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.\"

.TH libcurl-tutorial 3 "4 Mar 2009" "libcurl" "libcurl programming"

.SH NAME

libcurl-tutorial \- libcurl programming tutorial

.SH "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.

.SH "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.

.IP "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

.IP "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

.IP "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 supported 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-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 could be either on or off on

for different libcurls.

See also the "Features libcurl Provides" further down.

.IP "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.

.SH "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 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.

.SH "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 \fICURL_GLOBAL_ALL\fP will make it initialize all known

internal sub modules, and might be a good default option. The current two bits

that are specified are:

.RS

.IP "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 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.

.IP 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.

.RE

libcurl has a default protection mechanism that detects if

\fIcurl_global_init(3)\fP hasn't been called by the time

\fIcurl_easy_perform(3)\fP 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

\fIcurl_global_cleanup(3)\fP, which is the opposite of the init call. It will

then do the reversed operations to cleanup the resources the

\fIcurl_global_init(3)\fP call initialized.

Repeated calls to \fIcurl_global_init(3)\fP and \fIcurl_global_cleanup(3)\fP

should be avoided. They should only be called once each.

.SH "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

\fIcurl_version_info(3)\fP and checking out the details of the returned

struct, your program can figure out exactly what the currently running libcurl

supports.

.SH "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 handle. 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

\fIcurl_easy_setopt(3)\fP. 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 options you set in libcurl are "strings", pointers to data

terminated with a zero byte. When you set strings with

\fIcurl_easy_setopt(3)\fP, 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:

.nf

 curl_easy_setopt(handle, CURLOPT_URL, "http://domain.com/");

.fi

Let's assume for a while that you want to receive data as the URL identifies 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 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 application

and the function that gets invoked by libcurl. libcurl itself won't touch the

data you pass with \fICURLOPT_WRITEDATA\fP.

libcurl offers its own default internal callback that will take care of the data

if you don't set the callback with \fICURLOPT_WRITEFUNCTION\fP. 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 \fICURLOPT_WRITEDATA\fP 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 an open file with

\fICURLOPT_WRITEDATA\fP, it will crash. You should therefore avoid this to

make your program run fine virtually everywhere.

(\fICURLOPT_WRITEDATA\fP was formerly known as \fICURLOPT_FILE\fP. Both names

still work and do the same thing).

If you're using libcurl as a win32 DLL, you MUST use the

\fICURLOPT_WRITEFUNCTION\fP if you set \fICURLOPT_WRITEDATA\fP - 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);

\fIcurl_easy_perform(3)\fP 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.

.SH "Multi-threading Issues"

The first basic rule is that you must \fBnever\fP 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 handle 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 properly. For all

details, see this:

OpenSSL

 http://www.openssl.org/docs/crypto/threads.html#DESCRIPTION

GnuTLS

 http://www.gnu.org/software/gnutls/manual/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.

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.

.SH "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 1. It'll cause the library to spew out the entire protocol

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 appropriate RFC documents

at least briefly.

.SH "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_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]:

.nf

 /* in this example, file_size must be an curl_off_t variable */

 curl_easy_setopt(easyhandle, CURLOPT_INFILESIZE_LARGE, file_size);

.fi

When you call \fIcurl_easy_perform(3)\fP 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.

.SH "Passwords"

Many protocols use or even require that user name and password are provided

to be able to download or upload the data of your choice. libcurl offers

several ways to specify them.

Most protocols support that you specify the name and password in the URL

itself. libcurl will detect this and use them accordingly. This is written

like this:

 protocol://user:[email protected]/path/

If you need any odd letters in your user name or password, you should enter

them URL encoded, as %XX where XX is a two-digit hexadecimal number.

libcurl also provides options to set various passwords. The user name and

password as shown embedded in the URL can instead get set with the

CURLOPT_USERPWD option. The argument passed to libcurl should be a char * to

a string in the format "user:password". In a manner like this:

 curl_easy_setopt(easyhandle, CURLOPT_USERPWD, "myname:thesecret");

Another case where name and password might be needed at times, is for those

users who need to 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: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, 1L);

And a very basic example of how such a .netrc file may look like:

.nf

 machine myhost.mydomain.com

 login userlogin

 password secretword

.fi

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_KEYPASSWD, "keypassword");

.SH "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 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 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,

 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.

.SH "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 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:

.nf

    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! */

.fi

Simple enough, huh? Since you set the POST options with the

CURLOPT_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. 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.

.nf

 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 */

.fi

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 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 \fIcurl_formadd(3)\fP. 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.

.nf

 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);

.fi

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 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:

.nf

 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 */

.fi

Since all options on an easyhandle are "sticky", they remain the same until

changed even if you do call \fIcurl_easy_perform(3)\fP, 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!

.SH "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 CURLOPT_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:

.nf

 int progress_callback(void *clientp,

                       double dltotal,

                       double dlnow,

                       double ultotal,

                       double ulnow);

.fi

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.

.SH "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:

.nf

class AClass {

    static size_t write_data(void *ptr, size_t size, size_t nmemb,

                             void *ourpointer)

    {

      /* do what you want with the data */

    }

 }

.fi

.SH "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.

.IP "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: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.

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, CURLPROXY_SOCKS4);

.IP "Environment Variables"

libcurl automatically 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://][user:password@]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.

To explicitly disable libcurl's checking for and using the proxy environment

variables, set the proxy name to "" - an empty string - with CURLOPT_PROXY.

.IP "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).

.IP "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!

.IP "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 similar.

.SH "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 \fIcurl_easy_perform(3)\fP 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 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 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. 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 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 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.

.SH "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.

.IP "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.

.IP "Accept"

\&"*/*".

.IP "Expect"

When doing POST requests, libcurl sets this header to \&"100-continue" 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.

.SH "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.

.IP 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, "MYOWNREQUEST");

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.

.IP "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:

.nf

 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 */

.fi

\&... 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:

.nf

 headers = curl_slist_append(headers, "Accept: Agent-007");

 headers = curl_slist_append(headers, "Host: munged.host.line");

.fi

.IP "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.

.IP "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 automatically if the upload

data size is unknown.

.IP "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);

.IP "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:

.nf

 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 */

.fi

If you would instead want this operation (or chain of operations) to happen

_after_ the data transfer took place the option to \fIcurl_easy_setopt(3)\fP

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.

.IP "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.

.SH "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.