curl/docs/INTERNALS

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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.