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Updated for curl 7.6 on January 26, 2001
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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.
All changes to the sources are committed to the CVS repository as soon as
they're somewhat verified to work. Changes shall be commited as independently
as possible so that individual changes can be easier spotted and tracked
Tagging shall be used extensively, and by the time we release new archives we
should tag the sources with a name similar to the released version number.
Windows vs Unix
There are a few differences in how to program curl the unix way compared to
the Windows way. The four perhaps most notable details are:
1. Different function names for socket operations.
In curl, this is solved with defines and macros, so that the source looks
the same at all places except for the header file that defines them. The
macros in use are sclose(), sread() and swrite().
2. Windows requires a couple of init calls for the socket stuff
Those 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. The file descriptors for network communication and file operations are
not easily interchangable as in unix
We avoid this by not trying any funny tricks on file descriptors.
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)
We set stdout to binary under windows
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!
Generally speaking: always remember that this will be compiled on dozens of
operating systems. Don't walk on the edge.
As described elsewhere, libcurl is meant to get two different "layers" of
interfaces. At the present point only the high-level, the "easy", interface
has been fully implemented and 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.
All printf()-style functions use the supplied clones in lib/mprintf.c. This
makes sure we stay absolutely platform independent.
curl_easy_init() allocates an internal struct and makes some initializations.
The returned handle does not revail internals.
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:
It starts off in the lib/easy.c file by calling curl_transfer(), but the main
work is lib/url.c. The function first analyzes the URL, it separates the
different components and connects to the remote host. This may involve using
a proxy and/or using SSL. The Curl_gethost() function in lib/hostip.c is used
for looking up host names.
When connected, the proper protocol-specific function is called. The
functions are named after the protocols they handle. Curl_ftp(), Curl_http(),
Curl_dict(), etc. They all reside in their respective files (ftp.c, http.c
and dict.c).
The protocol-specific functions of course deal with protocol-specific
negotiations and setup. They have access to the Curl_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
Curl_Transfer() function (in lib/transfer.c) to setup the transfer and
returns. curl_transfer() then calls _Tranfer() in lib/transfer.c that
performs the entire file transfer.
During transfer, 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 completed, the 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 (and more) 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 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 Curl_if2ip() function can be used for getting the IP number of a
specified network interface, and it resides in lib/if2ip.c.
Curl_ftpsendf() is used for sending FTP commands to the remote server. It was
made a separate function to prevent us programmers from forgetting that they
must be CRLF terminated. They must also be sent in one single write() to make
firewalls and similar happy.
The kerberos support is mainly in lib/krb4.c and lib/security.c.
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 environment variables. Note that contrary
to the normal unix getenv(), this returns an allocated buffer that must be
free()ed after use.
lib/netrc.c holds the .netrc parser
lib/timeval.c features replacement functions for systems that don't have
gettimeofday() and a few support functions for timeval convertions.
A function named curl_version() that returns the full curl version string is
found in lib/version.c.
If authentication is requested but no password is given, a getpass_r() clone
exists in lib/getpass.c. libcurl offers a custom callback that can be used
instead of this, but it doesn't change much to us.
Library Symbols
All symbols used internally in libcurl must use a 'Curl_' prefix if they're
used in more than a single file. Single-file symbols must be made
static. Public (exported) symbols must use a 'curl_' prefix. (There are
exceptions, but they are destined to be changed to follow this pattern in the
Return Codes and Informationals
I've made things simple. Almost every function in libcurl returns a CURLcode,
that must be CURLE_OK if everything is OK or otherwise a suitable error code
as the curl/curl.h include file defines. The very spot that detects an error
must use the Curl_failf() function to set the human-readable error
In aiding the user to understand what's happening and to debug curl usage, we
must supply a fair amount of informational messages by using the Curl_infof()
function. Those messages are only displayed when the user explicitly asks for
them. They are best used when revealing information that isn't otherwise
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 URL-"globbing" support. Globbing in the sense that the {} and []
expansion stuff is there.
The client mostly messes 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.
When the operation is done, the ourWriteOut() function in src/writeout.c may
be called to report about the operation. That function is using the
curl_easy_getinfo() function to extract useful information from the curl
Recent versions may loop and do all that several times if many URLs were
specified on the command line or config file.
Memory Debugging
The file named lib/memdebug.c contains debug-versions of a few
functions. Functions such as malloc, free, fopen, fclose, etc that somehow
deal with resources that might give us problems if we "leak" them. The
functions in the memdebug system do nothing fancy, they do their normal
function and then log information about what they just did. The logged data
is then analyzed after a complete session, is a perl script present only in CVS (not part of the release
archives) that analyzes a log file generated by the memdebug system. It
detects if resources are allocated but never freed and other kinds of errors
related to resource management.
Use -DMALLOCDEBUG when compiling to enable memory debugging.
Test Suite
Since November 2000, a test suite has evolved. It is placed in its own
subdirectory directly off the root in the curl archive tree, and it contains
a bunch of scripts and a lot of test case data.
The main test script is that will invoke the two servers and before all the test cases are performed. The
test suite currently only runs on unix-like platforms.
You'll find a complete description of the test case data files in the README
file in the test directory.
The test suite automatically detects if curl was built with the memory
debugging enabled, and if it was it will detect memory leaks too.
Building Releases
There's no magic to this. When you consider everything stable enough to be
released, run the 'maketgz' script (using 'make distcheck' will give you a
pretty good view on the status of the current sources). maketgz prompts for
version number of the client and the library before it creates a release
You must have autoconf installed to build release archives.
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