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About this Document
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 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.
To get the more details on all options and functions described herein, please
refer to their respective man pages.
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
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
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()
call initialized.
Repeated calls to curl_global_init() and curl_global_cleanup() should be
avoided. They should only 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.
You set properties and options for this handle using curl_easy_setopt(). They
control how the subsequent transfer or transfers 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 informationals you set in libcurl are "strings", pointers to data
terminated with a zero byte. Keep in mind that when you set strings with
curl_easy_setopt(), libcurl will not copy the data. It will merely point to
the data. You MUST make sure that the data remains available for libcurl to
use until finished or until you use the same option again to point to
something else.
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, "");
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
Multi-threading issues
libcurl is completely thread safe, except for two issues: signals and alarm
handlers. Signals are needed for a SIGPIPE handler, and the alarm() syscall
is used to catch timeouts (mostly during DNS lookup).
If you are accessing HTTPS or FTPS URLs in a multi-threaded manner, you are
then of course using OpenSSL multi-threaded and it has itself a few
requirements on this. Basicly, you need to provide one or two functions to
allow it to function properly. For all details, see this:
When using multiple threads you should first ignore SIGPIPE in your main
thread and set the CURLOPT_NOSIGNAL option to TRUE for all handles.
Everything will work fine except that timeouts are not honored during the DNS
lookup - which you can work around by building libcurl with ares-support.
Ares is a library that provides asynchronous name resolves. Unfortunately,
ares does not yet support IPv6.
For SIGPIPE info see the UNIX Socket FAQ at
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 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.
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 appropriate RFC documents
at least briefly.
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. 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 off_t variable */
curl_easy_setopt(easyhandle, CURLOPT_INFILESIZE_LARGE, 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.
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:
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");
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 Considerations"
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 protocols such as HTTP. To make
curl use this file, use the CURLOPT_NETRC option:
curl_easy_setopt(easyhandle, CURLOPT_NETRC, TRUE);
And a very basic example of how such a .netrc file may look like:
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 transfers.
To pass the known private key password to libcurl:
curl_easy_setopt(easyhandle, CURLOPT_SSLKEYPASSWD, "keypassword");
HTTP Authentication
The previous chapter showed how to set user name and password for getting
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 what way libcurl will (attempt to) use. 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 unsecure.
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
curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH, CURLAUTH_DIGEST);
And when you send authentication to a proxy, you can also set authentication
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,
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.
We get many questions regarding how to issue HTTP POSTs with libcurl the
proper way. This chapter will thus include examples using both different
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, "");
curl_easy_perform(easyhandle); /* post away! */
Simple enough, huh? Since you set the POST options with the
CURLOPT_POSTFIELDS, this automaticly 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 prevents 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. Setting 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, 23);
/* 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 multipart formposts. Multipart
formposts were introduced as a better way to post (possibly large) binary
data and was first documented in the RFC1867. They're called multipart
because they're built by a chain of parts, each being a single unit. Each
part has its own name and contents. You can in fact create and post a
multipart formpost 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(). 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 upload the whole thing.
struct curl_httppost *post=NULL;
struct curl_httppost *last=NULL;
curl_formadd(&post, &last,
curl_formadd(&post, &last,
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "logotype-image",
/* Set the form info */
curl_easy_setopt(easyhandle, CURLOPT_HTTPPOST, post);
curl_easy_perform(easyhandle); /* post away! */
/* free the post data again */
Multipart formposts are chains of parts using MIME-style separators and
headers. It means that each one of these separate parts get a few headers 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 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",
curl_easy_perform(easyhandle); /* post away! */
curl_formfree(post); /* free post */
curl_slist_free_all(post); /* 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(), you may need to tell curl to
go back to a plain GET request if you intend to do such a one as your next
request. You force an easyhandle to back to GET by using the CURLOPT_HTTPGET
curl_easy_setopt(easyhandle, CURLOPT_HTTPGET, TRUE);
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 presents a progress meter in your
Switch on the progress meter by, oddly enough, set CURLOPT_NOPROGRESS to
FALSE. This option is set to TRUE 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
CURLOPT_PROGRESSDATA. libcurl won't touch it.
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.
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 HTTP proxies. Network clients or
user-agents ask the proxy for docuements, the proxy does the actual request
and then it returns them.
libcurl has full support for HTTP proxies, so 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.
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
transparantly, 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 is using 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, "");
Some proxies require user authentication before allowing a request, and
you pass that information similar to this:
curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "user:password");
If you want to, you can specify the host name only in the CURLOPT_PROXY
option, and set the port number separately with CURLOPT_PROXYPORT.
Environment Variables
libcurl automaticly checks and uses a set of environment variables to know
what proxies to use for certain protocols. The names of the variables are
following an ancient de facto standard and are built up as
"[protocol]_proxy" (note the lower casing). 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://]machine[:port]". Where the protocol:// 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.
SSL and Proxies
SSL is for secure point-to-point connections. This involves strong
encryption and similar things, which effectivly 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
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 adminstrators of proxies and is
rarely allowed.
Tell libcurl to use proxy tunneling like this:
curl_easy_setopt(easyhandle, CURLOPT_HTTPPROXYTUNNEL, TRUE);
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 basicly 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 brower 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 interpreted, people have successfully used the
Mozilla javascript engine in the past.
- Ask your admins to stop this, for a static proxy setup or similar.
Persistancy 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() 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 connection! 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 drasticly
reduces re-connection time.
FTP connections that are kept alive saves a lot of time, as the command-
response roundtrips 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 previously
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. It 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.
When the connection cache gets filled, libcurl must close an existing
connection in order to get room for the new one. To know which connection to
close, libcurl uses a "close policy" that you can affect with the
CURLOPT_CLOSEPOLICY option. There's only two polices implemented as of this
writing (libcurl 7.9.4) and they are:
CURLCLOSEPOLICY_LEAST_RECENTLY_USED simply close the one that hasn't been
used for the longest time. This is the default behavior.
CURLCLOSEPOLICY_OLDEST closes the oldest connection, the one that was
createst the longest time ago.
There are, or at least were, plans to support a close policy that would call
a user-specified callback to let the user be able to decide which connection
to dump when this is necessary and therefor is the CURLOPT_CLOSEFUNCTION an
existing option still today. Nothing ever uses this though and this will not
be used within the forseeable future either.
To force your upcoming request to not use an already existing connection (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 TRUE. 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
HTTP Headers Used by libcurl
When you use libcurl to do HTTP requeests, it'll pass along a series of
headers automaticly. It might be good for you to know and understand these
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.
"no-cache". Tells a possible proxy to not grap a copy from the cache but
to fetch a fresh one.
"image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, */*". Cloned from a
browser once a hundred years ago.
When doing multi-part formposts, libcurl will set this header to
"100-continue" to ask the server for an "OK" message before it proceeds
with sending the data part of the post.
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 have 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.
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, "MYOWNRUQUEST");
When using the custom request, you change the request keyword of the
actual request you are performing. Thus, by default you make 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 are 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:");
Delete Headers
If you replace an existing header with one with no contents, you will
prevent the header from being sent. Like if you want to completely prevent
the "Accept:" header to be sent, you can disable it with code similar to
headers = curl_slist_append(headers, "Accept:");
Both replacing and cancelling 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-Encoding:
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
automaticly if the upload data size is unknown.
HTTP Version
There's only one aspect left in the HTTP requests that we haven't yet
mentioned how to modify: the version field. All HTTP requests includes the
version number to tell the server which version we support. libcurl speak
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,
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
comands 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 requires data interchange and thus needs
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 operations) to
happen _after_ the data transfer took place the option to
curl_easy_setopt() 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_FTP_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 true, 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.
If you do what 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 one to
update them. Server use cookies to "track" users and to keep "sessions".
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 CURLOPT_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 dynamicly save whatever
cookies the remote server passes to you, and make sure those cookies are then
use 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 header file to read cookies from with
The CURLOPT_COOKIEFILE option also automaticly 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 able might be to use a file name you know doesn't exist.
If you 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
automaticly find out what kind of file it is and act accordingly.
The perhaps 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() is called. This enabled 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 custom 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-masquarading 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
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
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 FALSE.
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 CURLOPT_HEADER to
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
Or, you can set an entirely separate function to receive the headers, by
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
"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
libcurl is in itself not insecure. If used the right way, you can use libcurl
to transfer data pretty safely.
There are of course many things to consider that may loosen up this
Command Lines
If you use a command line tool (such as curl) that uses libcurl, and you
give option 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 processes.
To avoid this problem, never feed sensitive things to programs using
command line options.
.netrc is a pretty handy file/feature that allows you to login quickly and
automaticly 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, TELNET etc). It is very
easy for anyone on your network or a network nearby yours, to just fire up
a network analyzer tool and evesdrop on your passwords. Don't let the fact
that HTTP uses base64 encoded passwords fool you. They may not look
readable at a first glance, but they very easily "deciphered" by anyone
within seconds.
To avoid this problem, use protocols that don't let snoopers see your
password: HTTPS, FTPS and FTP-kerberos are a few examples. HTTP Digest
authentication allows this too, but isn't supported by libcurl as of this
Showing What You Do
On a related issue, be aware that even in situations like when you have
problems with libcurl and ask somone for help, everything 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 passwords of course) may in fact be used by intruders to gain
additional information of a potential target.
To avoid this problem, you must of course use your common sense. Often,
you can just edit out the senstive data or just rearch/replace your true
information with faked data.
SSL, Certificates and Other Tricks
[ seeding, passwords, keys, certificates, ENGINE, ca certs ]
Multiple Transfers Using the multi Interface
The easy interface as described in detail in this document is a synchronous
interface that transfers one file at a time and doesn't return until its
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.
[fill in lots of more multi stuff here]
[ sharing between handles, mutexes, pipelining ]
[1] = libcurl 7.10.3 and later have the ability to switch over to chunked
Tranfer-Encoding in cases were 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
[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.