libcurl-tutorial(3) Introduction to Library Functions libcurl-tutorial(3)

NAME

libcurl-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
focuses on the C interface but should apply fairly well on other
language bindings as well as they usually follow the C API pretty
closely.

This document refers to 'the user' as the person writing the source
code that uses libcurl. That would probably be you or someone in your
position. What is generally referred to as 'the program' is 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
assumes a Unix style build process. If you use a different build
system, you can still read this to get general information that may
apply to your environment as well.

Compiling the Program
Your compiler needs to know where the libcurl headers are
located. Therefore you must set your compiler's include path
to point to the directory where you installed them. The
'curl-config'[3] tool can be used to get this information:
$ curl-config --cflags

Linking the Program with libcurl
When having compiled the program, you need to link your object
files to create a single executable. For that to succeed, you
need to link with libcurl and possibly also with other
libraries that libcurl itself depends on. Like 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
supported SSL library was detected properly at build-time,
libcurl is 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

If SSL is supported, the keyword SSL is written to stdout,
possibly together with a 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 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
environments.

You program libcurl the same way on all platforms that libcurl runs
on. There are only a few minor details that differ. If you just make
sure to write your code portable enough, you can create a portable
program. libcurl should not 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 makes 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 makes 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.

CURL_GLOBAL_SSL
which only does anything on libcurls compiled and built
SSL-enabled. On these systems, this makes 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) has not 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 good.

When the program no longer uses libcurl, it should call
curl_global_cleanup(3), which is the opposite of the init
call. It performs 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
runtime 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.

Two Interfaces
libcurl first introduced the so called easy interface. All operations
in the easy interface are prefixed with 'curl_easy'. The easy
interface lets you do single transfers with a synchronous and
blocking function call.

libcurl also offers another interface that allows multiple
simultaneous transfers in a single thread, the so called multi
interface. More about that interface is detailed in a separate
chapter further down. You still need to understand the easy interface
first, so please continue reading for better understanding.

Handle the Easy libcurl
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
handle = 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
transfers using this handle are made. Options remain set in the
handle until set again to something different. They are sticky.
Multiple requests using the same handle use the same options.

If you at any point would like to blank all previously set options
for a single easy handle, you can call curl_easy_reset(3) and you can
also make a clone of an easy handle (with all its set options) using
curl_easy_duphandle(3).

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 do not
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(3) in a manner
similar to:

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

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(handle, CURLOPT_WRITEFUNCTION, write_data);
You can control what data your callback function gets in the fourth
argument by setting another property:
curl_easy_setopt(handle, 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 does not touch the data you pass with CURLOPT_WRITEDATA(3).

libcurl offers its own default internal callback that takes care of
the data if you do not set the callback with
CURLOPT_WRITEFUNCTION(3). It simply outputs 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(3) option.

Now, we need to take a step back and take a deep breath. Here is one
of those rare platform-dependent nitpicks. Did you spot it? On some
platforms[2], libcurl is not able to operate on file handles opened
by the program. Therefore, if you use the default callback and pass
in an open file handle with CURLOPT_WRITEDATA(3), libcurl crashes.
You should avoid this to make your program run fine virtually
everywhere.

(CURLOPT_WRITEDATA(3) was formerly known as CURLOPT_FILE. Both names
still work and do the same thing).

If you are using libcurl as a Windows DLL, you MUST use the
CURLOPT_WRITEFUNCTION(3) if you set CURLOPT_WRITEDATA(3) - or
experience crashes.

There are of course many more options you can set, and we get back to
a few of them later. Let's instead continue to the actual transfer:

success = curl_easy_perform(handle);

curl_easy_perform(3) connects to the remote site, does the necessary
commands and performs 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 same amount of bytes that was passed to it, libcurl aborts
the operation and returns 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 is not enough for you, you can use the CURLOPT_ERRORBUFFER(3) to
point libcurl to a buffer of yours where it stores a human readable
error message as well.

If you then want to transfer another file, the handle is ready to be
used again. It is even preferred and encouraged that you reuse an
existing handle if you intend to make another transfer. libcurl then
attempts to reuse a 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 takes care of all the details needed to get the file
moved from one machine to another.

Multi-threading Issues
libcurl is thread safe but there are a few exceptions. Refer to
libcurl-thread(3) for more information.

When It does not Work
There are 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 is one golden rule when these things occur: set the
CURLOPT_VERBOSE(3) option to 1. it causes 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
are 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(3) set 1.

Of course, there are bugs left. We need to know about them to be able
to fix them, so we are 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(3)
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(3) is not enough, you increase the level of debug
data your application receive by using the CURLOPT_DEBUGFUNCTION(3).

Getting some in-depth knowledge about the protocols involved is never
wrong, and if you are trying to do funny things, you might 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 similar to
uploading data to an HTTP server with a PUT request.

Of course, first you either create an easy handle or you reuse one
existing one. Then you set the URL to operate on just like before.
This is the remote URL, that we now 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 passes 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 sizenitems* 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(handle, CURLOPT_READFUNCTION, read_function);

curl_easy_setopt(handle, CURLOPT_READDATA, &filedata);
Tell libcurl that we want to upload:
curl_easy_setopt(handle, CURLOPT_UPLOAD, 1L);
A few protocols do not 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(3) for all known file
sizes like this[1]:

/* in this example, file_size must be an curl_off_t variable */
curl_easy_setopt(handle, CURLOPT_INFILESIZE_LARGE, file_size);

When you call curl_easy_perform(3) this time, it performs all the
necessary operations and when it has invoked the upload it calls 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
signals the end of the upload.

Passwords
Many protocols use or even require that username 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 detects this and use them accordingly. This is
written like this:
protocol://user:password@example.com/path/
If you need any odd letters in your username 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 username
and password as shown embedded in the URL can instead get set with
the CURLOPT_USERPWD(3) 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(handle, 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(3).
It is used quite similar to the CURLOPT_USERPWD(3) option like this:

curl_easy_setopt(handle, CURLOPT_PROXYUSERPWD, "myname:thesecret");

There is a long time Unix "standard" way of storing FTP usernames and
passwords, namely in the $HOME/.netrc file (on Windows, libcurl also
checks the %USERPROFILE% environment variable if %HOME% is unset, and
tries "_netrc" as name). 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 username 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(3)
option:

curl_easy_setopt(handle, CURLOPT_NETRC, 1L);

A 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 is not
optional, like when you are using an SSL private key for secure
transfers.

To pass the known private key password to libcurl:
curl_easy_setopt(handle, CURLOPT_KEYPASSWD, "keypassword");

HTTP Authentication
The previous chapter showed how to set username 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 uses
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 (SPNEGO). You can tell libcurl which one to
use with CURLOPT_HTTPAUTH(3) as in:

curl_easy_setopt(handle, CURLOPT_HTTPAUTH, CURLAUTH_DIGEST);


When you send authentication to a proxy, you can also set
authentication type the same way but instead with
CURLOPT_PROXYAUTH(3):

curl_easy_setopt(handle, 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(handle, 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 picks 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 thus includes 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(handle, CURLOPT_POSTFIELDS, data);
curl_easy_setopt(handle, CURLOPT_URL, "http://posthere.com/");

curl_easy_perform(handle); /* post away! */

Simple enough, huh? Since you set the POST options with the
CURLOPT_POSTFIELDS(3), this automatically switches the handle to use
POST in the upcoming request.

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.

struct curl_slist *headers=NULL;
headers = curl_slist_append(headers, "Content-Type: text/xml");

/* post binary data */
curl_easy_setopt(handle, CURLOPT_POSTFIELDS, binaryptr);

/* set the size of the postfields data */
curl_easy_setopt(handle, CURLOPT_POSTFIELDSIZE, 23L);

/* pass our list of custom made headers */
curl_easy_setopt(handle, CURLOPT_HTTPHEADER, headers);

curl_easy_perform(handle); /* 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 do not 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
RFC 1867 (updated in RFC 2388). They are called multi-part because
they are 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 a MIME API consisting in
several functions: using those, you can create and fill a multi-part
form. Function curl_mime_init(3) creates a multi-part body; you can
then append new parts to a multi-part body using
curl_mime_addpart(3).

There are three possible data sources for a part: memory using
curl_mime_data(3), file using curl_mime_filedata(3) and user-defined
data read callback using curl_mime_data_cb(3). curl_mime_name(3) sets
a part's (i.e.: form field) name, while curl_mime_filename(3) fills
in the remote filename. With curl_mime_type(3), you can tell the MIME
type of a part, curl_mime_headers(3) allows defining the part's
headers. When a multi-part body is no longer needed, you can destroy
it using curl_mime_free(3).

The following example sets two simple text parts with plain textual
contents, and then a file with binary contents and uploads the whole
thing.

curl_mime *multipart = curl_mime_init(handle);
curl_mimepart *part = curl_mime_addpart(multipart);
curl_mime_name(part, "name");
curl_mime_data(part, "daniel", CURL_ZERO_TERMINATED);
part = curl_mime_addpart(multipart);
curl_mime_name(part, "project");
curl_mime_data(part, "curl", CURL_ZERO_TERMINATED);
part = curl_mime_addpart(multipart);
curl_mime_name(part, "logotype-image");
curl_mime_filedata(part, "curl.png");

/* Set the form info */
curl_easy_setopt(handle, CURLOPT_MIMEPOST, multipart);

curl_easy_perform(handle); /* post away! */

/* free the post data again */
curl_mime_free(multipart);

To post multiple files for a single form field, you must supply each
file in a separate part, all with the same field name. Although
function curl_mime_subparts(3) implements nested multi-parts, this
way of multiple files posting is deprecated by RFC 7578, chapter 4.3.

To set the data source from an already opened FILE pointer, use:

curl_mime_data_cb(part, filesize, (curl_read_callback) fread,
(curl_seek_callback) fseek, NULL, filepointer);

A deprecated curl_formadd(3) function is still supported in libcurl.
It should however not be used anymore for new designs and programs
using it ought to be converted to the MIME API. It is however
described here as an aid to conversion.

Using curl_formadd, you add parts to the form. When you are done
adding parts, you post the whole form.

The MIME API example above is expressed as follows using this
function:

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(handle, CURLOPT_HTTPPOST, post);

curl_easy_perform(handle); /* 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
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 shows 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(handle); /* post away! */

curl_formfree(post); /* free post */
curl_slist_free_all(headers); /* free custom header list */

Since all options on an easy handle 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 easy handle to go back to
GET by using the CURLOPT_HTTPGET(3) option:
curl_easy_setopt(handle, CURLOPT_HTTPGET, 1L);
Just setting CURLOPT_POSTFIELDS(3) to "" or NULL does not stop
libcurl from doing a POST. It just makes it POST without any data to
send!

Converting from deprecated form API to MIME API
Four rules have to be respected in building the multi-part:

- The easy handle must be created before building the multi-part.

- The multi-part is always created by a call to
curl_mime_init(handle).

- Each part is created by a call to curl_mime_addpart(multipart).

- When complete, the multi-part must be bound to the easy handle
using CURLOPT_MIMEPOST(3) instead of CURLOPT_HTTPPOST(3).

Here are some example of curl_formadd calls to MIME API sequences:

curl_formadd(&post, &last,
CURLFORM_COPYNAME, "id",
CURLFORM_COPYCONTENTS, "daniel", CURLFORM_END);
CURLFORM_CONTENTHEADER, headers,
CURLFORM_END);

becomes:
part = curl_mime_addpart(multipart);
curl_mime_name(part, "id");
curl_mime_data(part, "daniel", CURL_ZERO_TERMINATED);
curl_mime_headers(part, headers, FALSE);

Setting the last curl_mime_headers(3) argument to TRUE would have
caused the headers to be automatically released upon destroyed the
multi-part, thus saving a clean-up call to curl_slist_free_all(3).

curl_formadd(&post, &last,
CURLFORM_PTRNAME, "logotype-image",
CURLFORM_FILECONTENT, "-",
CURLFORM_END);

becomes:
part = curl_mime_addpart(multipart);
curl_mime_name(part, "logotype-image");
curl_mime_data_cb(part, (curl_off_t) -1, fread, fseek, NULL, stdin);

curl_mime_name(3) always copies the field name. The special filename
"-" is not supported by curl_mime_filename(3): to read an open file,
use a callback source using fread(). The transfer is be chunk-encoded
since the data size is unknown.

curl_formadd(&post, &last,
CURLFORM_COPYNAME, "datafile[]",
CURLFORM_FILE, "file1",
CURLFORM_FILE, "file2",
CURLFORM_END);

becomes:
part = curl_mime_addpart(multipart);
curl_mime_name(part, "datafile[]");
curl_mime_filedata(part, "file1");
part = curl_mime_addpart(multipart);
curl_mime_name(part, "datafile[]");
curl_mime_filedata(part, "file2");

The deprecated multipart/mixed implementation of multiple files field
is translated to two distinct parts with the same name.

curl_easy_setopt(handle, CURLOPT_READFUNCTION, myreadfunc);
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "stream",
CURLFORM_STREAM, arg,
CURLFORM_CONTENTLEN, (curl_off_t) datasize,
CURLFORM_FILENAME, "archive.zip",
CURLFORM_CONTENTTYPE, "application/zip",
CURLFORM_END);

becomes:
part = curl_mime_addpart(multipart);
curl_mime_name(part, "stream");
curl_mime_data_cb(part, (curl_off_t) datasize,
myreadfunc, NULL, NULL, arg);
curl_mime_filename(part, "archive.zip");
curl_mime_type(part, "application/zip");

CURLOPT_READFUNCTION(3) callback is not used: it is replace by
directly setting the part source data from the callback read
function.

curl_formadd(&post, &last,
CURLFORM_COPYNAME, "memfile",
CURLFORM_BUFFER, "memfile.bin",
CURLFORM_BUFFERPTR, databuffer,
CURLFORM_BUFFERLENGTH, (long) sizeof databuffer,
CURLFORM_END);

becomes:
part = curl_mime_addpart(multipart);
curl_mime_name(part, "memfile");
curl_mime_data(part, databuffer, (curl_off_t) sizeof databuffer);
curl_mime_filename(part, "memfile.bin");

curl_mime_data(3) always copies the initial data: data buffer is thus
free for immediate reuse.

curl_formadd(&post, &last,
CURLFORM_COPYNAME, "message",
CURLFORM_FILECONTENT, "msg.txt",
CURLFORM_END);

becomes:
part = curl_mime_addpart(multipart);
curl_mime_name(part, "message");
curl_mime_filedata(part, "msg.txt");
curl_mime_filename(part, NULL);

Use of curl_mime_filedata(3) sets the remote filename as a side
effect: it is therefore necessary to clear it for
CURLFORM_FILECONTENT emulation.

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(3) 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 is then called on
irregular intervals with information about the current transfer.

Set the progress callback by using CURLOPT_PROGRESSFUNCTION(3). 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 is provided. The first
argument, the 'clientp' is the pointer you pass to libcurl with
CURLOPT_PROGRESSDATA(3). libcurl does not touch it.

libcurl with C++
There is 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 asks the proxy for it instead of trying to connect to the
actual remote host identified in the URL.

If you are using a SOCKS proxy, you may find that libcurl does not
quite support all operations through it.

For HTTP proxies: the fact that the proxy is an HTTP proxy puts
certain restrictions on what can actually happen. A requested URL
that might not be a HTTP URL is 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 important to
understand that all operations over an HTTP proxy use the HTTP
protocol. For example, you cannot 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(handle, 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(handle, CURLOPT_PROXYUSERPWD, "user:password");
If you want to, you can specify the hostname only in the
CURLOPT_PROXY(3) option, and set the port number separately
with CURLOPT_PROXYPORT(3).

Tell libcurl what kind of proxy it is with
CURLOPT_PROXYTYPE(3) (if not, it defaults to assuming an HTTP
proxy):
curl_easy_setopt(handle, CURLOPT_PROXYTYPE, CURLPROXY_SOCKS4);

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 old tradition 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 specifies which type of proxy it is, and
the optional port number specifies on which port the proxy
operates. If not specified, the internal default port number
is 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 was not 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(3).

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 an HTTP proxy is to ask the
proxy to tunnel everything through without being able to check
or fiddle with the traffic.

Opening an SSL connection over an HTTP proxy is therefore 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 dear 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 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 an 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(handle, 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 does not stand in the way for such
innovative actions either!

Proxy Auto-Config
Netscape first came up with this. It is basically a webpage
(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 does not 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.

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 keeps 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, 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 do not 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 attempts 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(3) option. Default is 5.
There is rarely 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, you can do that by setting CURLOPT_FRESH_CONNECT(3) to 1.
In a similar spirit, you can also forbid the upcoming request to be
"lying" around and possibly get reused after the request by setting
CURLOPT_FORBID_REUSE(3) to 1.

HTTP Headers Used by libcurl
When you use libcurl to do HTTP requests, it passes 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(3) 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-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 does 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.

CURLOPT_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(3) is there for you. It is simple to
use:

curl_easy_setopt(handle, 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 are the boss.

Modify Headers
HTTP-like protocols pass a series of headers to the server
when doing the request, and you are 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(handle, CURLOPT_HTTPHEADER, headers);

curl_easy_perform(handle); /* 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: do not 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 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-Encoding: chunked" when doing a non-GET HTTP
operation, libcurl switches 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 old servers do not 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(handle, 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 an FTP server means that you need
to send the commands exactly as the FTP server expects them
(RFC 959 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 judgment. Also
be aware that libcurl does its 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(handle, CURLOPT_QUOTE, headers);

curl_easy_perform(handle); /* 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(3) would instead be called
CURLOPT_POSTQUOTE(3) and used the exact same way.

The custom FTP commands are 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 are
issued and libcurl bails out with an error code
(CURLE_QUOTE_ERROR). Note that if you use CURLOPT_QUOTE(3) to
send commands before a transfer, no transfer actually takes
place when a quote command has failed.

If you set the CURLOPT_HEADER(3) to 1, you tell libcurl to get
information about the target file and output "headers" about
it. The headers are 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(3). If this option is
set, no actual file content transfer is performed.

FTP Custom CURLOPT_CUSTOMREQUEST
If you do want to list the contents of an FTP directory using
your own defined FTP command, CURLOPT_CUSTOMREQUEST(3) does
just that. "NLST" is the default one for listing directories
but you are 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 has not 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
"sessions".

Cookies are sent from server to clients with the header Set-Cookie:
and they are sent from clients to servers with the Cookie: header.

To just send whatever cookie you want to a server, you can use
CURLOPT_COOKIE(3) to set a cookie string like this:

curl_easy_setopt(handle, 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(3).

The CURLOPT_COOKIEFILE(3) option also automatically enables the
cookie parser in libcurl. Until the cookie parser is enabled, libcurl
does not parse or understand incoming cookies and they are just be
ignored. However, when the parser is enabled the cookies are
understood and the cookies are 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(3) does not 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 does
not exist.

If you would rather use existing cookies that you have previously
received with your Netscape or Mozilla browsers, you can make libcurl
use that cookie file as input. The CURLOPT_COOKIEFILE(3) is used for
that too, as libcurl automatically finds out what kind of file it is
and acts 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
filename with CURLOPT_COOKIEJAR(3), that filename is created and all
received cookies get 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
detail comes 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 does not 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(3) to zero.

In some cases, you want 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(3) option. If you set it to "-", libcurl uses your
system's "default IP address". If you want to use a particular IP,
you can set the full IP address, a hostname to resolve to an IP
address or even a local network interface name that libcurl gets the
IP address from.

When doing the "PORT" approach, libcurl attempts 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(3) to zero.

MIME API revisited for SMTP and IMAP
In addition to support HTTP multi-part form fields, the MIME API can
be used to build structured email messages and send them via SMTP or
append such messages to IMAP directories.

A structured email message may contain several parts: some are
displayed inline by the MUA, some are attachments. Parts can also be
structured as multi-part, for example to include another email
message or to offer several text formats alternatives. This can be
nested to any level.

To build such a message, you prepare the nth-level multi-part and
then include it as a source to the parent multi-part using function
curl_mime_subparts(3). Once it has been bound to its parent
multi-part, a nth-level multi-part belongs to it and should not be
freed explicitly.

Email messages data is not supposed to be non-ASCII and line length
is limited: fortunately, some transfer encodings are defined by the
standards to support the transmission of such incompatible data.
Function curl_mime_encoder(3) tells a part that its source data must
be encoded before being sent. It also generates the corresponding
header for that part. If the part data you want to send is already
encoded in such a scheme, do not use this function (this would
over-encode it), but explicitly set the corresponding part header.

Upon sending such a message, libcurl prepends it with the header list
set with CURLOPT_HTTPHEADER(3), as zero level mime part headers.

Here is an example building an email message with an inline
plain/html text alternative and a file attachment encoded in base64:

curl_mime *message = curl_mime_init(handle);

/* The inline part is an alternative proposing the html and the text
versions of the email. */
curl_mime *alt = curl_mime_init(handle);

/* HTML message. */
curl_mimepart *part = curl_mime_addpart(alt);
curl_mime_data(part, "<html><body><p>This is HTML</p></body></html>",
CURL_ZERO_TERMINATED);
curl_mime_type(part, "text/html");

/* Text message. */
part = curl_mime_addpart(alt);
curl_mime_data(part, "This is plain text message",
CURL_ZERO_TERMINATED);

/* Create the inline part. */
part = curl_mime_addpart(message);
curl_mime_subparts(part, alt);
curl_mime_type(part, "multipart/alternative");
struct curl_slist *headers = curl_slist_append(NULL,
"Content-Disposition: inline");
curl_mime_headers(part, headers, TRUE);

/* Add the attachment. */
part = curl_mime_addpart(message);
curl_mime_filedata(part, "manual.pdf");
curl_mime_encoder(part, "base64");

/* Build the mail headers. */
headers = curl_slist_append(NULL, "From: me@example.com");
headers = curl_slist_append(headers, "To: you@example.com");

/* Set these into the easy handle. */
curl_easy_setopt(handle, CURLOPT_HTTPHEADER, headers);
curl_easy_setopt(handle, CURLOPT_MIMEPOST, mime);

It should be noted that appending a message to an IMAP directory
requires the message size to be known prior upload. It is therefore
not possible to include parts with unknown data size in this context.

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(3) 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_HEADERDATA(3).

Or, you can set an entirely separate function to receive the headers,
by using CURLOPT_HEADERFUNCTION(3).

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
are not actually true headers, but in this case we pretend they are!
;-)

Post Transfer Information
See curl_easy_getinfo(3).

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 does not
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 allows 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 synchronizing many threads.

To complicate matters somewhat more, there are even two versions of
the multi interface. The event based one, also called multi_socket
and the "normal one" designed for using with select(). See the
libcurl-multi.3 man page for details on the multi_socket event based
API, this description here is for the select() oriented one.

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, one for each concurrent
transfer, and you set all the options just like you learned 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 have 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 only performs what can be
done now and then return control to your program. It is designed to
never block. You need to keep calling the function until all
transfers are completed.

The best usage of this interface is when you do a select() on all
possible 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 returns 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 long
timeouts on select() before you call curl_multi_perform(3) again.
curl_multi_timeout(3) is provided to help you get a suitable timeout
period.

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 in any curl function invoke.

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)) decreases.
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
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 reuse is
also shared. If you are 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
similar 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.

SEE ALSO

libcurl-easy(3), libcurl-errors(3), libcurl-multi(3), libcurl-url(3)

libcurl 2025-02-25 libcurl-tutorial(3)