.\" * | (__| |_| | _ <| |___
.\" * \___|\___/|_| \_\_____|
.\" *
-.\" * Copyright (C) 1998 - 2007, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" * Copyright (C) 1998 - 2014, Daniel Stenberg, <daniel@haxx.se>, et al.
.\" *
.\" * This software is licensed as described in the file COPYING, which
.\" * you should have received as part of this distribution. The terms
.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
.\" * KIND, either express or implied.
.\" *
-.\" * $Id$
.\" **************************************************************************
.\"
-.TH libcurl-tutorial 3 "27 Feb 2007" "libcurl" "libcurl programming"
+.TH libcurl-tutorial 3 "19 Sep 2014" "libcurl" "libcurl programming"
.SH NAME
libcurl-tutorial \- libcurl programming tutorial
.SH "Objective"
source code that you write that is using libcurl for transfers. The program
is outside libcurl and libcurl is outside of the program.
-To get the more details on all options and functions described herein, please
+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
+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"
$ curl-config --feature
And if SSL is supported, the keyword 'SSL' will be written to stdout,
-possibly together with a few other features that can be on and off on
-different libcurls.
+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"
on a large amount of different operating systems and environments.
You program libcurl the same way on all platforms that libcurl runs on. There
-are only very few minor considerations that differs. If you just make sure to
+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.
.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 is only needed to do once for each application so if your
+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
struct, your program can figure out exactly what the currently running libcurl
supports.
-.SH "Handle the Easy libcurl"
+.SH "Two Interfaces"
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.
-
+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.
+.SH "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
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.
+something different. They are sticky. Multiple requests using the same handle
+will 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 \fIcurl_easy_reset(3)\fP and you can also
+make a clone of an easy handle (with all its set options) using
+\fIcurl_easy_duphandle(3)\fP.
Many of the options you set in libcurl are "strings", pointers to data
-terminated with a zero byte. Keep in mind that when you set strings with
-\fIcurl_easy_setopt(3)\fP, libcurl will not copy the data. It will merely
-point to the data. You MUST make sure that the data remains available for
-libcurl to use until finished or until you use the same option again to point
-to something else.
+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:
+One of the most basic properties to set in the handle is the URL. You set your
+preferred URL to transfer with \fICURLOPT_URL(3)\fP in a manner similar to:
.nf
curl_easy_setopt(handle, CURLOPT_URL, "http://domain.com/");
curl_easy_setopt(easyhandle, CURLOPT_WRITEFUNCTION, write_data);
-You can control what data your function get in the forth argument by setting
-another property:
+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.
+data you pass with \fICURLOPT_WRITEDATA(3)\fP.
-libcurl offers its own default internal callback that'll 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.
+libcurl offers its own default internal callback that will take care of the
+data if you don't set the callback with \fICURLOPT_WRITEFUNCTION(3)\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(3)\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
+\fICURLOPT_WRITEDATA(3)\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).
+(\fICURLOPT_WRITEDATA(3)\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.
+\fICURLOPT_WRITEFUNCTION(3)\fP if you set \fICURLOPT_WRITEDATA(3)\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:
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.
+you, you can use the \fICURLOPT_ERRORBUFFER(3)\fP 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 share a libcurl handle (be
-it easy or multi or whatever) between multiple threads. Only use one handle in
-one thread at a time.
+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 timeouting name resolves (during DNS lookup) - when
-built without c-ares support and not on Windows..
+handlers. Signals are used for timing out name resolves (during DNS lookup) -
+when built without using either the c-ares or threaded resolver backends.
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
GnuTLS
- http://www.gnu.org/software/gnutls/manual/html_node/Multi_002dthreaded-applications.html
+ http://gnutls.org/manual/html_node/Thread-safety.html
NSS
-
- is claimed to be thread-safe already without anything required
+
+ is claimed to be thread-safe already without anything required.
+
+PolarSSL
+
+ Required actions unknown.
yassl
- Required actions unknown
+ 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. Unfortunately, c-ares does not yet fully support IPv6. On some
-platforms, libcurl simply will not function properly multi-threaded unless
-this option is set.
+Secure Transport
-Also, note that CURLOPT_DNS_USE_GLOBAL_CACHE is not thread-safe.
+ The engine is fully thread-safe, and no additional steps are required.
+
+When using multiple threads you should set the \fICURLOPT_NOSIGNAL(3)\fP
+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 \fICURLOPT_DNS_USE_GLOBAL_CACHE(3)\fP is not thread-safe.
.SH "When It Doesn't Work"
There will always be times when the transfer fails for some reason. You might
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.
+There's one golden rule when these things occur: set the
+\fICURLOPT_VERBOSE(3)\fP 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 \fICURLOPT_HEADER(3)\fP set 1.
-Of course there are bugs left. We need to get to know about them to be able
-to fix them, so we're quite dependent on your bug reports! When you do report
-suspected bugs in libcurl, please include as much details you possibly can: a
-protocol dump that CURLOPT_VERBOSE produces, library version, as much as
-possible of your code that uses libcurl, operating system name and version,
-compiler name and version etc.
+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 \fICURLOPT_VERBOSE(3)\fP 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.
+If \fICURLOPT_VERBOSE(3)\fP is not enough, you increase the level of debug
+data your application receive by using the \fICURLOPT_DEBUGFUNCTION(3)\fP.
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
curl_easy_setopt(easyhandle, CURLOPT_READFUNCTION, read_function);
- curl_easy_setopt(easyhandle, CURLOPT_INFILE, &filedata);
+ curl_easy_setopt(easyhandle, CURLOPT_READDATA, &filedata);
Tell libcurl that we want to upload:
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]:
+\fICURLOPT_INFILESIZE_LARGE(3)\fP for all known file sizes like this[1]:
.nf
- /* in this example, file_size must be an off_t variable */
+ /* in this example, file_size must be an curl_off_t variable */
curl_easy_setopt(easyhandle, CURLOPT_INFILESIZE_LARGE, file_size);
.fi
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:
+\fICURLOPT_USERPWD(3)\fP 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:
+another option for this, the \fICURLOPT_PROXYUSERPWD(3)\fP. It is used quite
+similar to the \fICURLOPT_USERPWD(3)\fP 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
+
+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 use this file, use the \fICURLOPT_NETRC(3)\fP option:
curl_easy_setopt(easyhandle, CURLOPT_NETRC, 1L);
The previous chapter showed how to set user name and password for getting
URLs that require authentication. When using the HTTP protocol, there are
many different ways a client can provide those credentials to the server and
-you can control what way libcurl will (attempt to) use. The default HTTP
+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:
+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 \fICURLOPT_HTTPAUTH(3)\fP 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:
+type the same way but instead with \fICURLOPT_PROXYAUTH(3)\fP:
curl_easy_setopt(easyhandle, CURLOPT_PROXYAUTH, CURLAUTH_NTLM);
.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.
+\fICURLOPT_POSTFIELDS(3)\fP, 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 prevents libcurl from
+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
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 was first documented in the RFC1867. They're called multi-part because
-they're built by a chain of parts, each being a single unit. Each part has its
-own name and contents. You can in fact create and post a 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.
+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 upload the whole thing.
+and then a file with binary contents and uploads the whole thing.
.nf
struct curl_httppost *post=NULL;
that describe the individual content-type, size etc. To enable your
application to handicraft this formpost even more, libcurl allows you to
supply your own set of custom headers to such an individual form part. You can
-of course supply headers to as many parts you like, but this little example
+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:
curl_easy_perform(easyhandle); /* post away! */
curl_formfree(post); /* free post */
- curl_slist_free_all(post); /* free custom header list */
+ 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 such a one as your
-next request. You force an easyhandle to back to GET by using the
-CURLOPT_HTTPGET option:
+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
+\fICURLOPT_HTTPGET(3)\fP 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!
+Just setting \fICURLOPT_POSTFIELDS(3)\fP 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 presents a progress meter in your
+that can be switched on and then makes it present a progress meter in your
terminal.
-Switch on the progress meter by, oddly enough, set CURLOPT_NOPROGRESS to
-zero. This option is set to 1 by default.
+Switch on the progress meter by, oddly enough, setting
+\fICURLOPT_NOPROGRESS(3)\fP 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:
+Set the progress callback by using \fICURLOPT_PROGRESSFUNCTION(3)\fP. And pass
+a pointer to a function that matches this prototype:
.nf
int progress_callback(void *clientp,
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.
+\fICURLOPT_PROGRESSDATA(3)\fP. libcurl won't touch it.
.SH "libcurl with C++"
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 is using the HTTP protocol. For example, you
+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.
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.
+If you want to, you can specify the host name only in the
+\fICURLOPT_PROXY(3)\fP option, and set the port number separately with
+\fICURLOPT_PROXYPORT(3)\fP.
-Tell libcurl what kind of proxy it is with CURLOPT_PROXYTYPE (if not, it will
-default to assume a HTTP proxy):
+Tell libcurl what kind of proxy it is with \fICURLOPT_PROXYTYPE(3)\fP (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.
+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
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.
+variables, set the proxy name to "" - an empty string - with
+\fICURLOPT_PROXY(3)\fP.
.IP "SSL and Proxies"
SSL is for secure point-to-point connections. This involves strong encryption
.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
+\&.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
+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
+- 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.
+- Read the Javascript code and rewrite the same logic in another language.
-- Implement a javascript interpreted, people have successfully used the
-Mozilla javascript engine in the past.
+- 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.
host again, will benefit from libcurl's session ID cache that drastically
reduces re-connection time.
-FTP connections that are kept alive saves a lot of time, as the command-
+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.
Each easy handle will attempt to keep the last few connections alive for a
while in case they are to be used again. You can set the size of this "cache"
-with the CURLOPT_MAXCONNECTS option. Default is 5. It is very seldom any
-point in changing this value, and if you think of changing this it is often
-just a matter of thinking again.
+with the \fICURLOPT_MAXCONNECTS(3)\fP 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.
+you're about to operate on), you can do that by setting
+\fICURLOPT_FRESH_CONNECT(3)\fP 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 \fICURLOPT_FORBID_REUSE(3)\fP 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 ones. You
-can replace or remove them by using the CURLOPT_HTTPHEADER option.
+automatically. It might be good for you to know and understand these. You
+can replace or remove them by using the \fICURLOPT_HTTPHEADER(3)\fP 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 "Pragma"
-\&"no-cache". Tells a possible proxy to not grab a copy from the cache but to
-fetch a fresh one.
-
.IP "Accept"
\&"*/*".
.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 have excellent proxy-support.
+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...)
.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:
+GET, HEAD or POST is not good enough for you, \fICURLOPT_CUSTOMREQUEST(3)\fP
+is there for you. It is very simple to use:
- curl_easy_setopt(easyhandle, CURLOPT_CUSTOMREQUEST, "MYOWNRUQUEST");
+ 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 GET request but you can
+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 are this easy:
+think fit. Adding headers is this easy:
.nf
struct curl_slist *headers=NULL; /* init to NULL is important */
.IP "Delete Headers"
If you replace an existing header with one with no contents, you will prevent
-the header from being sent. Like if you want to completely prevent the
-\&"Accept:" header to be sent, you can disable it with code similar to this:
+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:");
.IP "HTTP Version"
All HTTP requests includes the version number to tell the server which version
-we support. libcurl speak HTTP 1.1 by default. Some very old servers don't
+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:
.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.
+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 requires data interchange and thus needs
-a data-connection must be left to libcurl's own judgment. Also be aware
+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
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.
+would instead be called \fICURLOPT_POSTQUOTE(3)\fP 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.
+error code (CURLE_QUOTE_ERROR). Note that if you use \fICURLOPT_QUOTE(3)\fP 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
+If you set the \fICURLOPT_HEADER(3)\fP 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.
+combine with \fICURLOPT_NOBODY(3)\fP. If this option is set, no actual file
+content transfer will be performed.
.IP "FTP Custom CUSTOMREQUEST"
-If you do what list the contents of a FTP directory using your own defined FTP
-command, CURLOPT_CUSTOMREQUEST will do just that. "NLST" is the default one
-for listing directories but you're free to pass in your idea of a good
-alternative.
+If you do want to list the contents of a FTP directory using your own defined
+FTP command, \fICURLOPT_CUSTOMREQUEST(3)\fP 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
set. The conditions include that the domain name and path match and that the
cookie hasn't become too old.
-In real-world cases, servers send new cookies to replace existing one to
+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're sent from clients to servers with the Cookie: header.
-To just send whatever cookie you want to a server, you can use CURLOPT_COOKIE
-to set a cookie string like this:
+To just send whatever cookie you want to a server, you can use
+\fICURLOPT_COOKIE(3)\fP to set a cookie string like this:
curl_easy_setopt(easyhandle, CURLOPT_COOKIE, "name1=var1; name2=var2;");
In many cases, that is not enough. You might want to dynamically save
whatever cookies the remote server passes to you, and make sure those cookies
-are then use accordingly on later requests.
+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 header file to read cookies from with
-CURLOPT_COOKIEFILE.
-
-The CURLOPT_COOKIEFILE option also automatically enables the cookie parser in
-libcurl. Until the cookie parser is enabled, libcurl will not parse or
-understand incoming cookies and they will just be ignored. However, when the
-parser is enabled the cookies will be understood and the cookies will be kept
-in memory and used properly in subsequent requests when the same handle is
-used. Many times this is enough, and you may not have to save the cookies to
-disk at all. Note that the file you specify to CURLOPT_COOKIEFILE doesn't
-have to exist to enable the parser, so a common way to just enable the parser
-and not read able might be to use a file name you know doesn't exist.
-
-If you rather use existing cookies that you've previously received with your
-Netscape or Mozilla browsers, you can make libcurl use that cookie file as
-input. The CURLOPT_COOKIEFILE is used for that too, as libcurl will
-automatically find out what kind of file it is and act accordingly.
-
-The perhaps most advanced cookie operation libcurl offers, is saving the
+figure out which cookies to use. Set the header file to read cookies from with
+\fICURLOPT_COOKIEFILE(3)\fP.
+
+The \fICURLOPT_COOKIEFILE(3)\fP option also automatically enables the cookie
+parser in libcurl. Until the cookie parser is enabled, libcurl will not parse
+or understand incoming cookies and they will just be ignored. However, when
+the parser is enabled the cookies will be understood and the cookies will be
+kept in memory and used properly in subsequent requests when the same handle
+is used. Many times this is enough, and you may not have to save the cookies
+to disk at all. Note that the file you specify to \ICURLOPT_COOKIEFILE(3)\fP
+doesn't have to exist to enable the parser, so a common way to just enable the
+parser and not read any cookies is to use the name of a file you know doesn't
+exist.
+
+If you would rather use existing cookies that you've previously received with
+your Netscape or Mozilla browsers, you can make libcurl use that cookie file
+as input. The \fICURLOPT_COOKIEFILE(3)\fP is used for that too, as libcurl
+will automatically find out what kind of file it is and act accordingly.
+
+Perhaps the most advanced cookie operation libcurl offers, is saving the
entire internal cookie state back into a Netscape/Mozilla formatted cookie
file. We call that the cookie-jar. When you set a file name with
-CURLOPT_COOKIEJAR, that file name will be created and all received cookies
-will be stored in it when \fIcurl_easy_cleanup(3)\fP is called. This enabled
-cookies to get passed on properly between multiple handles without any
+\fICURLOPT_COOKIEJAR(3)\fP, that file name will be created and all received
+cookies will be stored in it when \fIcurl_easy_cleanup(3)\fP is called. This
+enables cookies to get passed on properly between multiple handles without any
information getting lost.
.SH "FTP Peculiarities We Need"
FTP transfers use a second TCP/IP connection for the data transfer. This is
usually a fact you can forget and ignore but at times this fact will come
-back to haunt you. libcurl offers several different ways to custom how the
+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
and does not exist nor work on all FTP servers.)
You can prevent libcurl from first trying the EPSV command by setting
-CURLOPT_FTP_USE_EPSV to zero.
+\fICURLOPT_FTP_USE_EPSV(3)\fP to zero.
In some cases, you will prefer to have the server connect back to you for the
second connection. This might be when the server is perhaps behind a firewall
or something and only allows connections on a single port. libcurl then
informs the remote server which IP address and port number to connect to.
-This is made with the CURLOPT_FTPPORT option. If you set it to "-", libcurl
-will use your system's "default IP address". If you want to use a particular
-IP, you can set the full IP address, a host name to resolve to an IP address
-or even a local network interface name that libcurl will get the IP address
-from.
+This is made with the \fICURLOPT_FTPPORT(3)\fP option. If you set it to "-",
+libcurl will use your system's "default IP address". If you want to use a
+particular IP, you can set the full IP address, a host name to resolve to an
+IP address or even a local network interface name that libcurl will get the IP
+address from.
When doing the "PORT" approach, libcurl will attempt to use the EPRT and the
LPRT before trying PORT, as they work with more protocols. You can disable
-this behavior by setting CURLOPT_FTP_USE_EPRT to zero.
+this behavior by setting \fICURLOPT_FTP_USE_EPRT(3)\fP to zero.
.SH "Headers Equal Fun"
Some protocols provide "headers", meta-data separated from the normal
-data. These headers are by default not included in the normal data stream,
-but you can make them appear in the data stream by setting CURLOPT_HEADER to
-1.
+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 \fICURLOPT_HEADER(3)\fP
+to 1.
What might be even more useful, is libcurl's ability to separate the headers
from the data and thus make the callbacks differ. You can for example set a
different pointer to pass to the ordinary write callback by setting
-CURLOPT_WRITEHEADER.
+\fICURLOPT_HEADERDATA(3)\fP.
-Or, you can set an entirely separate function to receive the headers, by
-using CURLOPT_HEADERFUNCTION.
+Or, you can set an entirely separate function to receive the headers, by using
+\fICURLOPT_HEADERFUNCTION(3)\fP.
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
.SH "Security Considerations"
-libcurl is in itself not insecure. If used the right way, you can use libcurl
-to transfer data pretty safely.
-
-There are of course many things to consider that may loosen up this
-situation:
+The libcurl project takes security seriously. The library is written with
+caution and precautions are taken to mitigate many kinds of risks encountered
+while operating with potentially malicious servers on the Internet. It is a
+powerful library, however, which allows application writers to make trade offs
+between ease of writing and exposure to potential risky operations. If
+used the right way, you can use libcurl to transfer data pretty safely.
+
+Many applications are used in closed networks where users and servers
+can be trusted, but many others are used on arbitrary servers and are fed
+input from potentially untrusted users. Following is a discussion about
+some risks in the ways in which applications commonly use libcurl and
+potential mitigations of those risks. It is by no means comprehensive, but
+shows classes of attacks that robust applications should consider. The
+Common Weakness Enumeration project at http://cwe.mitre.org/ is a good
+reference for many of these and similar types of weaknesses of which
+application writers should be aware.
.IP "Command Lines"
If you use a command line tool (such as curl) that uses libcurl, and you give
-option to the tool on the command line those options can very likely get read
+options to the tool on the command line those options can very likely get read
by other users of your system when they use 'ps' or other tools to list
currently running processes.
To avoid this problem, never feed sensitive things to programs using command
-line options.
+line options. Write them to a protected file and use the \-K option to
+avoid this.
.IP ".netrc"
\&.netrc is a pretty handy file/feature that allows you to login quickly and
.IP "Clear Text Passwords"
Many of the protocols libcurl supports send name and password unencrypted as
clear text (HTTP Basic authentication, FTP, TELNET etc). It is very easy for
-anyone on your network or a network nearby yours, to just fire up a network
+anyone on your network or a network nearby yours to just fire up a network
analyzer tool and eavesdrop on your passwords. Don't let the fact that HTTP
-uses base64 encoded passwords fool you. They may not look readable at a first
-glance, but they very easily "deciphered" by anyone within seconds.
-
-To avoid this problem, use protocols that don't let snoopers see your
-password: HTTPS, FTPS and FTP-kerberos are a few examples. HTTP Digest
-authentication allows this too, but isn't supported by libcurl as of this
-writing.
+Basic uses base64 encoded passwords fool you. They may not look readable at a
+first glance, but they very easily "deciphered" by anyone within seconds.
+
+To avoid this problem, use an authentication mechanism or other protocol that
+doesn't let snoopers see your password: Digest, CRAM-MD5, Kerberos, SPNEGO or
+NTLM authentication, HTTPS, FTPS, SCP and SFTP are a few examples.
+
+.IP "Redirects"
+The \fICURLOPT_FOLLOWLOCATION(3)\fP option automatically follows HTTP
+redirects sent by a remote server. These redirects can refer to any kind of
+URL, not just HTTP. A redirect to a file: URL would cause the libcurl to read
+(or write) arbitrary files from the local filesystem. If the application
+returns the data back to the user (as would happen in some kinds of CGI
+scripts), an attacker could leverage this to read otherwise forbidden data
+(e.g. file://localhost/etc/passwd).
+
+If authentication credentials are stored in the ~/.netrc file, or Kerberos
+is in use, any other URL type (not just file:) that requires
+authentication is also at risk. A redirect such as
+ftp://some-internal-server/private-file would then return data even when
+the server is password protected.
+
+In the same way, if an unencrypted SSH private key has been configured for
+the user running the libcurl application, SCP: or SFTP: URLs could access
+password or private-key protected resources,
+e.g. sftp://user@some-internal-server/etc/passwd
+
+The \fICURLOPT_REDIR_PROTOCOLS(3)\fP and \fICURLOPT_NETRC(3)\fP options can be
+used to mitigate against this kind of attack.
+
+A redirect can also specify a location available only on the machine running
+libcurl, including servers hidden behind a firewall from the attacker.
+e.g. http://127.0.0.1/ or http://intranet/delete-stuff.cgi?delete=all or
+tftp://bootp-server/pc-config-data
+
+Apps can mitigate against this by disabling \fICURLOPT_FOLLOWLOCATION(3)\fP
+and handling redirects itself, sanitizing URLs as necessary. Alternately, an
+app could leave \fICURLOPT_FOLLOWLOCATION(3)\fP enabled but set
+\fICURLOPT_REDIR_PROTOCOLS(3)\fP and install a
+\fICURLOPT_OPENSOCKETFUNCTION(3)\fP callback function in which addresses are
+sanitized before use.
+
+.IP "Private Resources"
+A user who can control the DNS server of a domain being passed in within a URL
+can change the address of the host to a local, private address which a
+server-side libcurl-using application could then use. e.g. the innocuous URL
+http://fuzzybunnies.example.com/ could actually resolve to the IP address of a
+server behind a firewall, such as 127.0.0.1 or 10.1.2.3. Apps can mitigate
+against this by setting a \fICURLOPT_OPENSOCKETFUNCTION(3)\fP and checking the
+address before a connection.
+
+All the malicious scenarios regarding redirected URLs apply just as well to
+non-redirected URLs, if the user is allowed to specify an arbitrary URL that
+could point to a private resource. For example, a web app providing a
+translation service might happily translate file://localhost/etc/passwd and
+display the result. Apps can mitigate against this with the
+\fICURLOPT_PROTOCOLS(3)\fP option as well as by similar mitigation techniques
+for redirections.
+
+A malicious FTP server could in response to the PASV command return an IP
+address and port number for a server local to the app running libcurl but
+behind a firewall. Apps can mitigate against this by using the
+\fICURLOPT_FTP_SKIP_PASV_IP(3)\fP option or \fICURLOPT_FTPPORT(3)\fP.
+
+.IP "IPv6 Addresses"
+libcurl will normally handle IPv6 addresses transparently and just as easily
+as IPv4 addresses. That means that a sanitizing function that filters out
+addressses like 127.0.0.1 isn't sufficient--the equivalent IPv6 addresses ::1,
+::, 0:00::0:1, ::127.0.0.1 and ::ffff:7f00:1 supplied somehow by an attacker
+would all bypass a naive filter and could allow access to undesired local
+resources. IPv6 also has special address blocks like link-local and site-local
+that generally shouldn't be accessed by a server-side libcurl-using
+application. A poorly-configured firewall installed in a data center,
+organization or server may also be configured to limit IPv4 connections but
+leave IPv6 connections wide open. In some cases, the CURL_IPRESOLVE_V4 option
+can be used to limit resolved addresses to IPv4 only and bypass these issues.
+
+.IP Uploads
+When uploading, a redirect can cause a local (or remote) file to be
+overwritten. Apps must not allow any unsanitized URL to be passed in for
+uploads. Also, \fICURLOPT_FOLLOWLOCATION(3)\fP should not be used on uploads.
+Instead, the app should handle redirects itself, sanitizing each URL first.
+
+.IP Authentication
+Use of \fICURLOPT_UNRESTRICTED_AUTH(3)\fP could cause authentication
+information to be sent to an unknown second server. Apps can mitigate against
+this by disabling \fICURLOPT_FOLLOWLOCATION(3)\fP and handling redirects
+itself, sanitizing where necessary.
+
+Use of the CURLAUTH_ANY option to \fICURLOPT_HTTPAUTH(3)\fP could result in
+user name and password being sent in clear text to an HTTP server. Instead,
+use CURLAUTH_ANYSAFE which ensures that the password is encrypted over the
+network, or else fail the request.
+
+Use of the CURLUSESSL_TRY option to \fICURLOPT_USE_SSL(3)\fP could result in
+user name and password being sent in clear text to an FTP server. Instead,
+use CURLUSESSL_CONTROL to ensure that an encrypted connection is used or else
+fail the request.
+
+.IP Cookies
+If cookies are enabled and cached, then a user could craft a URL which
+performs some malicious action to a site whose authentication is already
+stored in a cookie. e.g. http://mail.example.com/delete-stuff.cgi?delete=all
+Apps can mitigate against this by disabling cookies or clearing them
+between requests.
+
+.IP "Dangerous URLs"
+SCP URLs can contain raw commands within the scp: URL, which is a side effect
+of how the SCP protocol is designed. e.g.
+scp://user:pass@host/a;date >/tmp/test;
+Apps must not allow unsanitized SCP: URLs to be passed in for downloads.
+
+.IP "Denial of Service"
+A malicious server could cause libcurl to effectively hang by sending a
+trickle of data through, or even no data at all but just keeping the TCP
+connection open. This could result in a denial-of-service attack. The
+\fICURLOPT_TIMEOUT(3)\fP and/or \fICURLOPT_LOW_SPEED_LIMIT(3)\fP options can
+be used to mitigate against this.
+
+A malicious server could cause libcurl to effectively hang by starting to send
+data, then severing the connection without cleanly closing the TCP connection.
+The app could install a \fICURLOPT_SOCKOPTFUNCTION(3)\fP callback function and
+set the TCP SO_KEEPALIVE option to mitigate against this. Setting one of the
+timeout options would also work against this attack.
+
+A malicious server could cause libcurl to download an infinite amount of data,
+potentially causing all of memory or disk to be filled. Setting the
+\fICURLOPT_MAXFILESIZE_LARGE(3)\fP option is not sufficient to guard against
+this. Instead, the app should monitor the amount of data received within the
+write or progress callback and abort once the limit is reached.
+
+A malicious HTTP server could cause an infinite redirection loop, causing a
+denial-of-service. This can be mitigated by using the
+\fICURLOPT_MAXREDIRS(3)\fP option.
+
+.IP "Arbitrary Headers"
+User-supplied data must be sanitized when used in options like
+\fICURLOPT_USERAGENT(3)\fP, \fICURLOPT_HTTPHEADER(3)\fP,
+\fICURLOPT_POSTFIELDS(3)\fP and others that are used to generate structured
+data. Characters like embedded carriage returns or ampersands could allow the
+user to create additional headers or fields that could cause malicious
+transactions.
+
+.IP "Server-supplied Names"
+A server can supply data which the application may, in some cases, use as
+a file name. The curl command-line tool does this with --remote-header-name,
+using the Content-disposition: header to generate a file name. An application
+could also use CURLINFO_EFFECTIVE_URL to generate a file name from a
+server-supplied redirect URL. Special care must be taken to sanitize such
+names to avoid the possibility of a malicious server supplying one like
+"/etc/passwd", "\\autoexec.bat", "prn:" or even ".bashrc".
+
+.IP "Server Certificates"
+A secure application should never use the \fICURLOPT_SSL_VERIFYPEER(3)\fP
+option to disable certificate validation. There are numerous attacks that are
+enabled by apps that fail to properly validate server TLS/SSL certificates,
+thus enabling a malicious server to spoof a legitimate one. HTTPS without
+validated certificates is potentially as insecure as a plain HTTP connection.
.IP "Showing What You Do"
On a related issue, be aware that even in situations like when you have
problems with libcurl and ask someone for help, everything you reveal in order
to get best possible help might also impose certain security related
-risks. Host names, user names, paths, operating system specifics etc (not to
+risks. Host names, user names, paths, operating system specifics, etc. (not to
mention passwords of course) may in fact be used by intruders to gain
additional information of a potential target.
+Be sure to limit access to application logs if they could hold private or
+security-related data. Besides the obvious candidates like user names and
+passwords, things like URLs, cookies or even file names could also hold
+sensitive data.
+
To avoid this problem, you must of course use your common sense. Often, you
can just edit out the sensitive data or just search/replace your true
information with faked data.
-.SH "Multiple Transfers Using the multi Interface"
-
+.SH "The multi Interface"
The easy interface as described in detail in this document is a synchronous
-interface that transfers one file at a time and doesn't return until its
+interface that transfers one file at a time and doesn't return until it is
done.
-The multi interface on the other hand, allows your program to transfer
-multiple files in both directions at the same time, without forcing you to
-use multiple threads.
+The 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 in to
+multiple transfers at the same time by adding up multiple easy handles into
a "multi stack".
-You create the easy handles you want and you set all the options just like you
-have been told above, and then you create a multi handle with
-\fIcurl_multi_init(3)\fP and add all those easy handles to that multi handle
-with \fIcurl_multi_add_handle(3)\fP.
+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 \fIcurl_multi_init(3)\fP and add all those easy handles to
+that multi handle with \fIcurl_multi_add_handle(3)\fP.
When you've added the handles you have for the moment (you can still add new
-ones at any time), you start the transfers by call
+ones at any time), you start the transfers by calling
\fIcurl_multi_perform(3)\fP.
-\fIcurl_multi_perform(3)\fP is asynchronous. It will only execute as little as
-possible and then return back control to your program. It is designed to never
-block. If it returns CURLM_CALL_MULTI_PERFORM you better call it again soon,
-as that is a signal that it still has local data to send or remote data to
-receive.
+\fIcurl_multi_perform(3)\fP is asynchronous. It will only perform what can be
+done now and then return back 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
When you then call select(), it'll return when one of the file handles signal
action and you then call \fIcurl_multi_perform(3)\fP to allow libcurl to do
what it wants to do. Take note that libcurl does also feature some time-out
-code so we advice you to never use very long timeouts on select() before you
-call \fIcurl_multi_perform(3)\fP, which thus should be called unconditionally
-every now and then even if none of its file descriptors have signaled
-ready. Another precaution you should use: always call
-\fIcurl_multi_fdset(3)\fP immediately before the select() call since the
-current set of file descriptors may change when calling a curl function.
+code so we advise you to never use very long timeouts on select() before you
+call \fIcurl_multi_perform(3)\fP again. \fIcurl_multi_timeout(3)\fP is
+provided to help you get a suitable timeout period.
+
+Another precaution you should use: always call \fIcurl_multi_fdset(3)\fP
+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 \fIcurl_multi_remove_handle(3)\fP to remove individual easy
[ seeding, passwords, keys, certificates, ENGINE, ca certs ]
.SH "Sharing Data Between Easy Handles"
+You can share some data between easy handles when the easy interface is used,
+and some data is share automatically when you use the multi interface.
+
+When you add easy handles to a multi handle, these easy handles will
+automatically share a lot of the data that otherwise would be kept on a
+per-easy handle basis when the easy interface is used.
- [ fill in ]
+The DNS cache is shared between handles within a multi handle, making
+subsequent name resolving faster, and the connection pool that is kept to
+better allow persistent connections and connection re-use is also shared. If
+you're using the easy interface, you can still share these between specific
+easy handles by using the share interface, see \fIlibcurl-share(3)\fP.
+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.
.SH "Footnotes"
.IP "[1]"
libcurl 7.10.3 and later have the ability to switch over to chunked
-Transfer-Encoding in cases were HTTP uploads are done with data of an unknown
+Transfer-Encoding in cases where HTTP uploads are done with data of an unknown
size.
.IP "[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.
.IP "[3]"
-The curl-config tool is generated at build-time (on unix-like systems) and
+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.
+.IP "[4]"
+This behavior was different in versions before 7.17.0, where strings had to
+remain valid past the end of the \fIcurl_easy_setopt(3)\fP call.
+.SH "SEE ALSO"
+.BR libcurl-errors "(3), " libcurl-multi "(3), " libcurl-easy "(3) "
projects / platform / upstream / curl.git / blobdiff