each application, so if
your program already does this or of another library in use does it, you
should not tell libcurl to do this as well.
CURL_GLOBAL_SSL which only does anything on libcurls compiled and built
SSL-enabled. On these systems, this will make libcurl init OpenSSL properly
for this application. This is only needed to do once for each application so
if your program or another library already does this, this bit should not be
needed.
libcurl has a default protection mechanism that detects if curl_global_init()
hasn't been called by the time curl_easy_perform() is called and if that is
the case, libcurl runs the function itself with a guessed bit pattern. Please
note that depending solely on this is not considered nice nor very good.
When the program no longer uses libcurl, it should call
curl_global_cleanup(), which is the opposite of the init call. It will then
do the reversed operations to cleanup the resources the curl_global_init()
call initialized.
Repeated calls to curl_global_init() and curl_global_cleanup() should be
avoided. They should be called once each.
Handle the Easy libcurl
libcurl version 7 is oriented around the so called easy interface. All
operations in the easy interface are prefixed with 'curl_easy'.
Future libcurls will also offer the multi interface. More about that
interface, what it is targeted for and how to use it is still only debated on
the libcurl mailing list and developer web pages. Join up to discuss and
figure out!
To use the easy interface, you must first create yourself an easy handle. You
need one handle for each easy session you want to perform. Basicly, you
should use one handle for every thread you plan to use for transferring. You
must never share the same handle in multiple threads.
Get an easy handle with
easyhandle = curl_easy_init();
It returns an easy handle. Using that you proceed to the next step: setting
up your preferred actions. A handle is just a logic entity for the upcoming
transfer or series of transfers. One of the most basic properties to set in
the handle is the URL. You set your preferred URL to transfer with
CURLOPT_URL in a manner similar to:
curl_easy_setopt(easyhandle, CURLOPT_URL, "http://curl.haxx.se/");
Let's assume for a while that you want to receive data as the URL indentifies
a remote resource you want to get here. Since you write a sort of application
that needs this transfer, I assume that you would like to get the data passed
to you directly instead of simply getting it passed to stdout. So, you write
your own function that matches this prototype:
size_t write_data(void *buffer, size_t size, size_t nmemb, void *userp);
You tell libcurl to pass all data to this function by issuing a function
similar to this:
curl_easy_setopt(easyhandle, CURLOPT_WRITEFUNCTION, write_data);
You can control what data your function get in the forth argument by setting
another property:
curl_easy_setopt(easyhandle, CURLOPT_FILE, &internal_struct);
Using that property, you can easily pass local data between your application
and the function that gets invoked by libcurl. libcurl itself won't touch the
data you pass with CURLOPT_FILE.
libcurl offers its own default internal callback that'll take care of the
data if you don't set the callback with CURLOPT_WRITEFUNCTION. It will then
simply output the received data to stdout. You can have the default callback
write the data to a different file handle by passing a 'FILE *' to a file
opened for writing with the CURLOPT_FILE option.
Now, we need to take a step back and have a deep breath. Here's one of those
rare platform-dependent nitpicks. Did you spot it? On some platforms[2],
libcurl won't be able to operate on files opened by the program. Thus, if you
use the default callback and pass in a an open file with CURLOPT_FILE, it
will crash. You should therefore avoid this to make your program run fine
virtually everywhere.
There are of course many more options you can set, and we'll get back to a
few of them later. Let's instead continue to the actual transfer:
success = curl_easy_perform(easyhandle);
The curl_easy_perform() will connect to the remote site, do the necessary
commands and receive the transfer. Whenever it receives data, it calls the
callback function we previously set. The function may get one byte at a time,
or it may get many kilobytes at once. libcurl delivers as much as possible as
often as possible. Your callback function should return the number of bytes
it "took care of". If that is not the exact same amount of bytes that was
passed to it, libcurl will abort the operation and return with an error code.
When the transfer is complete, the function returns a return code that
informs you if it succeeded in its mission or not. If a return code isn't
enough for you, you can use the CURLOPT_ERRORBUFFER to point libcurl to a
buffer of yours where it'll store a human readable error message as well.
If you then want to transfer another file, the handle is ready to be used
again. Mind you, it is even preferred that you re-use an existing handle if
you intend to make another transfer. libcurl will then attempt to re-use the
previous
When It Doesn't Work
There will always be times when the transfer fails for some reason. You might
have set the wrong libcurl option or misunderstood what the libcurl option
actually does, or the remote server might return non-standard replies that
confuse the library which then confuses your program.
There's one golden rule when these things occur: set the CURLOPT_VERBOSE
option to TRUE. It'll cause the library to spew out the entire protocol
details it sends, some internal info and some received protcol data as well
(especially when using FTP). If you're using HTTP, adding the headers in the
received output to study is also a clever way to get a better understanding
wht the server behaves the way it does. Include headers in the normal body
output with CURLOPT_HEADER set TRUE.
Of course there are bugs left. We need to get to know about them to be able
to fix them, so we're quite dependent on your bug reports! When you do report
suspected bugs in libcurl, please include as much details you possibly can: a
protocol dump that CURLOPT_VERBOSE produces, library version, as much as
possible of your code that uses libcurl, operating system name and version,
compiler name and version etc.
Upload Data to a Remote Site
libcurl tries to keep a protocol independent approach to most transfers, thus
uploading to a remote FTP site is very similar to uploading data to a HTTP
server with a PUT request.
Of course, first you either create an easy handle or you re-use one existing
one. Then you set the URL to operate on just like before. This is the remote
URL, that we now will upload.
Since we write an application, we most likely want libcurl to get the upload
data by asking us for it. To make it do that, we set the read callback and
the custom pointer libcurl will pass to our read callback. The read callback
should have a prototype similar to:
size_t function(char *bufptr, size_t size, size_t nitems, void *userp);
Where bufptr is the pointer to a buffer we fill in with data to upload and
size*nitems is the size of the buffer and therefore also the maximum amount
of data we can return to libcurl in this call. The 'userp' pointer is the
custom pointer we set to point to a struct of ours to pass private data
between the application and the callback.
curl_easy_setopt(easyhandle, CURLOPT_READFUNCTION, read_function);
curl_easy_setopt(easyhandle, CURLOPT_INFILE, &filedata);
Tell libcurl that we want to upload:
curl_easy_setopt(easyhandle, CURLOPT_UPLOAD, TRUE);
A few protocols won't behave properly when uploads are done without any prior
knowledge of the expected file size. HTTP PUT is one example [1]. So, set the
upload file size using the CURLOPT_INFILESIZE like this:
curl_easy_setopt(easyhandle, CURLOPT_INFILESIZE, file_size);
When you call curl_easy_perform() this time, it'll perform all the necessary
operations and when it has invoked the upload it'll call your supplied
callback to get the data to upload. The program should return as much data as
possible in every invoke, as that is likely to make the upload perform as
fast as possible. The callback should return the number of bytes it wrote in
the buffer. Returning 0 will signal the end of the upload.
Passwords
Many protocols use or even require that user name and password are provided
to be able to download or upload the data of your choice. libcurl offers
several ways to specify them.
Most protocols support that you specify the name and password in the URL
itself. libcurl will detect this and use them accordingly. This is written
like this:
protocol://user:password@example.com/path/
If you need any odd letters in your user name or password, you should enter
them URL encoded, as %XX where XX is a two-digit hexadecimal number.
libcurl also provides options to set various passwords. The user name and
password as shown embedded in the URL can instead get set with the
CURLOPT_USERPWD option. The argument passed to libcurl should be a char * to
a string in the format "user:password:". In a manner like this:
curl_easy_setopt(easyhandle, CURLOPT_USERPWD, "myname:thesecret");
Another case where name and password might be needed at times, is for those
users who need to athenticate themselves to a proxy they use. libcurl offers
another option for this, the CURLOPT_PROXYUSERPWD. It is used quite similar
to the CURLOPT_USERPWD option like this:
curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "myname:thesecret");
[ more options, setting passsword callback ]
Showing Progress
libcurl with C++
There's basicly only one thing to keep in mind when using C++ instead of C
when interfacing libcurl:
"The Callbacks Must Be Plain C"
So if you want a write callback set in libcurl, you should put it within
'extern'. Similar to this:
extern "C" {
size_t write_data(void *ptr, size_t size, size_t nmemb,
void *ourpointer)
{
/* do what you want with the data */
}
}
This will of course effectively turn the callback code into C. There won't be
any "this" pointer available etc.
Security Considerations
Certificates and Other SSL Tricks
Future
-----
Footnotes:
[1] = HTTP PUT without knowing the size prior to transfer is indeed possible,
but libcurl does not support the chunked transfers on uploading that is
necessary for this feature to work. We'd gratefully appreciate patches
that bring this functionality...
[2] = This happens on Windows machines when libcurl is built and used as a
DLL. However, you can still do this on Windows if you link with a static
library.
[3] = The curl-config tool is generated at build-time (on unix-like systems)
and should be installed with the 'make install' or similar instruction
that installs the library, header files, man pages etc.
|