libcurl -- the guide
Date: Tue, 15 Jan 2002 09:44:05 +0100 (MET)
My initial attempts to write a tutorial-like guide for programming
with libcurl is now found here:
I include the current version here, feel free to comment and improve!
PROGRAMMING WITH LIBCURL
About this Document
This document will attempt to describe the general principle and some basic
approach 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.
Compiling the Program
Linking the Program with libcurl
SSL or Not
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
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
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()
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
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
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.
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
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.
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 *buffer, size_t size, size_t nitems, void *userp);
Where buffer is the pointer to a buffer we fill in with data to upload and
size*nitems is the size of the buffer. 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 . So, set the
upload file size using the CURLOPT_INFILESIZE like this:
curl_easy_setopt(easyhandle, CURLOPT_INFILESIZE, file_size);
So, then you call curl_easy_perform() this time, it'll perform all 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.
 = 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...
-- Daniel Stenberg -- curl groks URLs -- http://curl.haxx.se/Received on 2002-01-15