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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 do you not 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?
  1.15 How do I port libcurl to my OS?

2. Install Related Problems
  2.1 configure fails when using static libraries
  2.2 Does curl work/build with other SSL libraries?
  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 does my posting using -F not work?
  3.4 How do I tell curl to run custom FTP commands?
  3.5 How can I disable the Accept: */* 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 do 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 directory 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
  3.22 curl -X gives me HTTP problems

4. Running Problems
  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 does not exist?
  4.5 Why do I get return code XXX from an 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 cannot authenticate to a server that requires NTLM?
  4.10 My HTTP request using HEAD, PUT or DELETE does not work
  4.11 Why do 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 does not 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 does not curl return an error when the network cable is unplugged?
  4.20 curl does not return error for HTTP non-200 responses

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 initialization 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?
  5.16 I want a different time-out
  5.17 Can I write a server with libcurl?
  5.18 Does libcurl use threads?

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?
  7.4 Does PHP/CURL have dependencies?

8. Development
  8.1 Why does curl use C89?
  8.2 Will curl be rewritten?

==============================================================================

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 read as '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 client-side URL transfer library, supporting DICT, FILE, FTP, FTPS,
    GOPHER, GOPHERS, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, MQTT, POP3, POP3S,
    RTMP, RTMPS, RTSP, SCP, SFTP, SMB, SMBS, SMTP, SMTPS, TELNET, TFTP, WS
    and WSS.

libcurl supports HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading,
    Kerberos, SPNEGO, 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, HP-UX,
    IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, macOS,
    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 data using URL syntax.

Since curl uses libcurl, curl supports the same wide range of common
    Internet protocols that libcurl does.

We pronounce curl with an initial k sound. It rhymes with words like girl
  and earl. This is a short WAV file to help you:

https://media.merriam-webster.com/soundc11/c/curl0001.wav

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 for doing Internet data transfers
  using one or more of its supported Internet protocols.

You can use libcurl freely 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 website mirroring program. If you want to use curl to mirror
  something: fine, go ahead and write a script that wraps around curl or use
  libcurl to make it reality.

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, OS/2, macOS,
  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
  well at the side. curl's output can be piped into another program or
  redirected to another file for the next program to interpret.

We focus on protocol related issues and improvements. If you want to do more
  magic with the supported protocols than curl currently does, chances are
  good we will agree. If you want to add more protocols, we may agree.

If you want someone else to do all the work while you wait for us to
  implement it for you, that is not a 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
  effort in return. Simply go to the GitHub repository which resides at
  https://github.com/curl/curl, fork the project, and create pull requests
  with your proposed changes.

If you write the code, chances are better 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 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. We do this voluntarily, mostly in
  our spare time. Companies may pay individual developers to work on curl.
  This is not controlled by nor supervised in any way by the curl project.

We get help from companies. Haxx provides website, bandwidth, mailing lists
  etc, GitHub hosts the primary git repository and other services like the bug
  tracker at https://github.com/curl/curl. 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 with coding, documenting or testing etc.

See also: https://curl.se/sponsors.html

1.7 What about CURL from curl.com?

During the summer of 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
  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
  https://curl.se/mail/

Keeping curl-related questions and discussions on mailing lists allows
  others to join in and help, to share their ideas, to contribute their
  suggestions and to 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,
  submit all the details at https://hackerone.one/curl. On there we keep the
  issue private while we investigate, confirm it, work and validate a fix and
  agree on a time schedule for publication etc. That way we produce a fix in a
  timely manner before the flaw is announced to the world, reducing the impact
  the problem risks having on existing users.

Security issues can also be taking to the curl security team by emailing
  security at curl.se (closed list of receivers, mails are not disclosed).

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 website:
  https://curl.se/support.html

1.10 How many are using curl?

It is impossible to tell.

We do not know how many users that knowingly have installed and use curl.

We do not know how many users that use curl without knowing that they are in
  fact using it.

We do not know how many users that downloaded or installed curl and then
  never use it.

In 2020, we estimate that curl runs in roughly ten billion installations
  world wide.

1.11 Why do you not update ca-bundle.crt

In the cURL project we have decided not to attempt to keep this file updated
  (or even present) since deciding what to add to a ca cert bundle is an
  undertaking we have not been ready to accept, and the one we can get from
  Mozilla is perfectly fine so there is 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: https://curl.se/docs/caextract.html

1.12 I have a problem who, can I chat with?

There is a bunch of friendly people hanging out in the #curl channel on the
  IRC network libera.chat. If you are polite and nice, chances are good 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.

Apache Software Foundation gives a good explanation of ECCNs at
  https://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 (the authority that administers ECCN
  numbers), asking to confirm.

Comprehensible explanations of the meaning of such numbers and how to obtain
  them (resp.) are here

https://www.bis.doc.gov/licensing/exportingbasics.htm
  https://www.bis.doc.gov/licensing/do_i_needaneccn.html

An incomprehensible description of the two numbers above is here
  https://www.bis.doc.gov/index.php/documents/new-encryption/1653-ccl5-pt2-3

1.14 How do I submit my patch?

We strongly encourage you to submit changes and improvements directly as
  "pull requests" on GitHub: https://github.com/curl/curl/pulls

If you for any reason cannot or will not deal with GitHub, send your patch to
  the curl-library mailing list. We are many subscribers there and there are
  lots of people who can review patches, comment on them and "receive" them
  properly.

Lots of more details are found in the CONTRIBUTE.md and INTERNALS.md
  documents.

1.15 How do I port libcurl to my OS?

Here's a rough step-by-step:

1. copy a suitable lib/config-*.h file as a start to lib/config-[youros].h

2. edit lib/config-[youros].h to match your OS and setup

3. edit lib/curl_setup.h to include config-[youros].h when your OS is
     detected by the preprocessor, in the style others already exist

4. compile lib/*.c and make them into a library

2. Install Related Problems

2.1 configure fails when using static libraries

You may find that configure fails to properly detect the entire dependency
  chain of libraries when you provide static versions of the libraries that
  configure checks for.

The reason why static libraries is much harder to deal with is that for them
  we do not get any help but the script itself must know or check what more
  libraries that are needed (with shared libraries, that dependency "chain" is
  handled automatically). This is a error-prone process and one that also
  tends to vary over time depending on the release versions of the involved
  components and may also differ between operating systems.

For that reason, configure does few attempts to actually figure this out and
  you are instead encouraged to set LIBS and LDFLAGS accordingly when you
  invoke configure, and point out the needed libraries and set the necessary
  flags yourself.

2.2 Does curl work with other SSL libraries?

curl has been written to use a generic SSL function layer internally, and
  that SSL functionality can then be provided by one out of many different SSL
  backends.

curl can be built to use one of the following SSL alternatives: OpenSSL,
  libressl, BoringSSL, AWS-LC, GnuTLS, wolfSSL, NSS, mbedTLS, Secure
  Transport (native iOS/OS X), Schannel (native Windows), GSKit (native IBM
  i), BearSSL, or Rustls. They all have their pros and cons, and we try to
  maintain a comparison of them here: https://curl.se/docs/ssl-compared.html

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 an https:// server,
  it means that the instance of curl/libcurl that you are using was built
  without support for this protocol.

This could have happened if the configure script that was run at build time
  could not 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 does not find the SSL libs
  and/or include files.

Also, check out the other paragraph in this FAQ labeled "configure does not
  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 does my posting using -F not work?

You cannot arbitrarily use -F or -d, the choice between -F or -d depends on
  the HTTP operation you need curl to do and what the web server that will
  receive your post expects.

If the form you are trying to submit uses the type '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 do not 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 do not normally use curl to
  perform FTP commands without transferring anything. Therefore you must
  always specify a URL to transfer to/from even when doing custom FTP
  commands, or use -I which implies the "no body" option sent to libcurl.

3.5 How can I disable the Accept: */* 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 "Accept:" to disable that specific header.

3.6 Does curl support ASP, XML, XHTML or HTML version Y?

To curl, all contents are alike. It does not matter how the page was
  generated. It may be ASP, PHP, Perl, shell-script, SSI or plain HTML
  files. There is no difference to curl and it does not 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 are 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?

Many programming languages have interfaces/bindings that allow you to use
  curl without having to use the command line tool. If you are fluent in such
  a language, you may prefer to use one of these interfaces instead.

Find out more about which languages that support curl directly, and how to
  install and use them, in the libcurl section of the curl website:
  https://curl.se/libcurl/

All the various bindings to libcurl are made by other projects and people,
  outside of the cURL project. The cURL project itself only produces libcurl
  with its plain C API. If you do not find anywhere else to ask you can ask
  about bindings on the curl-library list too, but be prepared that people on
  that list may not know anything about bindings.

In December 2021, there were interfaces available for the following
  languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Delphi, Dylan, Eiffel,
  Euphoria, Falcon, Ferite, Gambas, glib/GTK+, Go, Guile, Harbour, Haskell,
  Java, Julia, Lisp, Lua, Mono, .NET, node.js, Object-Pascal, OCaml, Pascal,
  Perl, PHP, PostgreSQL, Python, R, Rexx, Ring, RPG, Ruby, Rust, Scheme,
  Scilab, S-Lang, Smalltalk, SP-Forth, SPL, Tcl, Visual Basic, Visual FoxPro,
  Q, wxwidgets, XBLite and Xoho. 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 good and you would 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 an HTTP proxy, the protocol spoken on the network will
  be HTTP, even if you specify an FTP URL. This effectively means that you
  normally cannot 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
  ports other than 443 (which is used for HTTPS access through proxies).

3.13 Why do 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 command prompts you must use double (") quotes, and if the
  option string contains inner double quotes you can escape them with a
  backslash.

For Windows powershell the arguments are not always passed on as expected
  because curl is not a powershell script. You may or may not be able to use
  single quotes. To escape inner double quotes seems to require a
  backslash-backtick escape sequence and the outer quotes as double quotes.

Please study the documentation for your particular environment. Examples in
  the curl docs will use a mix of both of these 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 does not support JavaScript,
  it cannot 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 exists 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 with may require that you can provide this in
  order to prove that you actually are who you claim to be. If the server
  does not require this, you do not 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
  (https://curl.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 directory 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" directory 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.

You can 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 website installation that is not 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/

You can also opt to add faked host name entries to curl with the --resolve
  option. That has the added benefit that things like redirects will also work
  properly. The above operation would instead be done as:

curl --resolve www.example.com:80:127.0.0.1 http://www.example.com/

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 do not 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 does not make a distinction internally of whether
  a particular protocol is not supported (i.e. 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/".

3.22 curl -X gives me HTTP problems

In normal circumstances, -X should hardly ever be used.

By default you use curl without explicitly saying which request method to
  use when the URL identifies an HTTP transfer. If you just pass in a URL like
  "curl http://example.com" it will use GET. If you use -d or -F curl will use
  POST, -I will cause a HEAD and -T will make it a PUT.

If for whatever reason you are not happy with these default choices that curl
  does for you, you can override those request methods by specifying -X
  [WHATEVER]. This way you can for example send a DELETE by doing "curl -X
  DELETE [URL]".

It is thus pointless to do "curl -XGET [URL]" as GET would be used
  anyway. In the same vein it is pointless to do "curl -X POST -d data
  [URL]"... But you can make a fun and somewhat rare request that sends a
  request-body in a GET request with something like "curl -X GET -d data
  [URL]"

Note that -X does not actually change curl's behavior as it only modifies the
  actual string sent in the request, but that may of course trigger a
  different set of events.

Accordingly, by using -XPOST on a command line that for example would follow
  a 303 redirect, you will effectively prevent curl from behaving
  correctly. Be aware.

4. Running Problems

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 percent sign specially and you
  need to use TWO percent signs for each single one you want to use in the
  URL.

If you want a literal percent sign 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
  percent sign 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, to be used in
  a URL specified to curl you must quote them.

An example that downloads two URLs (sequentially) would be:

curl '{curl,www}.haxx.se'

To be able to use those characters 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 does not exist?

curl asks remote servers for the page you specify. If the page does not 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 is simply how
  HTTP works.

By using the --fail option you can tell curl explicitly to not get any data
  if the HTTP return code does not say success.

4.5 Why do I get return code XXX from an HTTP server?

RFC 2616 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 as to 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 requested 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 do not 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
  does not 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 as cleartext across the network. All it takes for anyone to fetch
  them is to listen on the network. Eavesdropping is 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 have 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 cannot authenticate to a server that requires NTLM?

NTLM support requires OpenSSL, GnuTLS, mbedTLS, NSS, Secure Transport, or
  Microsoft Windows libraries at build-time to provide this functionality.

4.10 My HTTP request using HEAD, PUT or DELETE does not 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 are trying to interact with. This is not anything curl can do
  anything about.

4.11 Why do 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" ?

When you invoke curl and get an error 60 error back it means that curl
  could not verify that the server's certificate was good. curl verifies the
  certificate using the CA cert bundle and verifying for which names the
  certificate has been granted.

To completely disable the certificate verification, use -k. This does
  however enable man-in-the-middle attacks and makes the transfer INSECURE.
  We strongly advise against doing this for more than experiments.

If you get this failure with a CA cert bundle installed and used, the
  server's certificate might not be signed by one of the CA's in your CA
  store. 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.

At times, you find that the verification works in your favorite browser but
  fails in curl. When this happens, the reason is usually that the server
  sends an incomplete cert chain. The server is mandated to send all
  "intermediate certificates" but does not. This typically works with browsers
  anyway since they A) cache such certs and B) supports AIA which downloads
  such missing certificates on demand. This is a server misconfiguration. A
  good way to figure out if this is the case it to use the SSL Labs server
  test and check the certificate chain: https://www.ssllabs.com/ssltest/

Details are also in the SSLCERTS.md document, found online here:
  https://curl.se/docs/sslcerts.html

4.13 Why is curl -R on Windows one hour off?

Since curl 7.53.0 this issue should be fixed as long as curl was built with
  any modern compiler that allows for a 64-bit curl_off_t type. For older
  compilers or prior curl versions it may 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 more details read this:
  https://www.codeproject.com/Articles/1144/Beating-the-Daylight-Savings-Time-bug-and-getting

4.14 Redirects work in browser but not with curl

curl supports HTTP redirects well (see item 3.8). Browsers generally support
  at least two other ways to perform redirects that curl does not:

Meta tags. You can write an 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 an 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 write a script that parses
  the results and fetches the new URL.

4.15 FTPS does not 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 an FTP:// URL and the --ftp-ssl option (or one
  of its related flavors). This is the most common method, and the one
  mandated by RFC 4217. This kind of connection will then of course use 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 requests with a small
  request body) use the "Expect: 100-continue" header. This header allows the
  server to deny the operation early so that libcurl can bail out before having
  to send any data. This is useful in authentication cases and others.

However, many servers do not implement the Expect: stuff properly and if the
  server does not 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:
  https://support.microsoft.com/en-us/kb/175523/en-us

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 will not 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 will find that even if D:\blah.txt does exist, curl returns a 'file
  not found' error.

According to RFC 1738 (https://www.ietf.org/rfc/rfc1738.txt),
  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 does not 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 does not curl return an error when the network cable is unplugged?

Unplugging a 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 should not be affected, just possibly
  delayed. Eventually, the physical break will be fixed or the data will be
  re-routed around the physical problem through another path.

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 to 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.

TCP keep alive will not detect the network going down before the TCP/IP
  connection is established (e.g. during a DNS lookup) or using protocols that
  do not 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.

A libcurl-using application running in a known physical environment (e.g.
  an embedded device with only a single network connection) may want to act
  immediately if its lone network connection goes down. That can be achieved
  by having the application monitor the network connection on its own using an
  OS-specific mechanism, then signaling libcurl to abort (see also item 5.13).

4.20 curl does not return error for HTTP non-200 responses

Correct. Unless you use -f (--fail).

When doing HTTP transfers, curl will perform exactly what you are asking it
  to do and if successful it will not return an error. You can use curl to
  test your web server's "file not found" page (that gets 404 back), you can
  use it to check your authentication protected web pages (that gets a 401
  back) and so on.

The specific HTTP response code does not constitute a problem or error for
  curl. It simply sends and delivers HTTP as you asked and if that worked,
  everything is fine and dandy. The response code is generally providing more
  higher level error information that curl does not care about. The error was
  not in the HTTP transfer.

If you want your command line to treat error codes in the 400 and up range
  as errors and thus return a non-zero value and possibly show an error
  message, curl has a dedicated option for that: -f (CURLOPT_FAILONERROR in
  libcurl speak).

You can also use the -w option and the variable %{response_code} to extract
  the exact response code that was returned in the response.

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. Note that you must never share the same handle in
  multiple threads.

There may be some exceptions to thread safety depending on how libcurl was
  built. Please review the guidelines for thread safety to learn more:
  https://curl.se/libcurl/c/threadsafe.html

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 are 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.

When you use the easy interface the connection cache is kept within the easy
  handle. If you instead use the multi interface, the connection cache will be
  kept within the multi handle and will be shared among all the easy handles
  that are used within the same multi 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 are using Visual Studio, you need to instead
  add CURL_STATICLIB in the "Preprocessor Definitions" section.

If you get a 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 do not 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 runtime linker (ld.so) could not
  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.
  They are usually:

* Add an option to the linker command line that specify the hard-coded path
    the runtime 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 have
    put the library (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 number of 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 of four different 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. Besides, faking IP address would
  imply sending IP packets with a made-up source address, and then you normally
  get a problem with receiving the packet 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. You may also consider using
  https://www.torproject.org/ .

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 returns 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 are 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 or when you wish to abort the transfer.

5.14 Using C++ non-static functions for callbacks?

libcurl is a C library, it does not know anything about C++ member functions.

You can overcome this "limitation" with relative ease using a static
  member function that is passed a pointer to the class:

// f is the pointer to your object.
     static size_t 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 tends to be how is a program supposed to parse the
  directory listing. How does it know what's a file and what's a directory and
  what's a symlink etc. If the FTP server supports the MLSD command then it
  will return data in a machine-readable format that can be parsed for type.
  The types are specified by RFC 3659 section 7.5.1. If MLSD is not supported
  then you have to work with what you are given. The LIST output format is
  entirely at the server's own liking and the NLST output does not reveal any
  types and in many cases does not 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.

Example - List only directories.
  ftp.funet.fi supports MLSD and ftp.kernel.org does not:

curl -s ftp.funet.fi/pub/ -X MLSD | \
       perl -lne 'print if s/(?:^|;)type=dir;[^ ]+ (.+)$/$1/'

curl -s ftp.kernel.org/pub/linux/kernel/ | \
       perl -lne 'print if s/^d[-rwx]{9}(?: +[^ ]+){7} (.+)$/$1/'

If you need to parse LIST output in libcurl one such existing
  list parser is available at https://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.

5.16 I want a different time-out

Sometimes users realize that CURLOPT_TIMEOUT and CURLOPT_CONNECTIMEOUT are
  not sufficiently advanced or flexible to cover all the various use cases and
  scenarios applications end up with.

libcurl offers many more ways to time-out operations. A common alternative
  is to use the CURLOPT_LOW_SPEED_LIMIT and CURLOPT_LOW_SPEED_TIME options to
  specify the lowest possible speed to accept before to consider the transfer
  timed out.

The most flexible way is by writing your own time-out logic and using
  CURLOPT_XFERINFOFUNCTION (perhaps in combination with other callbacks) and
  use that to figure out exactly when the right condition is met when the
  transfer should get stopped.

5.17 Can I write a server with libcurl?

No. libcurl offers no functions or building blocks to build any kind of
  Internet protocol server. libcurl is only a client-side library. For server
  libraries, you need to continue your search elsewhere but there exist many
  good open source ones out there for most protocols you could want a server
  for. There are also really good stand-alone servers that have been tested
  and proven for many years. There is no need for you to reinvent them.

5.18 Does libcurl use threads?

Put simply: no, libcurl will execute in the same thread you call it in. All
  callbacks will be called in the same thread as the one you call libcurl in.

If you want to avoid your thread to be blocked by the libcurl call, you make
  sure you use the non-blocking multi API which will do transfers
  asynchronously - still in the same single thread.

libcurl will potentially internally use threads for name resolving, if it
  was built to work like that, but in those cases it will create the child
  threads by itself and they will only be used and then killed internally by
  libcurl and never exposed to the outside.

6. License Issues

curl and libcurl are released under a MIT/X derivative license. The license
  is 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 derivative 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 does not clash with other licenses.

6.5 Can I modify curl/libcurl for my program and keep the changes secret?

Yes

The MIT/X derivative 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: https://curl.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 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 re-use the same connection if it can.

7.4 Does PHP/CURL have dependencies?

PHP/CURL is a module that comes with the regular PHP package. It depends on
  and uses libcurl, so you need to have libcurl installed properly before
  PHP/CURL can be used.

8. Development

8.1 Why does curl use C89?

As with everything in curl, there is a history and we keep using what we have
 used before until someone brings up the subject and argues for and works on
 changing it.

We started out using C89 in the 1990s because that was the only way to write
 a truly portable C program and have it run as widely as possible. C89 was for
 a long time even necessary to make things work on otherwise considered modern
 platforms such as Windows. Today, we do not really know how many users that
 still require the use of a C89 compiler.

We will continue to use C89 for as long as nobody brings up a strong enough
 reason for us to change our minds. The core developers of the project do not
 feel restricted by this and we are not convinced that going C99 will offer us
 enough of a benefit to warrant the risk of cutting off a share of users.

8.2 Will curl be rewritten?

In one go: no. Little by little over time? Maybe.

Over the years, new languages and clever operating environments come and go.
 Every now and then the urge apparently arises to request that we rewrite curl
 in another language.

Some the most important properties in curl are maintaining the API and ABI
 for libcurl and keeping the behavior for the command line tool. As long as we
 can do that, everything else is up for discussion. To maintain the ABI, we
 probably have to maintain a certain amount of code in C, and to remain rock
 stable, we will never risk anything by rewriting a lot of things in one go.
 That said, we can certainly offer more and more optional backends written in
 other languages, as long as those backends can be plugged in at build-time.
 Backends can be written in any language, but should probably provide APIs
 usable from C to ease integration and transition.

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