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Updated: March 8, 2011 (http://curl.haxx.se/docs/faq.html)
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FAQ
1. Philosophy
1.1 What is cURL?
1.2 What is libcurl?
1.3 What is curl not?
1.4 When will you make curl do XXXX ?
1.5 Who makes curl?
1.6 What do you get for making curl?
1.7 What about CURL from curl.com?
1.8 I have a problem who do I mail?
1.9 Where do I buy commercial support for curl?
1.10 How many are using curl?
1.11 Why don't you update ca-bundle.crt
1.12 I have a problem who can I chat with?
1.13 curl's ECCN number?
1.14 How do I submit my patch?
2. Install Related Problems
2.1 configure doesn't find OpenSSL even when it is installed
2.1.1 native linker doesn't find OpenSSL
2.1.2 only the libssl lib is missing
2.2 Does curl work/build with other SSL libraries?
2.3 Where can I find a copy of LIBEAY32.DLL?
2.4 Does curl support SOCKS (RFC 1928) ?
3. Usage Problems
3.1 curl: (1) SSL is disabled, https: not supported
3.2 How do I tell curl to resume a transfer?
3.3 Why doesn't my posting using -F work?
3.4 How do I tell curl to run custom FTP commands?
3.5 How can I disable the Pragma: nocache header?
3.6 Does curl support ASP, XML, XHTML or HTML version Y?
3.7 Can I use curl to delete/rename a file through FTP?
3.8 How do I tell curl to follow HTTP redirects?
3.9 How do I use curl in my favorite programming language?
3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
3.11 How do I POST with a different Content-Type?
3.12 Why do FTP specific features over HTTP proxy fail?
3.13 Why does my single/double quotes fail?
3.14 Does curl support Javascript or PAC (automated proxy config)?
3.15 Can I do recursive fetches with curl?
3.16 What certificates do I need when I use SSL?
3.17 How do I list the root dir of an FTP server?
3.18 Can I use curl to send a POST/PUT and not wait for a response?
3.19 How do I get HTTP from a host using a specific IP address?
3.20 How to SFTP from my user's home directory?
3.21 Protocol xxx not supported or disabled in libcurl
4. Running Problems
4.1 Problems connecting to SSL servers.
4.2 Why do I get problems when I use & or % in the URL?
4.3 How can I use {, }, [ or ] to specify multiple URLs?
4.4 Why do I get downloaded data even though the web page doesn't exist?
4.5 Why do I get return code XXX from a HTTP server?
4.5.1 "400 Bad Request"
4.5.2 "401 Unauthorized"
4.5.3 "403 Forbidden"
4.5.4 "404 Not Found"
4.5.5 "405 Method Not Allowed"
4.5.6 "301 Moved Permanently"
4.6 Can you tell me what error code 142 means?
4.7 How do I keep user names and passwords secret in Curl command lines?
4.8 I found a bug!
4.9 Curl can't authenticate to the server that requires NTLM?
4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
4.11 Why does my HTTP range requests return the full document?
4.12 Why do I get "certificate verify failed" ?
4.13 Why is curl -R on Windows one hour off?
4.14 Redirects work in browser but not with curl!
4.15 FTPS doesn't work
4.16 My HTTP POST or PUT requests are slow!
4.17 Non-functional connect timeouts on Windows
4.18 file:// URLs containing drive letters (Windows, NetWare)
4.19 Why doesn't cURL return an error when the network cable is unplugged?
5. libcurl Issues
5.1 Is libcurl thread-safe?
5.2 How can I receive all data into a large memory chunk?
5.3 How do I fetch multiple files with libcurl?
5.4 Does libcurl do Winsock initing on win32 systems?
5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
5.6 What about Keep-Alive or persistent connections?
5.7 Link errors when building libcurl on Windows!
5.8 libcurl.so.X: open failed: No such file or directory
5.9 How does libcurl resolve host names?
5.10 How do I prevent libcurl from writing the response to stdout?
5.11 How do I make libcurl not receive the whole HTTP response?
5.12 Can I make libcurl fake or hide my real IP address?
5.13 How do I stop an ongoing transfer?
5.14 Using C++ non-static functions for callbacks?
5.15 How do I get an FTP directory listing?
6. License Issues
6.1 I have a GPL program, can I use the libcurl library?
6.2 I have a closed-source program, can I use the libcurl library?
6.3 I have a BSD licensed program, can I use the libcurl library?
6.4 I have a program that uses LGPL libraries, can I use libcurl?
6.5 Can I modify curl/libcurl for my program and keep the changes secret?
6.6 Can you please change the curl/libcurl license to XXXX?
6.7 What are my obligations when using libcurl in my commercial apps?
7. PHP/CURL Issues
7.1 What is PHP/CURL?
7.2 Who wrote PHP/CURL?
7.3 Can I perform multiple requests using the same handle?
==============================================================================
1. Philosophy
1.1 What is cURL?
cURL is the name of the project. The name is a play on 'Client for URLs',
originally with URL spelled in uppercase to make it obvious it deals with
URLs. The fact it can also be pronounced 'see URL' also helped, it works as
an abbreviation for "Client URL Request Library" or why not the recursive
version: "Curl URL Request Library".
The cURL project produces two products:
libcurl
A free and easy-to-use client-side URL transfer library, supporting DICT,
FILE, FTP, FTPS, GOPHER, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, POP3,
POP3S, RTMP, RTSP, SCP, SFTP, SMTP, SMTPS, TELNET and TFTP.
libcurl supports HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading,
kerberos, HTTP form based upload, proxies, cookies, user+password
authentication, file transfer resume, http proxy tunneling and more!
libcurl is highly portable, it builds and works identically on numerous
platforms, including Solaris, NetBSD, FreeBSD, OpenBSD, Darwin, HPUX,
IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, BeOs, Mac
OS X, Ultrix, QNX, OpenVMS, RISC OS, Novell NetWare, DOS, Symbian, OSF,
Android, Minix, IBM TPF and more...
libcurl is free, thread-safe, IPv6 compatible, feature rich, well
supported and fast.
curl
A command line tool for getting or sending files using URL syntax.
Since curl uses libcurl, curl supports the same wide range of common
Internet protocols that libcurl does.
We pronounce curl and cURL with an initial k sound: [kurl].
There are numerous sub-projects and related projects that also use the word
curl in the project names in various combinations, but you should take
notice that this FAQ is directed at the command-line tool named curl (and
libcurl the library), and may therefore not be valid for other curl-related
projects. (There is however a small section for the PHP/CURL in this FAQ.)
1.2 What is libcurl?
libcurl is a reliable and portable library which provides you with an easy
interface to a range of common Internet protocols.
You can use libcurl for free in your application, be it open source,
commercial or closed-source.
libcurl is most probably the most portable, most powerful and most often
used C-based multi-platform file transfer library on this planet - be it
open source or commercial.
1.3 What is curl not?
Curl is not a wget clone. That is a common misconception. Never, during
curl's development, have we intended curl to replace wget or compete on its
market. Curl is targeted at single-shot file transfers.
Curl is not a web site mirroring program. If you want to use curl to mirror
something: fine, go ahead and write a script that wraps around curl to make
it reality (like curlmirror.pl does).
Curl is not an FTP site mirroring program. Sure, get and send FTP with curl
but if you want systematic and sequential behavior you should write a
script (or write a new program that interfaces libcurl) and do it.
Curl is not a PHP tool, even though it works perfectly well when used from
or with PHP (when using the PHP/CURL module).
Curl is not a program for a single operating system. Curl exists, compiles,
builds and runs under a wide range of operating systems, including all
modern Unixes (and a bunch of older ones too), Windows, Amiga, BeOS, OS/2,
OS X, QNX etc.
1.4 When will you make curl do XXXX ?
We love suggestions of what to change in order to make curl and libcurl
better. We do however believe in a few rules when it comes to the future of
curl:
* Curl -- the command line tool -- is to remain a non-graphical command line
tool. If you want GUIs or fancy scripting capabilities, you should look
for another tool that uses libcurl.
* We do not add things to curl that other small and available tools already
do very fine at the side. Curl's output is fine to pipe into another
program or redirect to another file for the next program to interpret.
* We focus on protocol related issues and improvements. If you wanna do more
magic with the supported protocols than curl currently does, chances are
big we will agree. If you wanna add more protocols, we may very well
agree.
* If you want someone else to make all the work while you wait for us to
implement it for you, that is not a very friendly attitude. We spend a
considerable time already on maintaining and developing curl. In order to
get more out of us, you should consider trading in some of your time and
efforts in return.
* If you write the code, chances are bigger that it will get into curl
faster.
1.5 Who makes curl?
curl and libcurl are not made by any single individual. Daniel Stenberg is
project leader and main developer, but other persons' submissions are
important and crucial. Anyone can contribute and post their changes and
improvements and have them inserted in the main sources (of course on the
condition that developers agree on that the fixes are good).
The full list of all contributors is found in the docs/THANKS file.
curl is developed by a community, with Daniel at the wheel.
1.6 What do you get for making curl?
Project cURL is entirely free and open. No person gets paid for developing
(lib)curl on full or even part time. We do this voluntarily on our spare
time. Occasionally companies pay individual developers to work on curl, but
that's up to each company and developer. It is not controlled by nor
supervised in any way by the project.
We still get help from companies. Haxx provides web site, bandwidth, mailing
lists etc and sourceforge.net hosts project services we take advantage from,
like the bug tracker. Also again, some companies have sponsored certain
parts of the development in the past and I hope some will continue to do so
in the future.
If you want to support our project, consider a donation or a banner-program
or even better: by helping us coding, documenting, testing etc.
1.7 What about CURL from curl.com?
During the summer 2001, curl.com was busy advertising their client-side
programming language for the web, named CURL.
We are in no way associated with curl.com or their CURL programming
language.
Our project name curl has been in effective use since 1998. We were not the
first computer related project to use the name "curl" and do not claim any
first-hand rights to the name.
We recognize that we will be living in parallel with curl.com and wish them
every success.
1.8 I have a problem who do I mail?
Please do not mail any single individual unless you really need to. Keep
curl-related questions on a suitable mailing list. All available mailing
lists are listed in the MANUAL document and online at
http://curl.haxx.se/mail/
Keeping curl-related questions and discussions on mailing lists allows
others to join in and help, to share their ideas, contribute their
suggestions and spread their wisdom. Keeping discussions on public mailing
lists also allows for others to learn from this (both current and future
users thanks to the web based archives of the mailing lists), thus saving us
from having to repeat ourselves even more. Thanks for respecting this.
If you have found or simply suspect a security problem in curl or libcurl,
mail curl-security at haxx.se (closed list of receivers, mails are not
disclosed) and tell. Then we can produce a fix in a timely manner before the
flaw is announced to the world, thus lessen the impact the problem will have
on existing users.
1.9 Where do I buy commercial support for curl?
curl is fully open source. It means you can hire any skilled engineer to fix
your curl-related problems.
We list available alternatives on the curl web site:
http://curl.haxx.se/support.html
1.10 How many are using curl?
It is impossible to tell.
We don't know how many users that knowingly have installed and use curl.
We don't know how many users that use curl without knowing that they are in
fact using it.
We don't know how many users that downloaded or installed curl and then
never use it.
Some facts to use as input to the math:
curl packages are downloaded from the curl.haxx.se and mirrors over a
million times per year. curl is installed by default with most Linux
distributions. curl is installed by default with Mac OS X. curl and libcurl
as used by numerous applications that include libcurl binaries in their
distribution packages (like Adobe Acrobat Reader and Google Earth).
More than a hundred known named companies use curl in commercial
environments and products and more than a hundred known named open source
projects depend on (lib)curl.
In a poll on the curl web site mid-2005, more than 50% of the 300+ answers
estimated a user base of one million users or more.
In March 2005, the "Linux Counter project" estimated a total Linux user base
of some 29 millions, while Netcraft detected some 4 million "active" Linux
based web servers. A guess is that a fair amount of these Linux
installations have curl installed.
The Debian project maintains statistics on packages installed by people
who have voluntarily run their package counting application. In mid-2010,
libcurl3 was installed on over 55000 such systems (62% of reporting systems)
and was one of the 320 most popular installed packages (out of about 107000
possible packages).
All this taken together, there is no doubt that there are millions of
(lib)curl users.
http://curl.haxx.se/docs/companies.html
http://curl.haxx.se/docs/programs.html
http://curl.haxx.se/libcurl/using/apps.html
http://counter.li.org/estimates.php
http://news.netcraft.com/archives/2005/03/14/fedora_makes_rapid_progress.html
http://qa.debian.org/popcon.php?package=curl
1.11 Why don't you update ca-bundle.crt
The ca-bundle.crt file that used to be bundled with curl was very outdated
(it being last modified year 2000 should tell) and must be replaced with a
much more modern and up-to-date version by anyone who wants to verify peers
anyway. It is no longer provided, the last curl release that shipped it was
curl 7.18.0.
In the cURL project we've decided not to attempt to keep this file updated
(or even present anymore) since deciding what to add to a ca cert bundle is
an undertaking we've not been ready to accept, and the one we can get from
Mozilla is perfectly fine so there's no need to duplicate that work.
Today, with many services performed over HTTPS, every operating system
should come with a default ca cert bundle that can be deemed somewhat
trustworthy and that collection (if reasonably updated) should be deemed to
be a lot better than a private curl version.
If you want the most recent collection of ca certs that Mozilla Firefox
uses, we recommend that you extract the collection yourself from Mozilla
Firefox (by running 'make ca-bundle), or by using our online service setup
for this purpose: http://curl.haxx.se/docs/caextract.html
1.12 I have a problem who can I chat with?
There's a bunch of friendly people hanging out in the #curl channel on the
IRC network irc.freenode.net. If you're polite and nice, chances are big
that you can get -- or provide -- help instantly.
1.13 curl's ECCN number?
The US government restricts exports of software that contains or uses
cryptography. When doing so, the Export Control Classification Number (ECCN)
is used to identify the level of export control etc.
ASF gives a good explanation at http://www.apache.org/dev/crypto.html
We believe curl's number might be ECCN 5D002, another possibility is
5D992. It seems necessary to write them, asking to confirm.
Comprehensible explanations of the meaning of such numbers and how to
obtain them (resp.) are here
http://www.bis.doc.gov/licensing/exportingbasics.htm
http://www.bis.doc.gov/licensing/do_i_needaneccn.html
An incomprehensible description of the two numbers above is here
http://www.access.gpo.gov/bis/ear/pdf/ccl5-pt2.pdf
1.14 How do I submit my patch?
When you have made a patch or a change of whatever sort, and want to submit
that to the project, there are a few different ways we prefer:
o send a patch to the curl-library mailing list. We're many subscribers
there and there are lots of people who can review patches, comment on them
and "receive" them properly.
o if your patch changes or fixes a bug, you can also opt to submit a bug
report in the bug tracker and attach your patch there. There are less
people involved there.
Lots of more details are found in the CONTRIBUTE and INTERNALS docs.
2. Install Related Problems
2.1 configure doesn't find OpenSSL even when it is installed
This may be because of several reasons.
2.1.1 native linker doesn't find openssl
Affected platforms:
Solaris (native cc compiler)
HPUX (native cc compiler)
SGI IRIX (native cc compiler)
SCO UNIX (native cc compiler)
When configuring curl, I specify --with-ssl. OpenSSL is installed in
/usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
CRYPTO_lock in -lcrypto
Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
-lcrypto, so ld can't find the library. This is due to a bug in the GNU
autoconf tool.
Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
./configure places the -L/usr/local/ssl/lib early enough in the command
line to make things work
2.1.2 only the libssl lib is missing
If all include files and the libcrypto lib is present, with only the
libssl being missing according to configure, this is mostly likely because
a few functions are left out from the libssl.
If the function names missing include RSA or RSAREF you can be certain
that this is because libssl requires the RSA and RSAREF libs to build.
See the INSTALL file section that explains how to add those libs to
configure. Make sure that you remove the config.cache file before you
rerun configure with the new flags.
2.2 Does curl work/build with other SSL libraries?
Curl has been written to use OpenSSL, GnuTLS, yassl, NSS, PolarSSL, axTLS or
qssl, although there should not be many problems using a different
library. If anyone does "port" curl to use a different SSL library, we are
of course very interested in getting the patch!
2.3 Where can I find a copy of LIBEAY32.DLL?
That is an OpenSSL binary built for Windows.
Curl uses OpenSSL to do the SSL stuff. The LIBEAY32.DLL is what curl needs
on a windows machine to do https://. Check out the curl web site to find
accurate and up-to-date pointers to recent OpenSSL DLLs and other binary
packages.
2.4 Does curl support SOCKS (RFC 1928) ?
Yes, SOCKS 4 and 5 are supported.
3. Usage problems
3.1 curl: (1) SSL is disabled, https: not supported
If you get this output when trying to get anything from a https:// server,
it means that the instance of curl/libcurl that you're using was built
without support for this protocol.
This could've happened if the configure script that was run at build time
couldn't find all libs and include files curl requires for SSL to work. If
the configure script fails to find them, curl is simply built without SSL
support.
To get the https:// support into a curl that was previously built but that
reports that https:// is not supported, you should dig through the document
and logs and check out why the configure script doesn't find the SSL libs
and/or include files.
Also, check out the other paragraph in this FAQ labelled "configure doesn't
find OpenSSL even when it is installed".
3.2 How do I tell curl to resume a transfer?
Curl supports resumed transfers both ways on both FTP and HTTP.
Try the -C option.
3.3 Why doesn't my posting using -F work?
You can't simply use -F or -d at your choice. The web server that will
receive your post assumes one of the formats. If the form you're trying to
"fake" sets the type to 'multipart/form-data', then and only then you must
use the -F type. In all the most common cases, you should use -d which then
causes a posting with the type 'application/x-www-form-urlencoded'.
This is described in some detail in the MANUAL and TheArtOfHttpScripting
documents, and if you don't understand it the first time, read it again
before you post questions about this to the mailing list. Also, try reading
through the mailing list archives for old postings and questions regarding
this.
3.4 How do I tell curl to run custom FTP commands?
You can tell curl to perform optional commands both before and/or after a
file transfer. Study the -Q/--quote option.
Since curl is used for file transfers, you don't use curl to just perform
FTP commands without transferring anything. Therefore you must always specify
a URL to transfer to/from even when doing custom FTP commands.
3.5 How can I disable the Pragma: nocache header?
You can change all internally generated headers by adding a replacement with
the -H/--header option. By adding a header with empty contents you safely
disable that one. Use -H "Pragma:" to disable that specific header.
3.6 Does curl support ASP, XML, XHTML or HTML version Y?
To curl, all contents are alike. It doesn't matter how the page was
generated. It may be ASP, PHP, Perl, shell-script, SSI or plain
HTML-files. There's no difference to curl and it doesn't even know what kind
of language that generated the page.
See also item 3.14 regarding javascript.
3.7 Can I use curl to delete/rename a file through FTP?
Yes. You specify custom FTP commands with -Q/--quote.
One example would be to delete a file after you have downloaded it:
curl -O ftp://download.com/coolfile -Q '-DELE coolfile'
or rename a file after upload:
curl -T infile ftp://upload.com/dir/ -Q "-RNFR infile" -Q "-RNTO newname"
3.8 How do I tell curl to follow HTTP redirects?
Curl does not follow so-called redirects by default. The Location: header
that informs the client about this is only interpreted if you're using the
-L/--location option. As in:
curl -L http://redirector.com
Not all redirects are HTTP ones, see 4.14
3.9 How do I use curl in my favorite programming language?
There exist many language interfaces/bindings for curl that integrates it
better with various languages. If you are fluid in a script language, you
may very well opt to use such an interface instead of using the command line
tool.
Find out more about which languages that support curl directly, and how to
install and use them, in the libcurl section of the curl web site:
http://curl.haxx.se/libcurl/
In October 2009, there were interfaces available for the following
languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Dylan, Eiffel, Euphoria,
Ferite, Gambas, glib/GTK+, Haskell, ILE/RPG, Java, Lisp, Lua, Mono, .NET,
Object-Pascal, O'Caml, Pascal, Perl, PHP, PostgreSQL, Python, R, Rexx, Ruby,
Scheme, S-Lang, Smalltalk, SP-Forth, SPL, Tcl, Visual Basic, Visual FoxPro,
Q, wxwidgets and XBLite. By the time you read this, additional ones may have
appeared!
3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
Curl adheres to the HTTP spec, which basically means you can play with *any*
protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and
XML-RPC are all such ones. You can use -X to set custom requests and -H to
set custom headers (or replace internally generated ones).
Using libcurl is of course just as fine and you'd just use the proper
library options to do the same.
3.11 How do I POST with a different Content-Type?
You can always replace the internally generated headers with -H/--header.
To make a simple HTTP POST with text/xml as content-type, do something like:
curl -d "datatopost" -H "Content-Type: text/xml" [URL]
3.12 Why do FTP specific features over HTTP proxy fail?
Because when you use a HTTP proxy, the protocol spoken on the network will
be HTTP, even if you specify a FTP URL. This effectively means that you
normally can't use FTP specific features such as FTP upload and FTP quote
etc.
There is one exception to this rule, and that is if you can "tunnel through"
the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
and is generally not available as proxy admins usually disable tunneling to
other ports than 443 (which is used for HTTPS access through proxies).
3.13 Why does my single/double quotes fail?
To specify a command line option that includes spaces, you might need to
put the entire option within quotes. Like in:
curl -d " with spaces " url.com
or perhaps
curl -d ' with spaces ' url.com
Exactly what kind of quotes and how to do this is entirely up to the shell
or command line interpreter that you are using. For most unix shells, you
can more or less pick either single (') or double (") quotes. For
Windows/DOS prompts I believe you're forced to use double (") quotes.
Please study the documentation for your particular environment. Examples in
the curl docs will use a mix of both these ones as shown above. You must
adjust them to work in your environment.
Remember that curl works and runs on more operating systems than most single
individuals have ever tried.
3.14 Does curl support Javascript or PAC (automated proxy config)?
Many web pages do magic stuff using embedded Javascript. Curl and libcurl
have no built-in support for that, so it will be treated just like any other
contents.
.pac files are a netscape invention and are sometimes used by organizations
to allow them to differentiate which proxies to use. The .pac contents is
just a Javascript program that gets invoked by the browser and that returns
the name of the proxy to connect to. Since curl doesn't support Javascript,
it can't support .pac proxy configuration either.
Some workarounds usually suggested to overcome this Javascript dependency:
- 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.
3.15 Can I do recursive fetches with curl?
No. curl itself has no code that performs recursive operations, such as
those performed by wget and similar tools.
There exist wrapper scripts with that functionality (for example the
curlmirror perl script), and you can write programs based on libcurl to do
it, but the command line tool curl itself cannot.
3.16 What certificates do I need when I use SSL?
There are three different kinds of "certificates" to keep track of when we
talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.
- Client certificate. The server you communicate may require that you can
provide this in order to prove that you actually are who you claim to be.
If the server doesn't require this, you don't need a client certificate.
A client certificate is always used together with a private key, and the
private key has a pass phrase that protects it.
- Server certificate. The server you communicate with has a server
certificate. You can and should verify this certificate to make sure that
you are truly talking to the real server and not a server impersonating
it.
- Certificate Authority certificate ("CA cert"). You often have several CA
certs in a CA cert bundle that can be used to verify a server certificate
that was signed by one of the authorities in the bundle. curl does not
come with a CA cert bundle but most curl installs provide one. You can
also override the default.
The server certificate verification process is made by using a Certificate
Authority certificate ("CA cert") that was used to sign the server
certificate. Server certificate verification is enabled by default in curl
and libcurl and is often the reason for problems as explained in FAQ entry
4.12 and the SSLCERTS document
(http://curl.haxx.se/docs/sslcerts.html). Server certificates that are
"self-signed" or otherwise signed by a CA that you do not have a CA cert
for, cannot be verified. If the verification during a connect fails, you
are refused access. You then need to explicitly disable the verification
to connect to the server.
3.17 How do I list the root dir of an FTP server?
There are two ways. The way defined in the RFC is to use an encoded slash
in the first path part. List the "/tmp" dir like this:
curl ftp://ftp.sunet.se/%2ftmp/
or the not-quite-kosher-but-more-readable way, by simply starting the path
section of the URL with a slash:
curl ftp://ftp.sunet.se//tmp/
3.18 Can I use curl to send a POST/PUT and not wait for a response?
No.
But you could easily write your own program using libcurl to do such stunts.
3.19 How do I get HTTP from a host using a specific IP address?
For example, you may be trying out a web site installation that isn't yet in
the DNS. Or you have a site using multiple IP addresses for a given host
name and you want to address a specific one out of the set.
Set a custom Host: header that identifies the server name you want to reach
but use the target IP address in the URL:
curl --header "Host: www.example.com" http://127.0.0.1/
3.20 How to SFTP from my user's home directory?
Contrary to how FTP works, SFTP and SCP URLs specify the exact directory to
work with. It means that if you don't specify that you want the user's home
directory, you get the actual root directory.
To specify a file in your user's home directory, you need to use the correct
URL syntax which for sftp might look similar to:
curl -O -u user:password sftp://example.com/~/file.txt
and for SCP it is just a different protocol prefix:
curl -O -u user:password scp://example.com/~/file.txt
3.21 Protocol xxx not supported or disabled in libcurl
When passing on a URL to curl to use, it may respond that the particular
protocol is not supported or disabled. The particular way this error message
is phrased is because curl doesn't make a distinction internally of whether
a particular protocol is not supported (ie never got any code added that
knows how to speak that protocol) or if it was explicitly disabled. curl can
be built to only support a given set of protocols, and the rest would then
be disabled or not supported.
Note that this error will also occur if you pass a wrongly spelled protocol
part as in "htpt://example.com" or as in the less evident case if you prefix
the protocol part with a space as in " http://example.com/".
4. Running Problems
4.1 Problems connecting to SSL servers.
It took a very long time before we could sort out why curl had problems to
connect to certain SSL servers when using SSLeay or OpenSSL v0.9+. The
error sometimes showed up similar to:
16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:
It turned out to be because many older SSL servers don't deal with SSLv3
requests properly. To correct this problem, tell curl to select SSLv2 from
the command line (-2/--sslv2).
There have also been examples where the remote server didn't like the SSLv2
request and instead you had to force curl to use SSLv3 with -3/--sslv3.
4.2 Why do I get problems when I use & or % in the URL?
In general unix shells, the & symbol is treated specially and when used, it
runs the specified command in the background. To safely send the & as a part
of a URL, you should quote the entire URL by using single (') or double (")
quotes around it. Similar problems can also occur on some shells with other
characters, including ?*!$~(){}<>\|;`. When in doubt, quote the URL.
An example that would invoke a remote CGI that uses &-symbols could be:
curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
In Windows, the standard DOS shell treats the %-symbol specially and you
need to use TWO %-symbols for each single one you want to use in the URL.
Also note that if you want the literal %-symbol to be part of the data you
pass in a POST using -d/--data you must encode it as '%25' (which then also
needs the %-symbol doubled on Windows machines).
4.3 How can I use {, }, [ or ] to specify multiple URLs?
Because those letters have a special meaning to the shell, and to be used in
a URL specified to curl you must quote them.
An example that downloads two URLs (sequentially) would do:
curl '{curl,www}.haxx.se'
To be able to use those letters as actual parts of the URL (without using
them for the curl URL "globbing" system), use the -g/--globoff option:
curl -g 'www.site.com/weirdname[].html'
4.4 Why do I get downloaded data even though the web page doesn't exist?
Curl asks remote servers for the page you specify. If the page doesn't exist
at the server, the HTTP protocol defines how the server should respond and
that means that headers and a "page" will be returned. That's simply how
HTTP works.
By using the --fail option you can tell curl explicitly to not get any data
if the HTTP return code doesn't say success.
4.5 Why do I get return code XXX from a HTTP server?
RFC2616 clearly explains the return codes. This is a short transcript. Go
read the RFC for exact details:
4.5.1 "400 Bad Request"
The request could not be understood by the server due to malformed
syntax. The client SHOULD NOT repeat the request without modifications.
4.5.2 "401 Unauthorized"
The request requires user authentication.
4.5.3 "403 Forbidden"
The server understood the request, but is refusing to fulfill it.
Authorization will not help and the request SHOULD NOT be repeated.
4.5.4 "404 Not Found"
The server has not found anything matching the Request-URI. No indication
is given of whether the condition is temporary or permanent.
4.5.5 "405 Method Not Allowed"
The method specified in the Request-Line is not allowed for the resource
identified by the Request-URI. The response MUST include an Allow header
containing a list of valid methods for the requested resource.
4.5.6 "301 Moved Permanently"
If you get this return code and an HTML output similar to this:
<H1>Moved Permanently</H1> The document has moved <A
HREF="http://same_url_now_with_a_trailing_slash/">here</A>.
it might be because you request a directory URL but without the trailing
slash. Try the same operation again _with_ the trailing URL, or use the
-L/--location option to follow the redirection.
4.6 Can you tell me what error code 142 means?
All curl error codes are described at the end of the man page, in the
section called "EXIT CODES".
Error codes that are larger than the highest documented error code means
that curl has exited due to a crash. This is a serious error, and we
appreciate a detailed bug report from you that describes how we could go
ahead and repeat this!
4.7 How do I keep user names and passwords secret in Curl command lines?
This problem has two sides:
The first part is to avoid having clear-text passwords in the command line
so that they don't appear in 'ps' outputs and similar. That is easily
avoided by using the "-K" option to tell curl to read parameters from a file
or stdin to which you can pass the secret info. curl itself will also
attempt to "hide" the given password by blanking out the option - this
doesn't work on all platforms.
To keep the passwords in your account secret from the rest of the world is
not a task that curl addresses. You could of course encrypt them somehow to
at least hide them from being read by human eyes, but that is not what
anyone would call security.
Also note that regular HTTP (using Basic authentication) and FTP passwords
are sent in clear across the network. All it takes for anyone to fetch them
is to listen on the network. Eavesdropping is very easy. Use more secure
authentication methods (like Digest, Negotiate or even NTLM) or consider the
SSL-based alternatives HTTPS and FTPS.
4.8 I found a bug!
It is not a bug if the behavior is documented. Read the docs first.
Especially check out the KNOWN_BUGS file, it may be a documented bug!
If it is a problem with a binary you've downloaded or a package for your
particular platform, try contacting the person who built the package/archive
you have.
If there is a bug, read the BUGS document first. Then report it as described
in there.
4.9 Curl can't authenticate to the server that requires NTLM?
NTLM support requires OpenSSL, GnuTLS, NSS or Microsoft Windows libraries at
build-time to provide this functionality.
NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You
should not use such ones.
4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
Many web servers allow or demand that the administrator configures the
server properly for these requests to work on the web server.
Some servers seem to support HEAD only on certain kinds of URLs.
To fully grasp this, try the documentation for the particular server
software you're trying to interact with. This is not anything curl can do
anything about.
4.11 Why does my HTTP range requests return the full document?
Because the range may not be supported by the server, or the server may
choose to ignore it and return the full document anyway.
4.12 Why do I get "certificate verify failed" ?
You invoke curl 7.10 or later to communicate on a https:// URL and get an
error back looking something similar to this:
curl: (35) SSL: error:14090086:SSL routines:
SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
Then it means that curl couldn't verify that the server's certificate was
good. Curl verifies the certificate using the CA cert bundle that comes with
the curl installation.
To disable the verification (which makes it act like curl did before 7.10),
use -k. This does however enable man-in-the-middle attacks.
If you get this failure but are having a CA cert bundle installed and used,
the server's certificate is not signed by one of the CA's in the bundle. It
might for example be self-signed. You then correct this problem by obtaining
a valid CA cert for the server. Or again, decrease the security by disabling
this check.
Details are also in the SSLCERTS file in the release archives, found online
here: http://curl.haxx.se/docs/sslcerts.html
4.13 Why is curl -R on Windows one hour off?
During daylight savings time, when -R is used, curl will set a time that
appears one hour off. This happens due to a flaw in how Windows stores and
uses file modification times and it is not easily worked around. For details
on this problem, read this: http://www.codeproject.com/datetime/dstbugs.asp
4.14 Redirects work in browser but not with curl!
curl supports HTTP redirects fine (see item 3.8). Browsers generally support
at least two other ways to perform directs that curl does not:
- Meta tags. You can write a HTML tag that will cause the browser to
redirect to another given URL after a certain time.
- Javascript. You can write a Javascript program embedded in a HTML page
that redirects the browser to another given URL.
There is no way to make curl follow these redirects. You must either
manually figure out what the page is set to do, or you write a script that
parses the results and fetches the new URL.
4.15 FTPS doesn't work
curl supports FTPS (sometimes known as FTP-SSL) both implicit and explicit
mode.
When a URL is used that starts with FTPS://, curl assumes implicit SSL on
the control connection and will therefore immediately connect and try to
speak SSL. FTPS:// connections default to port 990.
To use explicit FTPS, you use a FTP:// URL and the --ftp-ssl option (or one
of its related flavours). This is the most common method, and the one
mandated by RFC4217. This kind of connection then of course uses the
standard FTP port 21 by default.
4.16 My HTTP POST or PUT requests are slow!
libcurl makes all POST and PUT requests (except for POST requests with a
very tiny request body) use the "Expect: 100-continue" header. This header
allows the server to deny the operation early so that libcurl can bail out
already before having to send any data. This is useful in authentication
cases and others.
However, many servers don't implement the Expect: stuff properly and if the
server doesn't respond (positively) within 1 second libcurl will continue
and send off the data anyway.
You can disable libcurl's use of the Expect: header the same way you disable
any header, using -H / CURLOPT_HTTPHEADER, or by forcing it to use HTTP 1.0.
4.17 Non-functional connect timeouts
In most Windows setups having a timeout longer than 21 seconds make no
difference, as it will only send 3 TCP SYN packets and no more. The second
packet sent three seconds after the first and the third six seconds after
the second. No more than three packets are sent, no matter how long the
timeout is set.
See option TcpMaxConnectRetransmissions on this page:
http://support.microsoft.com/?scid=kb%3Ben-us%3B175523&x=6&y=7
Also, even on non-Windows systems there may run a firewall or anti-virus
software or similar that accepts the connection but does not actually do
anything else. This will make (lib)curl to consider the connection connected
and thus the connect timeout won't trigger.
4.18 file:// URLs containing drive letters (Windows, NetWare)
When using cURL to try to download a local file, one might use a URL
in this format:
file://D:/blah.txt
You'll find that even if D:\blah.txt does exist, cURL returns a 'file
not found' error.
According to RFC 1738 (http://www.faqs.org/rfcs/rfc1738.html),
file:// URLs must contain a host component, but it is ignored by
most implementations. In the above example, 'D:' is treated as the
host component, and is taken away. Thus, cURL tries to open '/blah.txt'.
If your system is installed to drive C:, that will resolve to 'C:\blah.txt',
and if that doesn't exist you will get the not found error.
To fix this problem, use file:// URLs with *three* leading slashes:
file:///D:/blah.txt
Alternatively, if it makes more sense, specify 'localhost' as the host
component:
file://localhost/D:/blah.txt
In either case, cURL should now be looking for the correct file.
4.19 Why doesn't cURL return an error when the network cable is unplugged?
Unplugging the cable is not an error situation. The TCP/IP protocol stack
was designed to be fault tolerant, so even though there may be a physical
break somewhere the connection shouldn't be affected, just possibly
delayed. Eventually, the physical break will be fixed or the data will be
re-routed around the physical problem.
In such cases, the TCP/IP stack is responsible for detecting when the
network connection is irrevocably lost. Since with some protocols it is
perfectly legal for the client wait indefinitely for data, the stack may
never report a problem, and even when it does, it can take up to 20 minutes
for it to detect an issue. The curl option --keepalive-time enables
keep-alive support in the TCP/IP stack which makes it periodically probe the
connection to make sure it is still available to send data. That should
reliably detect any TCP/IP network failure.
But even that won't detect the network going down before the TCP/IP
connection is established (e.g. during a DNS lookup) or using protocols that
don't use TCP. To handle those situations, curl offers a number of timeouts
on its own. --speed-limit/--speed-time will abort if the data transfer rate
falls too low, and --connect-timeout and --max-time can be used to put an
overall timeout on the connection phase or the entire transfer.
5. libcurl Issues
5.1 Is libcurl thread-safe?
Yes.
We have written the libcurl code specifically adjusted for multi-threaded
programs. libcurl will use thread-safe functions instead of non-safe ones if
your system has such.
If you use a OpenSSL-powered libcurl in a multi-threaded environment, you
need to provide one or two locking functions:
http://www.openssl.org/docs/crypto/threads.html
If you use a GnuTLS-powered libcurl in a multi-threaded environment, you
need to provide locking function(s) for libgcrypt (which is used by GnuTLS
for the crypto functions).
http://www.gnu.org/software/gnutls/manual/html_node/Multi_002dthreaded-applications.html
No special locking is needed with a NSS-powered libcurl. NSS is thread-safe.
5.2 How can I receive all data into a large memory chunk?
[ See also the examples/getinmemory.c source ]
You are in full control of the callback function that gets called every time
there is data received from the remote server. You can make that callback do
whatever you want. You do not have to write the received data to a file.
One solution to this problem could be to have a pointer to a struct that you
pass to the callback function. You set the pointer using the
CURLOPT_WRITEDATA option. Then that pointer will be passed to the callback
instead of a FILE * to a file:
/* imaginary struct */
struct MemoryStruct {
char *memory;
size_t size;
};
/* imaginary callback function */
size_t
WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
{
size_t realsize = size * nmemb;
struct MemoryStruct *mem = (struct MemoryStruct *)data;
mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
if (mem->memory) {
memcpy(&(mem->memory[mem->size]), ptr, realsize);
mem->size += realsize;
mem->memory[mem->size] = 0;
}
return realsize;
}
5.3 How do I fetch multiple files with libcurl?
libcurl has excellent support for transferring multiple files. You should
just repeatedly set new URLs with curl_easy_setopt() and then transfer it
with curl_easy_perform(). The handle you get from curl_easy_init() is not
only reusable, but you're even encouraged to reuse it if you can, as that
will enable libcurl to use persistent connections.
5.4 Does libcurl do Winsock initialization on win32 systems?
Yes, if told to in the curl_global_init() call.
5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
that DLL use the FILE * (as the DLL and the client application cannot access
each others' variable memory areas). If you set CURLOPT_WRITEDATA you must
also use CURLOPT_WRITEFUNCTION as well to set a function that writes the
file, even if that simply writes the data to the specified FILE *.
Similarly, if you use CURLOPT_READDATA you must also specify
CURLOPT_READFUNCTION.
5.6 What about Keep-Alive or persistent connections?
curl and libcurl have excellent support for persistent connections when
transferring several files from the same server. Curl will attempt to reuse
connections for all URLs specified on the same command line/config file, and
libcurl will reuse connections for all transfers that are made using the
same libcurl handle.
5.7 Link errors when building libcurl on Windows!
You need to make sure that your project, and all the libraries (both static
and dynamic) that it links against, are compiled/linked against the same run
time library.
This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
options to the command line compiler. /MD (linking against MSVCRT dll) seems
to be the most commonly used option.
When building an application that uses the static libcurl library, you must
add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for
dynamic import symbols. If you're using Visual Studio, you need to instead
add CURL_STATICLIB in the "Preprocessor Definitions" section.
If you get linker error like "unknown symbol __imp__curl_easy_init ..." you
have linked against the wrong (static) library. If you want to use the
libcurl.dll and import lib, you don't need any extra CFLAGS, but use one of
the import libraries below. These are the libraries produced by the various
lib/Makefile.* files:
Target: static lib. import lib for libcurl*.dll.
-----------------------------------------------------------
MingW: libcurl.a libcurldll.a
MSVC (release): libcurl.lib libcurl_imp.lib
MSVC (debug): libcurld.lib libcurld_imp.lib
Borland: libcurl.lib libcurl_imp.lib
5.8 libcurl.so.X: open failed: No such file or directory
This is an error message you might get when you try to run a program linked
with a shared version of libcurl and your run-time linker (ld.so) couldn't
find the shared library named libcurl.so.X. (Where X is the number of the
current libcurl ABI, typically 3 or 4).
You need to make sure that ld.so finds libcurl.so.X. You can do that
multiple ways, and it differs somewhat between different operating systems,
but they are usually:
* Add an option to the linker command line that specify the hard-coded path
the run-time linker should check for the lib (usually -R)
* Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
should check for libs
* Adjust the system's config to check for libs in the directory where you've
put the dir (like Linux's /etc/ld.so.conf)
'man ld.so' and 'man ld' will tell you more details
5.9 How does libcurl resolve host names?
libcurl supports a large a number of different name resolve functions. One
of them is picked at build-time and will be used unconditionally. Thus, if
you want to change name resolver function you must rebuild libcurl and tell
it to use a different function.
- The non-ipv6 resolver that can use one out of four host name resolve calls
(depending on what your system supports):
A - gethostbyname()
B - gethostbyname_r() with 3 arguments
C - gethostbyname_r() with 5 arguments
D - gethostbyname_r() with 6 arguments
- The ipv6-resolver that uses getaddrinfo()
- The c-ares based name resolver that uses the c-ares library for resolves.
Using this offers asynchronous name resolves.
- The threaded resolver (default option on Windows). It uses:
A - gethostbyname() on plain ipv4 hosts
B - getaddrinfo() on ipv6-enabled hosts
Also note that libcurl never resolves or reverse-lookups addresses given as
pure numbers, such as 127.0.0.1 or ::1.
5.10 How do I prevent libcurl from writing the response to stdout?
libcurl provides a default built-in write function that writes received data
to stdout. Set the CURLOPT_WRITEFUNCTION to receive the data, or possibly
set CURLOPT_WRITEDATA to a different FILE * handle.
5.11 How do I make libcurl not receive the whole HTTP response?
You make the write callback (or progress callback) return an error and
libcurl will then abort the transfer.
5.12 Can I make libcurl fake or hide my real IP address?
No. libcurl operates on a higher level than so. Besides, faking IP address
would imply sending IP packages with a made-up source address, and then you
normally get a problem with intercepting the packages sent back as they
would then not be routed to you!
If you use a proxy to access remote sites, the sites will not see your local
IP address but instead the address of the proxy.
Also note that on many networks NATs or other IP-munging techniques are used
that makes you see and use a different IP address locally than what the
remote server will see you coming from.
5.13 How do I stop an ongoing transfer?
With the easy interface you make sure to return the correct error code from
one of the callbacks, but none of them are instant. There is no function you
can call from another thread or similar that will stop it immediately.
Instead you need to make sure that one of the callbacks you use return an
appropriate value that will stop the transfer.
Suitable callbacks that you can do this with include the progress callback,
the read callback and the write callback.
If you're using the multi interface, you can also stop a transfer by
removing the particular easy handle from the multi stack. At any moment you
think the transfer is done.
5.14 Using C++ non-static functions for callbacks?
libcurl is a C library, it doesn't know anything about C++ member functions.
You can overcome this "limitation" with a relative ease using a static
member function that is passed a pointer to the class:
// f is the pointer to your object.
static YourClass::func(void *buffer, size_t sz, size_t n, void *f)
{
// Call non-static member function.
static_cast<YourClass*>(f)->nonStaticFunction();
}
// This is how you pass pointer to the static function:
curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass:func);
curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);
5.15 How do I get an FTP directory listing?
If you end the FTP URL you request with a slash, libcurl will provide you
with a directory listing of that given directory. You can also set
CURLOPT_CUSTOMREQUEST to alter what exact listing command libcurl would use
to list the files.
The follow-up question that tend to follow the previous one, is how a
program is supposed to parse the directory listing. How does it know what's
a file and what's a dir and what's a symlink etc. The harsh reality is that
FTP provides no such fine and easy-to-parse output. The output format FTP
servers respond to LIST commands are entirely at the server's own liking and
the NLST output doesn't reveal any types and in many cases don't even
include all the directory entries. Also, both LIST and NLST tend to hide
unix-style hidden files (those that start with a dot) by default so you need
to do "LIST -a" or similar to see them.
The application thus needs to parse the LIST output. One such existing
list parser is available at http://cr.yp.to/ftpparse.html Versions of
libcurl since 7.21.0 also provide the ability to specify a wildcard to
download multiple files from one FTP directory.
6. License Issues
Curl and libcurl are released under a MIT/X derivate license. The license is
very liberal and should not impose a problem for your project. This section
is just a brief summary for the cases we get the most questions. (Parts of
this section was much enhanced by Bjorn Reese.)
We are not lawyers and this is not legal advice. You should probably consult
one if you want true and accurate legal insights without our prejudice. Note
especially that this section concerns the libcurl license only; compiling in
features of libcurl that depend on other libraries (e.g. OpenSSL) may affect
the licensing obligations of your application.
6.1 I have a GPL program, can I use the libcurl library?
Yes!
Since libcurl may be distributed under the MIT/X derivate license, it can be
used together with GPL in any software.
6.2 I have a closed-source program, can I use the libcurl library?
Yes!
libcurl does not put any restrictions on the program that uses the library.
6.3 I have a BSD licensed program, can I use the libcurl library?
Yes!
libcurl does not put any restrictions on the program that uses the library.
6.4 I have a program that uses LGPL libraries, can I use libcurl?
Yes!
The LGPL license doesn't clash with other licenses.
6.5 Can I modify curl/libcurl for my program and keep the changes secret?
Yes!
The MIT/X derivate license practically allows you to do almost anything with
the sources, on the condition that the copyright texts in the sources are
left intact.
6.6 Can you please change the curl/libcurl license to XXXX?
No.
We have carefully picked this license after years of development and
discussions and a large amount of people have contributed with source code
knowing that this is the license we use. This license puts the restrictions
we want on curl/libcurl and it does not spread to other programs or
libraries that use it. It should be possible for everyone to use libcurl or
curl in their projects, no matter what license they already have in use.
6.7 What are my obligations when using libcurl in my commercial apps?
Next to none. All you need to adhere to is the MIT-style license (stated in
the COPYING file) which basically says you have to include the copyright
notice in "all copies" and that you may not use the copyright holder's name
when promoting your software.
You do not have to release any of your source code.
You do not have to reveal or make public any changes to the libcurl source
code.
You do not have to broadcast to the world that you are using libcurl within
your app.
All we ask is that you disclose "the copyright notice and this permission
notice" somewhere. Most probably like in the documentation or in the section
where other third party dependencies already are mentioned and acknowledged.
As can be seen here: http://curl.haxx.se/docs/companies.html and elsewhere,
more and more companies are discovering the power of libcurl and take
advantage of it even in commercial environments.
7. PHP/CURL Issues
7.1 What is PHP/CURL?
The module for PHP that makes it possible for PHP programs to access curl-
functions from within PHP.
In the cURL project we call this module PHP/CURL to differentiate it from
curl the command line tool and libcurl the library. The PHP team however
does not refer to it like this (for unknown reasons). They call it plain
CURL (often using all caps) or sometimes ext/curl, but both cause much
confusion to users which in turn gives us a higher question load.
7.2 Who wrote PHP/CURL?
PHP/CURL is a module that comes with the regular PHP package. It depends and
uses libcurl, so you need to have libcurl installed properly first before
PHP/CURL can be used. PHP/CURL was initially written by Sterling Hughes.
7.3 Can I perform multiple requests using the same handle?
Yes - at least in PHP version 4.3.8 and later (this has been known to not
work in earlier versions, but the exact version when it started to work is
unknown to me).
After a transfer, you just set new options in the handle and make another
transfer. This will make libcurl to re-use the same connection if it can.
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