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INTERNALS
The project is kind of split in two. The library and the client. The client
part uses the library, but the library is meant to be designed to allow other
applications to use it.
Thus, the largest amount of code and complexity is in the library part.
Windows vs Unix
===============
There are a few differences in how to program curl the unix way compared to
the Windows way. The four most notable details are:
1. Different function names for close(), read(), write()
2. Windows requires a couple of init calls
3. The file descriptors for network communication and file operations are
not easily interchangable as in unix
4. When writing data to stdout, Windows makes end-of-lines the DOS way, thus
destroying binary data, although you do want that conversion if it is
text coming through... (sigh)
In curl, (1) and (2) are done with defines and macros, so that the source
looks the same at all places except for the header file that defines them.
(3) is simply avoided by not trying any funny tricks on file descriptors.
(4) is left alone, giving windows users problems when they pipe binary data
through stdout...
Inside the source code, I do make an effort to avoid '#ifdef WIN32'. All
conditionals that deal with features *should* instead be in the format
'#ifdef HAVE_THAT_WEIRD_FUNCTION'. Since Windows can't run configure scripts,
I maintain two config-win32.h files (one in / and one in src/) that are
supposed to look exactly as a config.h file would have looked like on a
Windows machine!
Library
=======
There is a few entry points to the library, namely each publicly defined
function that libcurl offers to applications. All of those functions are
rather small and easy-to-follow, accept the one single and do-it-all named
curl_urlget() (entry point in lib/url.c).
curl_urlget() takes a variable amount of arguments, and they must all be
passed in pairs, the parameter-ID and the parameter-value. The list of
arguments must be ended with a end-of-arguments parameter-ID.
The function then continues to analyze the URL, get the different components
and connects to the remote host. This may involve using a proxy and/or using
SSL. The GetHost() function in lib/hostip.c is used for looking up host
names.
When connected, the proper function is called. The functions are named after
the protocols they handle. ftp(), http(), dict(), etc. They all reside in
their respective files (ftp.c, http.c and dict.c).
The protocol-specific functions deal with protocol-specific negotiations and
setup. They have access to the sendf() (from lib/sendf.c) function to send
printf-style formatted data to the remote host and when they're ready to make
the actual file transfer they call the Transfer() function (in
lib/download.c) to do the transfer. All printf()-style functions use the
supplied clones in lib/mprintf.c.
While transfering, the progress functions in lib/progress.c are called at a
frequent interval. The speedcheck functions in lib/speedcheck.c are also used
to verify that the transfer is as fast as required.
When the operation is done, the writeout() function in lib/writeout.c may be
called to report about the operation as specified previously in the arguments
to curl_urlget().
HTTP(S)
HTTP offers a lot and is the protocol in curl that uses the most lines of
code. There is a special file (lib/formdata.c) that offers all the multipart
post functions.
base64-functions for user+password stuff is in (lib/base64.c) and all
functions for parsing and sending cookies are found in
(lib/cookie.c).
HTTPS uses in almost every means the same procedure as HTTP, with only two
exceptions: the connect procedure is different and the function used
FTP
The if2ip() function can be used for getting the IP number of a specified
network interface, and it resides in lib/if2ip.c
TELNET
Telnet is implemented in lib/telnet.c.
FILE
The file:// protocol is dealt with in lib/file.c.
LDAP
Everything LDAP is in lib/ldap.c.
GENERAL
URL encoding and decoding, called escaping and unescaping in the source code,
is found in lib/escape.c.
While transfering data in Transfer() a few functions might get
used. get_date() in lib/getdate.c is for HTTP date comparisons.
lib/getenv.c is for reading environment variables in a neat platform
independent way. That's used in the client, but also in lib/url.c when
checking the PROXY variables.
lib/netrc.c keeps the .netrc parser
lib/timeval.c features replacement functions for systems that don't have
A function named curl_version() that returns the full curl version string is
found in lib/version.c.
Client
======
main() resides in src/main.c together with most of the client
code. src/hugehelp.c is automatically generated by the mkhelp.pl perl script
to display the complete "manual" and the src/urlglob.c file holds the
functions used for the multiple-URL support.
The client mostly mess around to setup its config struct properly, then it
calls the curl_urlget() function in the library and when it gets back control
it checks status and exits.
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