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curl/docs/FAQ
Daniel Stenberg a3eb91ffb1 shortened the "what is libcurl" text
2001-01-15 14:59:07 +00:00

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Updated: January 15, 2001 (http://curl.haxx.se/docs/faq.shtml)
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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 ?
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 javascript, 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?
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"
4.6 Can you tell me what error code 142 means?
4.7 How do I keep usernames 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?
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?
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?
==============================================================================
1. Philosophy
1.1 What is cURL?
cURL (or simply just 'curl') is a command line tool for getting or sending
files using URL syntax. 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.
Curl supports a range of common internet protocols, currently including
HTTP, HTTPS, FTP, GOPHER, LDAP, DICT, TELNET and FILE.
We spell it cURL or just curl.
1.2 What is libcurl?
libcurl is a reliable, higly portable multiprotocol file transfer library.
Any application is free to use libcurl, even commercial or closed-source
ones.
1.3 What is cURL not?
Curl is *not*, I repeat, *not* a wget clone even though that is a very
common misconception. Never, during curl's development, have I 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 wanna 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 behaviour 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.
Curl is not a single-OS program. Curl exists, compiles, builds and runs
under a wide range of operating systems, including all modern Unixes,
Windows, Amiga, BeOS, OS/2, OS X, QNX etc.
1.4 When will you make curl do XXXX ?
I love suggestions of what to change in order to make curl and libcurl
better. I do however believe in a few rules when it comes to the future of
curl:
* It is to remain a command line tool. If you want GUIs or fancy scripting
capabilities, you're free to write another tool that uses libcurl and that
offers this. There's no point in having one single tool that does every
imaginable thing. That's also one of the great advantages of having the
core of curl as a library: libcurl.
* I 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.
* I focus on protocol related issues and improvements. If you wanna do more
magic with the supported protocols than curl currently does, chances are
big I will agree. If you wanna add more protocols, I may very well
agree.
* If you want me to make all the work while you wait for me to implement it
for you, that is not a very friendly attitude. I spend a considerable time
already on maintaining and developing curl. In order to get more out of
me, I trust you will offer some of your time and efforts in return.
* If you write the code, chances are bigger that it will get into curl
faster.
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
Solution submitted by: Bob Allison <allisonb@users.sourceforge.net>
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 RSASEF 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, although I doubt there would be much
problems using a different library. I just don't know any other free one and
that has limited my possibilities to develop against anything else.
If anyone does "port" curl to use a commercial SSL library, I am 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 page to find
accurate and up-to-date pointers to recent OpenSSL DDLs and other binary
packages.
2.4. Does cURL support Socks (RFC 1928) ?
No. Nobody has wanted it that badly yet. I would appriciate patches that
brings this functionality.
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 configure script couldn't find all libs and include files
it 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 labeled "configure doesn't
find OpenSSL even when it is installed".
3.2. How do I tell curl to resume a transfer?
Curl supports resume both ways on FTP, download ways on 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', than 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'.
I have described this in some detail in the README.curl file, and if you
don't understand it the first time, read it again before you post questions
about this to the mailing list. I would also suggest that you read 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 transfering 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 javascript, 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.
Javascript is slightly different since that is code embedded in the HTML
that is sent for the client to interpret and curl has no javascript
interpreter.
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'
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
4. Running Problems
4.1. Problems connecting to SSL servers.
It took a very long time before I 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).
I have also seen 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 & letter is treated special and when used it
runs the specified command in the background. To safely send the & as a part
of a URL, you should qoute the entire URL by using single (') or double (")
quotes around it.
An example that would invoke a remote CGI that uses &-letters could be:
curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
In win32, the standard DOS shell treats the %-letter specially and you may
need to quote the string properly when % is used in it.
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
(included in curl 7.6 and later):
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. I'll make a short transcript
here. 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.
4.6. Can you tell me what error code 142 means?
All error codes that are larger than the highest documented error code means
that curl has existed due to a timeout. There was no nice way for curl to
abort from such a condition and that's why it got this undocumented
error. This should not occur in releases after 7.4.1.
4.7. How do I keep usernames and passwords secret in Curl command lines?
I see this problem as two parts:
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 tho tell curl to read parameters from a
file or stdin to which you can pass the secret info.
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 and FTP passwords are sent in clear across the
network. All it takes for anyone to fetch them is to listen on the network.
Evesdropping is very easy.
4.8 I found a bug!
It is not a bug if the behaviour is documented. Read the docs first.
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, post a bug report in the Curl Bug Track System over at
http://sourceforge.net/bugs/?group_id=976
Always include as many details you can think of, including curl version,
operating system name and version and complete instructions how to repeat
the bug.
4.9. Curl can't authenticate to the server that requires NTLM?
NTLM is a Microsoft proprietary protocol. Unfortunately, curl does not
currently support that.
5. libcurl Issues
5.1. Is libcurl thread-safe?
Yes.
We have written the libcurl code specificly adjusted for multi-threaded
programs. libcurl will use thread-safe functions instead of non-safe ones if
your system has such.
I am very interested in once and for all getting some kind of report or
README file from those who have used libcurl in a threaded environment,
since I haven't and I get this question more and more frequently!
5.2 How can I receive all data into a large memory chunk?
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 receivied 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
curl_easy_setopt(CURLOPT_FILE) function. 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)
{
register int 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?
The easy interface of libcurl does not support multiple requests using the
same connection. The only available way to do multiple requests is to
init/perform/cleanup for each request.
5.4 Does libcurl do Winsock initing on win32 systems?
No.
On win32 systems, you need to init the winsock stuff manually, libcurl will
not do that for you. WSAStartup() and WSACleanup() should be used
accordingly. The reason for this is of course that a single application may
use several different libraries and parts, and there's no reason for every
single library to do this.
6. License Issues
NOTE: This section is now updated to concern curl 7.5.2 or later!
Curl and libcurl are released under a MIT/X derivate license *or* the MPL,
the Mozilla Public License. To get a really good answer to your license
conflict questions, you should study the MPL and MIT/X licenses and the
license you are about to use and check for clashes yourself. 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.)
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 don'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 carefully picked this license years ago 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. The recent dual
license modification should make it possible for everyone to use libcurl or
curl in their projects, no matter what license they already have in use.
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