libcurl-tutorial(3) libcurl programming libcurl-tutorial(3)NAMElibcurl-tutorial - libcurl programming tutorial
Objective
This document attempts to describe the general principles and some
basic approaches to consider when programming with libcurl. The text
will focus mainly on the 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 referred 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.
To get more details on all options and functions described herein,
please refer to their respective man pages.
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-con‐
fig'[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 the OpenSSL libraries, 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 a sup‐
ported SSL library 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-con‐
fig' 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 could
be either on or off on for different libcurls.
See also the "Features libcurl Provides" further down.
autoconf macro
When you write your configure script to detect libcurl and setup
variables accordingly, we offer a prewritten macro that probably
does everything you need in this area. See
docs/libcurl/libcurl.m4 file - it includes docs on how to use
it.
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 envi‐
ronments.
You program libcurl the same way on all platforms that libcurl runs on.
There are only very few minor considerations that differ. 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 initialize. 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 initialize the
win32 socket stuff. Without having that initialized prop‐
erly, 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
initialize the SSL library properly for this application.
This only needs to be done 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(3) hasn't been called by the time curl_easy_perform(3)
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(3), which is the opposite of the init call. It will
then do the reversed operations to cleanup the resources the
curl_global_init(3) call initialized.
Repeated calls to curl_global_init(3) and curl_global_cleanup(3) should
be avoided. They should only be called once each.
Features libcurl Provides
It is considered best-practice to determine libcurl features at run-
time rather than at build-time (if possible of course). By calling
curl_version_info(3) and checking out the details of the returned
struct, your program can figure out exactly what the currently running
libcurl supports.
Handle the Easy libcurl
libcurl first introduced the so called easy interface. All operations
in the easy interface are prefixed with 'curl_easy'.
Recent libcurl versions also offer the multi interface. More about that
interface, what it is targeted for and how to use it is detailed in a
separate chapter further down. You still need to understand the easy
interface first, so please continue reading for better understanding.
To use the easy interface, you must first create yourself an easy han‐
dle. You need one handle for each easy session you want to perform.
Basically, 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.
You set properties and options for this handle using
curl_easy_setopt(3). They control how the subsequent transfer or trans‐
fers will be made. Options remain set in the handle until set again to
something different. Alas, multiple requests using the same handle will
use the same options.
Many of the options you set in libcurl are "strings", pointers to data
terminated with a zero byte. When you set strings with
curl_easy_setopt(3), libcurl makes its own copy so that they don't need
to be kept around in your application after being set[4].
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(handle, CURLOPT_URL, "http://domain.com/");
Let's assume for a while that you want to receive data as the URL iden‐
tifies 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 func‐
tion similar to this:
curl_easy_setopt(easyhandle, CURLOPT_WRITEFUNCTION, write_data);
You can control what data your callback function gets in the fourth
argument by setting another property:
curl_easy_setopt(easyhandle, CURLOPT_WRITEDATA, &internal_struct);
Using that property, you can easily pass local data between your appli‐
cation and the function that gets invoked by libcurl. libcurl itself
won't touch the data you pass with CURLOPT_WRITEDATA.
libcurl offers its own default internal callback that will 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_WRITEDATA
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 plat‐
forms[2], libcurl won't be able to operate on files opened by the pro‐
gram. Thus, if you use the default callback and pass in an open file
with CURLOPT_WRITEDATA, it will crash. You should therefore avoid this
to make your program run fine virtually everywhere.
(CURLOPT_WRITEDATA was formerly known as CURLOPT_FILE. Both names still
work and do the same thing).
If you're using libcurl as a win32 DLL, you MUST use the CURLOPT_WRITE‐
FUNCTION if you set CURLOPT_WRITEDATA - or you will experience crashes.
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);
curl_easy_perform(3) 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 connection.
For some protocols, downloading a file can involve a complicated
process of logging in, setting the transfer mode, changing the current
directory and finally transferring the file data. libcurl takes care of
all that complication for you. Given simply the URL to a file, libcurl
will take care of all the details needed to get the file moved from one
machine to another.
Multi-threading Issues
The first basic rule is that you must never simultaneously share a
libcurl handle (be it easy or multi or whatever) between multiple
threads. Only use one handle in one thread at any time. You can pass
the handles around among threads, but you must never use a single han‐
dle from more than one thread at any given time.
libcurl is completely thread safe, except for two issues: signals and
SSL/TLS handlers. Signals are used for timing out name resolves (during
DNS lookup) - when built without c-ares support and not on Windows.
If you are accessing HTTPS or FTPS URLs in a multi-threaded manner, you
are then of course using the underlying SSL library multi-threaded and
those libs might have their own requirements on this issue. Basically,
you need to provide one or two functions to allow it to function prop‐
erly. For all details, see this:
OpenSSL
http://www.openssl.org/docs/crypto/threads.html#DESCRIPTION
GnuTLS
http://www.gnu.org/software/gnutls/man‐
ual/html_node/Multi_002dthreaded-applications.html
NSS
is claimed to be thread-safe already without anything required.
PolarSSL
Required actions unknown.
yassl
Required actions unknown.
axTLS
Required actions unknown.
Secure Transport
The engine is fully thread-safe, and no additional steps are required.
When using multiple threads you should set the CURLOPT_NOSIGNAL option
to 1 for all handles. Everything will or might work fine except that
timeouts are not honored during the DNS lookup - which you can work
around by building libcurl with c-ares support. c-ares is a library
that provides asynchronous name resolves. On some platforms, libcurl
simply will not function properly multi-threaded unless this option is
set.
Also, note that CURLOPT_DNS_USE_GLOBAL_CACHE is not thread-safe.
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 pro‐
gram.
There's one golden rule when these things occur: set the CURLOPT_VER‐
BOSE option to 1. It'll cause the library to spew out the entire proto‐
col details it sends, some internal info and some received protocol
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 why the server behaves the way it does. Include
headers in the normal body output with CURLOPT_HEADER set 1.
Of course, there are bugs left. We need 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 many details as 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.
If CURLOPT_VERBOSE is not enough, you increase the level of debug data
your application receive by using the CURLOPT_DEBUGFUNCTION.
Getting some in-depth knowledge about the protocols involved is never
wrong, and if you're trying to do funny things, you might very well
understand libcurl and how to use it better if you study the appropri‐
ate RFC documents at least briefly.
Upload Data to a Remote Site
libcurl tries to keep a protocol independent approach to most trans‐
fers, 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 maxi‐
mum 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_READDATA, &filedata);
Tell libcurl that we want to upload:
curl_easy_setopt(easyhandle, CURLOPT_UPLOAD, 1L);
A few protocols won't behave properly when uploads are done without any
prior knowledge of the expected file size. So, set the upload file size
using the CURLOPT_INFILESIZE_LARGE for all known file sizes like
this[1]:
/* in this example, file_size must be an curl_off_t variable */
curl_easy_setopt(easyhandle, CURLOPT_INFILESIZE_LARGE, file_size);
When you call curl_easy_perform(3) 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 num‐
ber 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 pro‐
vided 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 num‐
ber.
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 authenticate 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:these‐
cret");
There's a long time UNIX "standard" way of storing ftp user names and
passwords, namely in the $HOME/.netrc file. The file should be made
private so that only the user may read it (see also the "Security Con‐
siderations" chapter), as it might contain the password in plain text.
libcurl has the ability to use this file to figure out what set of user
name and password to use for a particular host. As an extension to the
normal functionality, libcurl also supports this file for non-FTP pro‐
tocols such as HTTP. To make curl use this file, use the CURLOPT_NETRC
option:
curl_easy_setopt(easyhandle, CURLOPT_NETRC, 1L);
And a very basic example of how such a .netrc file may look like:
machine myhost.mydomain.com
login userlogin
password secretword
All these examples have been cases where the password has been
optional, or at least you could leave it out and have libcurl attempt
to do its job without it. There are times when the password isn't
optional, like when you're using an SSL private key for secure trans‐
fers.
To pass the known private key password to libcurl:
curl_easy_setopt(easyhandle, CURLOPT_KEYPASSWD, "keypassword");
HTTP Authentication
The previous chapter showed how to set user name and password for get‐
ting URLs that require authentication. When using the HTTP protocol,
there are many different ways a client can provide those credentials to
the server and you can control which way libcurl will (attempt to) use
them. The default HTTP authentication method is called 'Basic', which
is sending the name and password in clear-text in the HTTP request,
base64-encoded. This is insecure.
At the time of this writing, libcurl can be built to use: Basic,
Digest, NTLM, Negotiate, GSS-Negotiate and SPNEGO. You can tell libcurl
which one to use with CURLOPT_HTTPAUTH as in:
curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH, CURLAUTH_DIGEST);
And when you send authentication to a proxy, you can also set authenti‐
cation type the same way but instead with CURLOPT_PROXYAUTH:
curl_easy_setopt(easyhandle, CURLOPT_PROXYAUTH, CURLAUTH_NTLM);
Both these options allow you to set multiple types (by ORing them
together), to make libcurl pick the most secure one out of the types
the server/proxy claims to support. This method does however add a
round-trip since libcurl must first ask the server what it supports:
curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH,
CURLAUTH_DIGEST|CURLAUTH_BASIC);
For convenience, you can use the 'CURLAUTH_ANY' define (instead of a
list with specific types) which allows libcurl to use whatever method
it wants.
When asking for multiple types, libcurl will pick the available one it
considers "best" in its own internal order of preference.
HTTP POSTing
We get many questions regarding how to issue HTTP POSTs with libcurl
the proper way. This chapter will thus include examples using both dif‐
ferent versions of HTTP POST that libcurl supports.
The first version is the simple POST, the most common version, that
most HTML pages using the <form> tag uses. We provide a pointer to the
data and tell libcurl to post it all to the remote site:
char *data="name=daniel&project=curl";
curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, data);
curl_easy_setopt(easyhandle, CURLOPT_URL, "http://posthere.com/");
curl_easy_perform(easyhandle); /* post away! */
Simple enough, huh? Since you set the POST options with the CUR‐
LOPT_POSTFIELDS, this automatically switches the handle to use POST in
the upcoming request.
Ok, so what if you want to post binary data that also requires you to
set the Content-Type: header of the post? Well, binary posts prevent
libcurl from being able to do strlen() on the data to figure out the
size, so therefore we must tell libcurl the size of the post data. Set‐
ting headers in libcurl requests are done in a generic way, by building
a list of our own headers and then passing that list to libcurl.
struct curl_slist *headers=NULL;
headers = curl_slist_append(headers, "Content-Type: text/xml");
/* post binary data */
curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, binaryptr);
/* set the size of the postfields data */
curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDSIZE, 23L);
/* pass our list of custom made headers */
curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers);
curl_easy_perform(easyhandle); /* post away! */
curl_slist_free_all(headers); /* free the header list */
While the simple examples above cover the majority of all cases where
HTTP POST operations are required, they don't do multi-part formposts.
Multi-part formposts were introduced as a better way to post (possibly
large) binary data and were first documented in the RFC1867 (updated in
RFC2388). They're called multi-part because they're built by a chain of
parts, each part being a single unit of data. Each part has its own
name and contents. You can in fact create and post a multi-part form‐
post with the regular libcurl POST support described above, but that
would require that you build a formpost yourself and provide to
libcurl. To make that easier, libcurl provides curl_formadd(3). Using
this function, you add parts to the form. When you're done adding
parts, you post the whole form.
The following example sets two simple text parts with plain textual
contents, and then a file with binary contents and uploads the whole
thing.
struct curl_httppost *post=NULL;
struct curl_httppost *last=NULL;
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "name",
CURLFORM_COPYCONTENTS, "daniel", CURLFORM_END);
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "project",
CURLFORM_COPYCONTENTS, "curl", CURLFORM_END);
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "logotype-image",
CURLFORM_FILECONTENT, "curl.png", CURLFORM_END);
/* Set the form info */
curl_easy_setopt(easyhandle, CURLOPT_HTTPPOST, post);
curl_easy_perform(easyhandle); /* post away! */
/* free the post data again */
curl_formfree(post);
Multipart formposts are chains of parts using MIME-style separators and
headers. It means that each one of these separate parts get a few head‐
ers set that describe the individual content-type, size etc. To enable
your application to handicraft this formpost even more, libcurl allows
you to supply your own set of custom headers to such an individual form
part. You can of course supply headers to as many parts as you like,
but this little example will show how you set headers to one specific
part when you add that to the post handle:
struct curl_slist *headers=NULL;
headers = curl_slist_append(headers, "Content-Type: text/xml");
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "logotype-image",
CURLFORM_FILECONTENT, "curl.xml",
CURLFORM_CONTENTHEADER, headers,
CURLFORM_END);
curl_easy_perform(easyhandle); /* post away! */
curl_formfree(post); /* free post */
curl_slist_free_all(headers); /* free custom header list */
Since all options on an easyhandle are "sticky", they remain the same
until changed even if you do call curl_easy_perform(3), you may need to
tell curl to go back to a plain GET request if you intend to do one as
your next request. You force an easyhandle to go back to GET by using
the CURLOPT_HTTPGET option:
curl_easy_setopt(easyhandle, CURLOPT_HTTPGET, 1L);
Just setting CURLOPT_POSTFIELDS to "" or NULL will *not* stop libcurl
from doing a POST. It will just make it POST without any data to send!
Showing Progress
For historical and traditional reasons, libcurl has a built-in progress
meter that can be switched on and then makes it present a progress
meter in your terminal.
Switch on the progress meter by, oddly enough, setting CUR‐
LOPT_NOPROGRESS to zero. This option is set to 1 by default.
For most applications however, the built-in progress meter is useless
and what instead is interesting is the ability to specify a progress
callback. The function pointer you pass to libcurl will then be called
on irregular intervals with information about the current transfer.
Set the progress callback by using CURLOPT_PROGRESSFUNCTION. And pass a
pointer to a function that matches this prototype:
int progress_callback(void *clientp,
double dltotal,
double dlnow,
double ultotal,
double ulnow);
If any of the input arguments is unknown, a 0 will be passed. The first
argument, the 'clientp' is the pointer you pass to libcurl with CUR‐
LOPT_PROGRESSDATA. libcurl won't touch it.
libcurl with C++
There's basically only one thing to keep in mind when using C++ instead
of C when interfacing libcurl:
The callbacks CANNOT be non-static class member functions
Example C++ code:
class AClass {
static size_t write_data(void *ptr, size_t size, size_t nmemb,
void *ourpointer)
{
/* do what you want with the data */
}
}
Proxies
What "proxy" means according to Merriam-Webster: "a person authorized
to act for another" but also "the agency, function, or office of a
deputy who acts as a substitute for another".
Proxies are exceedingly common these days. Companies often only offer
Internet access to employees through their proxies. Network clients or
user-agents ask the proxy for documents, the proxy does the actual
request and then it returns them.
libcurl supports SOCKS and HTTP proxies. When a given URL is wanted,
libcurl will ask the proxy for it instead of trying to connect to the
actual host identified in the URL.
If you're using a SOCKS proxy, you may find that libcurl doesn't quite
support all operations through it.
For HTTP proxies: the fact that the proxy is a HTTP proxy puts certain
restrictions on what can actually happen. A requested URL that might
not be a HTTP URL will be still be passed to the HTTP proxy to deliver
back to libcurl. This happens transparently, and an application may not
need to know. I say "may", because at times it is very important to
understand that all operations over a HTTP proxy use the HTTP protocol.
For example, you can't invoke your own custom FTP commands or even
proper FTP directory listings.
Proxy Options
To tell libcurl to use a proxy at a given port number:
curl_easy_setopt(easyhandle, CURLOPT_PROXY, "proxy-
host.com:8080");
Some proxies require user authentication before allowing a
request, and you pass that information similar to this:
curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "user:pass‐
word");
If you want to, you can specify the host name only in the CUR‐
LOPT_PROXY option, and set the port number separately with CUR‐
LOPT_PROXYPORT.
Tell libcurl what kind of proxy it is with CURLOPT_PROXYTYPE (if
not, it will default to assume a HTTP proxy):
curl_easy_setopt(easyhandle, CURLOPT_PROXYTYPE, CURL‐
PROXY_SOCKS4);
Environment Variables
libcurl automatically checks and uses a set of environment vari‐
ables to know what proxies to use for certain protocols. The
names of the variables are following an ancient de facto stan‐
dard and are built up as "[protocol]_proxy" (note the lower cas‐
ing). Which makes the variable 'http_proxy' checked for a name
of a proxy to use when the input URL is HTTP. Following the same
rule, the variable named 'ftp_proxy' is checked for FTP URLs.
Again, the proxies are always HTTP proxies, the different names
of the variables simply allows different HTTP proxies to be
used.
The proxy environment variable contents should be in the format
"[protocol://][user:password@]machine[:port]". Where the proto‐
col:// part is simply ignored if present (so http://proxy and
bluerk://proxy will do the same) and the optional port number
specifies on which port the proxy operates on the host. If not
specified, the internal default port number will be used and
that is most likely *not* the one you would like it to be.
There are two special environment variables. 'all_proxy' is what
sets proxy for any URL in case the protocol specific variable
wasn't set, and 'no_proxy' defines a list of hosts that should
not use a proxy even though a variable may say so. If 'no_proxy'
is a plain asterisk ("*") it matches all hosts.
To explicitly disable libcurl's checking for and using the proxy
environment variables, set the proxy name to "" - an empty
string - with CURLOPT_PROXY.
SSL and Proxies
SSL is for secure point-to-point connections. This involves
strong encryption and similar things, which effectively makes it
impossible for a proxy to operate as a "man in between" which
the proxy's task is, as previously discussed. Instead, the only
way to have SSL work over a HTTP proxy is to ask the proxy to
tunnel trough everything without being able to check or fiddle
with the traffic.
Opening an SSL connection over a HTTP proxy is therefor a matter
of asking the proxy for a straight connection to the target host
on a specified port. This is made with the HTTP request CONNECT.
("please mr proxy, connect me to that remote host").
Because of the nature of this operation, where the proxy has no
idea what kind of data that is passed in and out through this
tunnel, this breaks some of the very few advantages that come
from using a proxy, such as caching. Many organizations prevent
this kind of tunneling to other destination port numbers than
443 (which is the default HTTPS port number).
Tunneling Through Proxy
As explained above, tunneling is required for SSL to work and
often even restricted to the operation intended for SSL; HTTPS.
This is however not the only time proxy-tunneling might offer
benefits to you or your application.
As tunneling opens a direct connection from your application to
the remote machine, it suddenly also re-introduces the ability
to do non-HTTP operations over a HTTP proxy. You can in fact use
things such as FTP upload or FTP custom commands this way.
Again, this is often prevented by the administrators of proxies
and is rarely allowed.
Tell libcurl to use proxy tunneling like this:
curl_easy_setopt(easyhandle, CURLOPT_HTTPPROXYTUNNEL, 1L);
In fact, there might even be times when you want to do plain
HTTP operations using a tunnel like this, as it then enables you
to operate on the remote server instead of asking the proxy to
do so. libcurl will not stand in the way for such innovative
actions either!
Proxy Auto-Config
Netscape first came up with this. It is basically a web page
(usually using a .pac extension) with a Javascript that when
executed by the browser with the requested URL as input, returns
information to the browser on how to connect to the URL. The
returned information might be "DIRECT" (which means no proxy
should be used), "PROXY host:port" (to tell the browser where
the proxy for this particular URL is) or "SOCKS host:port" (to
direct the browser to a SOCKS proxy).
libcurl has no means to interpret or evaluate Javascript and
thus it doesn't support this. If you get yourself in a position
where you face this nasty invention, the following advice have
been mentioned and used in the past:
- Depending on the Javascript complexity, write up a script that
translates it to another language and execute that.
- Read the Javascript code and rewrite the same logic in another
language.
- Implement a Javascript interpreter; people have successfully
used the Mozilla Javascript engine in the past.
- Ask your admins to stop this, for a static proxy setup or sim‐
ilar.
Persistence Is The Way to Happiness
Re-cycling the same easy handle several times when doing multiple
requests is the way to go.
After each single curl_easy_perform(3) operation, libcurl will keep the
connection alive and open. A subsequent request using the same easy
handle to the same host might just be able to use the already open con‐
nection! This reduces network impact a lot.
Even if the connection is dropped, all connections involving SSL to the
same host again, will benefit from libcurl's session ID cache that
drastically reduces re-connection time.
FTP connections that are kept alive save a lot of time, as the command-
response round-trips are skipped, and also you don't risk getting
blocked without permission to login again like on many FTP servers only
allowing N persons to be logged in at the same time.
libcurl caches DNS name resolving results, to make lookups of a previ‐
ously looked up name a lot faster.
Other interesting details that improve performance for subsequent
requests may also be added in the future.
Each easy handle will attempt to keep the last few connections alive
for a while in case they are to be used again. You can set the size of
this "cache" with the CURLOPT_MAXCONNECTS option. Default is 5. There
is very seldom any point in changing this value, and if you think of
changing this it is often just a matter of thinking again.
To force your upcoming request to not use an already existing connec‐
tion (it will even close one first if there happens to be one alive to
the same host you're about to operate on), you can do that by setting
CURLOPT_FRESH_CONNECT to 1. In a similar spirit, you can also forbid
the upcoming request to be "lying" around and possibly get re-used
after the request by setting CURLOPT_FORBID_REUSE to 1.
HTTP Headers Used by libcurl
When you use libcurl to do HTTP requests, it'll pass along a series of
headers automatically. It might be good for you to know and understand
these. You can replace or remove them by using the CURLOPT_HTTPHEADER
option.
Host This header is required by HTTP 1.1 and even many 1.0 servers
and should be the name of the server we want to talk to. This
includes the port number if anything but default.
Accept "*/*".
Expect When doing POST requests, libcurl sets this header to "100-con‐
tinue" to ask the server for an "OK" message before it proceeds
with sending the data part of the post. If the POSTed data
amount is deemed "small", libcurl will not use this header.
Customizing Operations
There is an ongoing development today where more and more protocols are
built upon HTTP for transport. This has obvious benefits as HTTP is a
tested and reliable protocol that is widely deployed and has excellent
proxy-support.
When you use one of these protocols, and even when doing other kinds of
programming you may need to change the traditional HTTP (or FTP or...)
manners. You may need to change words, headers or various data.
libcurl is your friend here too.
CUSTOMREQUEST
If just changing the actual HTTP request keyword is what you
want, like when GET, HEAD or POST is not good enough for you,
CURLOPT_CUSTOMREQUEST is there for you. It is very simple to
use:
curl_easy_setopt(easyhandle, CURLOPT_CUSTOMREQUEST, "MYOWNRE‐
QUEST");
When using the custom request, you change the request keyword of
the actual request you are performing. Thus, by default you make
a GET request but you can also make a POST operation (as
described before) and then replace the POST keyword if you want
to. You're the boss.
Modify Headers
HTTP-like protocols pass a series of headers to the server when
doing the request, and you're free to pass any amount of extra
headers that you think fit. Adding headers is this easy:
struct curl_slist *headers=NULL; /* init to NULL is important */
headers = curl_slist_append(headers, "Hey-server-hey: how are you?");
headers = curl_slist_append(headers, "X-silly-content: yes");
/* pass our list of custom made headers */
curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers);
curl_easy_perform(easyhandle); /* transfer http */
curl_slist_free_all(headers); /* free the header list */
... and if you think some of the internally generated headers,
such as Accept: or Host: don't contain the data you want them to
contain, you can replace them by simply setting them too:
headers = curl_slist_append(headers, "Accept: Agent-007");
headers = curl_slist_append(headers, "Host: munged.host.line");
Delete Headers
If you replace an existing header with one with no contents, you
will prevent the header from being sent. For instance, if you
want to completely prevent the "Accept:" header from being sent,
you can disable it with code similar to this:
headers = curl_slist_append(headers, "Accept:");
Both replacing and canceling internal headers should be done
with careful consideration and you should be aware that you may
violate the HTTP protocol when doing so.
Enforcing chunked transfer-encoding
By making sure a request uses the custom header "Transfer-Encod‐
ing: chunked" when doing a non-GET HTTP operation, libcurl will
switch over to "chunked" upload, even though the size of the
data to upload might be known. By default, libcurl usually
switches over to chunked upload automatically if the upload data
size is unknown.
HTTP Version
All HTTP requests includes the version number to tell the server
which version we support. libcurl speaks HTTP 1.1 by default.
Some very old servers don't like getting 1.1-requests and when
dealing with stubborn old things like that, you can tell libcurl
to use 1.0 instead by doing something like this:
curl_easy_setopt(easyhandle, CURLOPT_HTTP_VERSION,
CURL_HTTP_VERSION_1_0);
FTP Custom Commands
Not all protocols are HTTP-like, and thus the above may not help
you when you want to make, for example, your FTP transfers to
behave differently.
Sending custom commands to a FTP server means that you need to
send the commands exactly as the FTP server expects them (RFC959
is a good guide here), and you can only use commands that work
on the control-connection alone. All kinds of commands that
require data interchange and thus need a data-connection must be
left to libcurl's own judgement. Also be aware that libcurl will
do its very best to change directory to the target directory
before doing any transfer, so if you change directory (with CWD
or similar) you might confuse libcurl and then it might not
attempt to transfer the file in the correct remote directory.
A little example that deletes a given file before an operation:
headers = curl_slist_append(headers, "DELE file-to-remove");
/* pass the list of custom commands to the handle */
curl_easy_setopt(easyhandle, CURLOPT_QUOTE, headers);
curl_easy_perform(easyhandle); /* transfer ftp data! */
curl_slist_free_all(headers); /* free the header list */
If you would instead want this operation (or chain of opera‐
tions) to happen _after_ the data transfer took place the option
to curl_easy_setopt(3) would instead be called CURLOPT_POSTQUOTE
and used the exact same way.
The custom FTP command will be issued to the server in the same
order they are added to the list, and if a command gets an error
code returned back from the server, no more commands will be
issued and libcurl will bail out with an error code
(CURLE_QUOTE_ERROR). Note that if you use CURLOPT_QUOTE to send
commands before a transfer, no transfer will actually take place
when a quote command has failed.
If you set the CURLOPT_HEADER to 1, you will tell libcurl to get
information about the target file and output "headers" about it.
The headers will be in "HTTP-style", looking like they do in
HTTP.
The option to enable headers or to run custom FTP commands may
be useful to combine with CURLOPT_NOBODY. If this option is set,
no actual file content transfer will be performed.
FTP Custom CUSTOMREQUEST
If you do want to list the contents of a FTP directory using
your own defined FTP command, CURLOPT_CUSTOMREQUEST will do just
that. "NLST" is the default one for listing directories but
you're free to pass in your idea of a good alternative.
Cookies Without Chocolate Chips
In the HTTP sense, a cookie is a name with an associated value. A
server sends the name and value to the client, and expects it to get
sent back on every subsequent request to the server that matches the
particular conditions set. The conditions include that the domain name
and path match and that the cookie hasn't become too old.
In real-world cases, servers send new cookies to replace existing ones
to update them. Server use cookies to "track" users and to keep "ses‐
sions".
Cookies are sent from server to clients with the header Set-Cookie: and
they're sent from clients to servers with the Cookie: header.
To just send whatever cookie you want to a server, you can use CUR‐
LOPT_COOKIE to set a cookie string like this:
curl_easy_setopt(easyhandle, CURLOPT_COOKIE, "name1=var1;
name2=var2;");
In many cases, that is not enough. You might want to dynamically save
whatever cookies the remote server passes to you, and make sure those
cookies are then used accordingly on later requests.
One way to do this, is to save all headers you receive in a plain file
and when you make a request, you tell libcurl to read the previous
headers to figure out which cookies to use. Set the header file to read
cookies from with CURLOPT_COOKIEFILE.
The CURLOPT_COOKIEFILE option also automatically enables the cookie
parser in libcurl. Until the cookie parser is enabled, libcurl will not
parse or understand incoming cookies and they will just be ignored.
However, when the parser is enabled the cookies will be understood and
the cookies will be kept in memory and used properly in subsequent
requests when the same handle is used. Many times this is enough, and
you may not have to save the cookies to disk at all. Note that the file
you specify to CURLOPT_COOKIEFILE doesn't have to exist to enable the
parser, so a common way to just enable the parser and not read any
cookies is to use the name of a file you know doesn't exist.
If you would rather use existing cookies that you've previously
received with your Netscape or Mozilla browsers, you can make libcurl
use that cookie file as input. The CURLOPT_COOKIEFILE is used for that
too, as libcurl will automatically find out what kind of file it is and
act accordingly.
Perhaps the most advanced cookie operation libcurl offers, is saving
the entire internal cookie state back into a Netscape/Mozilla formatted
cookie file. We call that the cookie-jar. When you set a file name with
CURLOPT_COOKIEJAR, that file name will be created and all received
cookies will be stored in it when curl_easy_cleanup(3) is called. This
enables cookies to get passed on properly between multiple handles
without any information getting lost.
FTP Peculiarities We Need
FTP transfers use a second TCP/IP connection for the data transfer.
This is usually a fact you can forget and ignore but at times this fact
will come back to haunt you. libcurl offers several different ways to
customize how the second connection is being made.
libcurl can either connect to the server a second time or tell the
server to connect back to it. The first option is the default and it is
also what works best for all the people behind firewalls, NATs or IP-
masquerading setups. libcurl then tells the server to open up a new
port and wait for a second connection. This is by default attempted
with EPSV first, and if that doesn't work it tries PASV instead. (EPSV
is an extension to the original FTP spec and does not exist nor work on
all FTP servers.)
You can prevent libcurl from first trying the EPSV command by setting
CURLOPT_FTP_USE_EPSV to zero.
In some cases, you will prefer to have the server connect back to you
for the second connection. This might be when the server is perhaps
behind a firewall or something and only allows connections on a single
port. libcurl then informs the remote server which IP address and port
number to connect to. This is made with the CURLOPT_FTPPORT option. If
you set it to "-", libcurl will use your system's "default IP address".
If you want to use a particular IP, you can set the full IP address, a
host name to resolve to an IP address or even a local network interface
name that libcurl will get the IP address from.
When doing the "PORT" approach, libcurl will attempt to use the EPRT
and the LPRT before trying PORT, as they work with more protocols. You
can disable this behavior by setting CURLOPT_FTP_USE_EPRT to zero.
Headers Equal Fun
Some protocols provide "headers", meta-data separated from the normal
data. These headers are by default not included in the normal data
stream, but you can make them appear in the data stream by setting CUR‐
LOPT_HEADER to 1.
What might be even more useful, is libcurl's ability to separate the
headers from the data and thus make the callbacks differ. You can for
example set a different pointer to pass to the ordinary write callback
by setting CURLOPT_WRITEHEADER.
Or, you can set an entirely separate function to receive the headers,
by using CURLOPT_HEADERFUNCTION.
The headers are passed to the callback function one by one, and you can
depend on that fact. It makes it easier for you to add custom header
parsers etc.
"Headers" for FTP transfers equal all the FTP server responses. They
aren't actually true headers, but in this case we pretend they are! ;-)
Post Transfer Information
[ curl_easy_getinfo ]
Security Considerations
The libcurl project takes security seriously. The library is written
with caution and precautions are taken to mitigate many kinds of risks
encountered while operating with potentially malicious servers on the
Internet. It is a powerful library, however, which allows application
writers to make trade offs between ease of writing and exposure to
potential risky operations. If used the right way, you can use libcurl
to transfer data pretty safely.
Many applications are used in closed networks where users and servers
can be trusted, but many others are used on arbitrary servers and are
fed input from potentially untrusted users. Following is a discussion
about some risks in the ways in which applications commonly use libcurl
and potential mitigations of those risks. It is by no means comprehen‐
sive, but shows classes of attacks that robust applications should con‐
sider. The Common Weakness Enumeration project at http://cwe.mitre.org/
is a good reference for many of these and similar types of weaknesses
of which application writers should be aware.
Command Lines
If you use a command line tool (such as curl) that uses libcurl,
and you give options to the tool on the command line those
options can very likely get read by other users of your system
when they use 'ps' or other tools to list currently running pro‐
cesses.
To avoid this problem, never feed sensitive things to programs
using command line options. Write them to a protected file and
use the -K option to avoid this.
.netrc .netrc is a pretty handy file/feature that allows you to login
quickly and automatically to frequently visited sites. The file
contains passwords in clear text and is a real security risk. In
some cases, your .netrc is also stored in a home directory that
is NFS mounted or used on another network based file system, so
the clear text password will fly through your network every time
anyone reads that file!
To avoid this problem, don't use .netrc files and never store
passwords in plain text anywhere.
Clear Text Passwords
Many of the protocols libcurl supports send name and password
unencrypted as clear text (HTTP Basic authentication, FTP, TEL‐
NET etc). It is very easy for anyone on your network or a net‐
work nearby yours to just fire up a network analyzer tool and
eavesdrop on your passwords. Don't let the fact that HTTP Basic
uses base64 encoded passwords fool you. They may not look read‐
able at a first glance, but they very easily "deciphered" by
anyone within seconds.
To avoid this problem, use HTTP authentication methods or other
protocols that don't let snoopers see your password: HTTP with
Digest, NTLM or GSS authentication, HTTPS, FTPS, SCP, SFTP and
FTP-Kerberos are a few examples.
Redirects
The CURLOPT_FOLLOWLOCATION option automatically follows HTTP
redirects sent by a remote server. These redirects can refer to
any kind of URL, not just HTTP. A redirect to a file: URL would
cause the libcurl to read (or write) arbitrary files from the
local filesystem. If the application returns the data back to
the user (as would happen in some kinds of CGI scripts), an
attacker could leverage this to read otherwise forbidden data
(e.g. file://localhost/etc/passwd).
If authentication credentials are stored in the ~/.netrc file,
or Kerberos is in use, any other URL type (not just file:) that
requires authentication is also at risk. A redirect such as
ftp://some-internal-server/private-file would then return data
even when the server is password protected.
In the same way, if an unencrypted SSH private key has been con‐
figured for the user running the libcurl application, SCP: or
SFTP: URLs could access password or private-key protected
resources, e.g. sftp://user@some-internal-server/etc/passwd
The CURLOPT_REDIR_PROTOCOLS and CURLOPT_NETRC options can be
used to mitigate against this kind of attack.
A redirect can also specify a location available only on the
machine running libcurl, including servers hidden behind a fire‐
wall from the attacker. e.g. http://127.0.0.1/ or
http://intranet/delete-stuff.cgi?delete=all or tftp://bootp-
server/pc-config-data
Apps can mitigate against this by disabling CURLOPT_FOLLOWLOCA‐
TION and handling redirects itself, sanitizing URLs as neces‐
sary. Alternately, an app could leave CURLOPT_FOLLOWLOCATION
enabled but set CURLOPT_REDIR_PROTOCOLS and install a CUR‐
LOPT_OPENSOCKETFUNCTION callback function in which addresses are
sanitized before use.
Private Resources
A user who can control the DNS server of a domain being passed
in within a URL can change the address of the host to a local,
private address which a server-side libcurl-using application
could then use. e.g. the innocuous URL http://fuzzybunnies.exam‐
ple.com/ could actually resolve to the IP address of a server
behind a firewall, such as 127.0.0.1 or 10.1.2.3. Apps can mit‐
igate against this by setting a CURLOPT_OPENSOCKETFUNCTION and
checking the address before a connection.
All the malicious scenarios regarding redirected URLs apply just
as well to non-redirected URLs, if the user is allowed to spec‐
ify an arbitrary URL that could point to a private resource. For
example, a web app providing a translation service might happily
translate file://localhost/etc/passwd and display the result.
Apps can mitigate against this with the CURLOPT_PROTOCOLS option
as well as by similar mitigation techniques for redirections.
A malicious FTP server could in response to the PASV command
return an IP address and port number for a server local to the
app running libcurl but behind a firewall. Apps can mitigate
against this by using the CURLOPT_FTP_SKIP_PASV_IP option or
CURLOPT_FTPPORT.
IPv6 Addresses
libcurl will normally handle IPv6 addresses transparently and
just as easily as IPv4 addresses. That means that a sanitizing
function that filters out addressses like 127.0.0.1 isn't suffi‐
cient--the equivalent IPv6 addresses ::1, ::, 0:00::0:1,
::127.0.0.1 and ::ffff:7f00:1 supplied somehow by an attacker
would all bypass a naive filter and could allow access to unde‐
sired local resources. IPv6 also has special address blocks
like link-local and site-local that generally shouldn't be
accessed by a server-side libcurl-using application. A poorly-
configured firewall installed in a data center, organization or
server may also be configured to limit IPv4 connections but
leave IPv6 connections wide open. In some cases, the CURL_IPRE‐
SOLVE_V4 option can be used to limit resolved addresses to IPv4
only and bypass these issues.
Uploads
When uploading, a redirect can cause a local (or remote) file to
be overwritten. Apps must not allow any unsanitized URL to be
passed in for uploads. Also, CURLOPT_FOLLOWLOCATION should not
be used on uploads. Instead, the app should handle redirects
itself, sanitizing each URL first.
Authentication
Use of CURLOPT_UNRESTRICTED_AUTH could cause authentication
information to be sent to an unknown second server. Apps can
mitigate against this by disabling CURLOPT_FOLLOWLOCATION and
handling redirects itself, sanitizing where necessary.
Use of the CURLAUTH_ANY option to CURLOPT_HTTPAUTH could result
in user name and password being sent in clear text to an HTTP
server. Instead, use CURLAUTH_ANYSAFE which ensures that the
password is encrypted over the network, or else fail the
request.
Use of the CURLUSESSL_TRY option to CURLOPT_USE_SSL could result
in user name and password being sent in clear text to an FTP
server. Instead, use CURLUSESSL_CONTROL to ensure that an
encrypted connection is used or else fail the request.
Cookies
If cookies are enabled and cached, then a user could craft a URL
which performs some malicious action to a site whose authentica‐
tion is already stored in a cookie. e.g. http://mail.exam‐
ple.com/delete-stuff.cgi?delete=all Apps can mitigate against
this by disabling cookies or clearing them between requests.
Dangerous URLs
SCP URLs can contain raw commands within the scp: URL, which is
a side effect of how the SCP protocol is designed. e.g.
scp://user:pass@host/a;date >/tmp/test; Apps must not allow
unsanitized SCP: URLs to be passed in for downloads.
Denial of Service
A malicious server could cause libcurl to effectively hang by
sending a trickle of data through, or even no data at all but
just keeping the TCP connection open. This could result in a
denial-of-service attack. The CURLOPT_TIMEOUT and/or CUR‐
LOPT_LOW_SPEED_LIMIT options can be used to mitigate against
this.
A malicious server could cause libcurl to effectively hang by
starting to send data, then severing the connection without
cleanly closing the TCP connection. The app could install a
CURLOPT_SOCKOPTFUNCTION callback function and set the TCP
SO_KEEPALIVE option to mitigate against this. Setting one of
the timeout options would also work against this attack.
A malicious server could cause libcurl to download an infinite
amount of data, potentially causing all of memory or disk to be
filled. Setting the CURLOPT_MAXFILESIZE_LARGE option is not suf‐
ficient to guard against this. Instead, the app should monitor
the amount of data received within the write or progress call‐
back and abort once the limit is reached.
A malicious HTTP server could cause an infinite redirection
loop, causing a denial-of-service. This can be mitigated by
using the CURLOPT_MAXREDIRS option.
Arbitrary Headers
User-supplied data must be sanitized when used in options like
CURLOPT_USERAGENT, CURLOPT_HTTPHEADER, CURLOPT_POSTFIELDS and
others that are used to generate structured data. Characters
like embedded carriage returns or ampersands could allow the
user to create additional headers or fields that could cause
malicious transactions.
Server-supplied Names
A server can supply data which the application may, in some
cases, use as a file name. The curl command-line tool does this
with --remote-header-name, using the Content-disposition: header
to generate a file name. An application could also use
CURLINFO_EFFECTIVE_URL to generate a file name from a server-
supplied redirect URL. Special care must be taken to sanitize
such names to avoid the possibility of a malicious server sup‐
plying one like "/etc/passwd", "\autoexec.bat", "prn:" or even
".bashrc".
Server Certificates
A secure application should never use the CURLOPT_SSL_VERIFYPEER
option to disable certificate validation. There are numerous
attacks that are enabled by apps that fail to properly validate
server TLS/SSL certificates, thus enabling a malicious server to
spoof a legitimate one. HTTPS without validated certificates is
potentially as insecure as a plain HTTP connection.
Showing What You Do
On a related issue, be aware that even in situations like when
you have problems with libcurl and ask someone for help, every‐
thing you reveal in order to get best possible help might also
impose certain security related risks. Host names, user names,
paths, operating system specifics, etc. (not to mention pass‐
words of course) may in fact be used by intruders to gain addi‐
tional information of a potential target.
Be sure to limit access to application logs if they could hold
private or security-related data. Besides the obvious candi‐
dates like user names and passwords, things like URLs, cookies
or even file names could also hold sensitive data.
To avoid this problem, you must of course use your common sense.
Often, you can just edit out the sensitive data or just
search/replace your true information with faked data.
Multiple Transfers Using the multi Interface
The easy interface as described in detail in this document is a syn‐
chronous interface that transfers one file at a time and doesn't return
until it is done.
The multi interface, on the other hand, allows your program to transfer
multiple files in both directions at the same time, without forcing you
to use multiple threads. The name might make it seem that the multi
interface is for multi-threaded programs, but the truth is almost the
reverse. The multi interface can allow a single-threaded application
to perform the same kinds of multiple, simultaneous transfers that
multi-threaded programs can perform. It allows many of the benefits of
multi-threaded transfers without the complexity of managing and syn‐
chronizing many threads.
To use this interface, you are better off if you first understand the
basics of how to use the easy interface. The multi interface is simply
a way to make multiple transfers at the same time by adding up multiple
easy handles into a "multi stack".
You create the easy handles you want and you set all the options just
like you have been told above, and then you create a multi handle with
curl_multi_init(3) and add all those easy handles to that multi handle
with curl_multi_add_handle(3).
When you've added the handles you have for the moment (you can still
add new ones at any time), you start the transfers by calling
curl_multi_perform(3).
curl_multi_perform(3) is asynchronous. It will only execute as little
as possible and then return back control to your program. It is
designed to never block.
The best usage of this interface is when you do a select() on all pos‐
sible file descriptors or sockets to know when to call libcurl again.
This also makes it easy for you to wait and respond to actions on your
own application's sockets/handles. You figure out what to select() for
by using curl_multi_fdset(3), that fills in a set of fd_set variables
for you with the particular file descriptors libcurl uses for the
moment.
When you then call select(), it'll return when one of the file handles
signal action and you then call curl_multi_perform(3) to allow libcurl
to do what it wants to do. Take note that libcurl does also feature
some time-out code so we advise you to never use very long timeouts on
select() before you call curl_multi_perform(3), which thus should be
called unconditionally every now and then even if none of its file
descriptors have signaled ready. Another precaution you should use:
always call curl_multi_fdset(3) immediately before the select() call
since the current set of file descriptors may change when calling a
curl function.
If you want to stop the transfer of one of the easy handles in the
stack, you can use curl_multi_remove_handle(3) to remove individual
easy handles. Remember that easy handles should be
curl_easy_cleanup(3)ed.
When a transfer within the multi stack has finished, the counter of
running transfers (as filled in by curl_multi_perform(3)) will
decrease. When the number reaches zero, all transfers are done.
curl_multi_info_read(3) can be used to get information about completed
transfers. It then returns the CURLcode for each easy transfer, to
allow you to figure out success on each individual transfer.
SSL, Certificates and Other Tricks
[ seeding, passwords, keys, certificates, ENGINE, ca certs ]
Sharing Data Between Easy Handles
You can share some data between easy handles when the easy interface is
used, and some data is share automatically when you use the multi
interface.
When you add easy handles to a multi handle, these easy handles will
automatically share a lot of the data that otherwise would be kept on a
per-easy handle basis when the easy interface is used.
The DNS cache is shared between handles within a multi handle, making
subsequent name resolving faster, and the connection pool that is kept
to better allow persistent connections and connection re-use is also
shared. If you're using the easy interface, you can still share these
between specific easy handles by using the share interface, see
libcurl-share(3).
Some things are never shared automatically, not within multi handles,
like for example cookies so the only way to share that is with the
share interface.
Footnotes
[1] libcurl 7.10.3 and later have the ability to switch over to
chunked Transfer-Encoding in cases where HTTP uploads are done
with data of an unknown size.
[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 sim‐
ilar instruction that installs the library, header files, man
pages etc.
[4] This behavior was different in versions before 7.17.0, where
strings had to remain valid past the end of the
curl_easy_setopt(3) call.
libcurl 4 Mar 2009 libcurl-tutorial(3)
