Newer
Older
}
return realsize;
}
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 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
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5.8 libcurl.so.3: 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.3.
You need to make sure that ld.so finds libcurl.so.3. 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
Daniel Stenberg
committed
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.
Daniel Stenberg
committed
- 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 but it currently has no IPv6
support.
Daniel Stenberg
committed
- The Windows threaded resolver. It use:
A - gethostbyname() on plain ipv4 windows hosts
B - getaddrinfo() on ipv6-enabled windows 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?
There are several ways, 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 also stop a transfer by removing
the particular easy handle from the multi stack.
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::staticFunction(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:staticFunction);
curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);
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
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.
6.1 I have a GPL program, can I use the libcurl library?
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?
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?
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?
The LGPL license doesn't clash with other licenses.
6.5 Can I modify curl/libcurl for my program and keep the changes secret?
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?
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 reveal or make public that you are using libcurl within
your app.
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 write 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.