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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 for the socket stuff
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) is made with defines and macros, so that the source looks the
same at all places except for the header file that defines them.
(2) must be made by the application that uses libcurl, in curl that means
src/main.c has some code #ifdef'ed to do just that.
(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!
As described elsewhere, libcurl is meant to get two different "layers" of
interface. At the present point only the high-level, the "easy", interface
has been fully implemented and thus documented. We assume the easy-interface
in this description, the low-level interface will be documented when fully
There are plenty of 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. All the ones prefixed with 'curl_easy' are
put in the lib/easy.c file.
curl_easy_setopt() takes a three arguments, where the option stuff must be
passed in pairs, the parameter-ID and the parameter-value. The list of
options is documented in the man page.
curl_easy_perform() does a whole lot of things.
The function analyzes 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 (or at the user's choice, a specified callback might get
called). 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_easy_setopt().
When completed curl_easy_cleanup() should be called to free up used
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
HTTPS uses in almost every means the same procedure as HTTP, with only two
exceptions: the connect procedure is different and the function used to read
or write from the socket is different, although the latter fact is hidden in
the source by the use of curl_read() for reading and curl_write() for writing
data to the remote server.
The if2ip() function can be used for getting the IP number of a specified
network interface, and it resides in lib/if2ip.c. It is only used for the FTP
PORT command.
Telnet is implemented in lib/telnet.c.
The file:// protocol is dealt with in lib/file.c.
Everything LDAP is in lib/ldap.c.
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. curl_getdate() in lib/getdate.c is for HTTP date comparisons (and
lib/getenv.c offers curl_getenv() which 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.
main() resides in src/main.c together with most of the client
code. src/hugehelp.c is automatically generated by the 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_easy_*() functions of the library and when it gets back
control after the curl_easy_perform() it cleans up the library, checks status
and exits.
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