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Updated for curl 7.9.1 on November 2, 2001
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The project is split in two. The library and the client. The client part uses
the library, but the library is designed to allow other applications to use
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, We make an effort to avoid '#ifdef [Your OS]'. All
conditionals that deal with features *should* instead be in the format
'#ifdef HAVE_THAT_WEIRD_FUNCTION'. Since Windows can't run configure scripts,
we maintain two config-win32.h files (one in lib/ 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.
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_global_init_() and curl_global_cleanup() should be called by the
application to initialize and clean up global stuff in the library. As of
today, it can handle the global SSL initing if SSL is enabled and it can init
the socket layer on windows machines. libcurl itself has no "global" scope.
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 reveal internals. This is the 'SessionHandle'
struct which works as an "anchor" struct for all curl_easy functions. All
connections performed will get connect-specific data allocated that should be
used for things related to particular connections/requests.
curl_easy_setopt() takes 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. This function mainly sets things in
the 'SessionHandle' struct.
curl_easy_perform() does a whole lot of things:
It starts off in the lib/easy.c file by calling Curl_perform() and the main
work then continues in lib/url.c. The flow continues with a call to
Curl_connect() to connect to the remote site.
o Curl_connect()
... 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 Curl_connect is done, we are connected to the remote site. Then it is
time to tell the server to get a document/file. Curl_do() arranges this.
This function makes sure there's an allocated and initiated 'connectdata'
struct that is used for this particular connection only (although there may
be several requests performed on the same connect). A bunch of things are
inited/inherited from the SessionHandle struct.
o Curl_do()
Curl_do() makes sure 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). HTTPS is handled by Curl_http() and FTPS by
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
Starting in 7.9.1, if this DO function fails and the connection is being
re-used, libcurl will then close this connection, setup a new connection
and re-issue the DO request on that. This is because there is no way to be
perfectly sure that we have discovered a dead connection before the DO
function and thus we might wrongly be re-using a connection that was closed
by the remote peer.
o Transfer()
Curl_perform() then calls Transfer() 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.
o Curl_done()
Called after a transfer is done. This function takes care of everything
that has to be done after a transfer. This function attempts to leave
matters in a state so that Curl_do() should be possible to call again on
the same connection (in a persistent connection case). It might also soon
be closed with Curl_disconnect().
o Curl_disconnect()
When doing normal connections and transfers, no one ever tries to close any
connections so this is not normally called when curl_easy_perform() is
used. This function is only used when we are certain that no more transfers
is going to be made on the connection. It can be also closed by force, or
it can be called to make sure that libcurl doesn't keep too many
connections alive at the same time (there's a default amount of 5 but that
can be changed with the CURLOPT_MAXCONNECTS option).
This function cleans up all resources that are associated with a single
Curl_perform() is the function that does the main "connect - do - transfer -
done" loop. It loops if there's a Location: to follow.
When completed, the curl_easy_cleanup() should be called to free up used
resources. It runs Curl_disconnect() on all open connectons.
A quick roundup on internal function sequences (many of these call
protocol-specific function-pointers):
curl_connect - connects to a remote site and does initial connect fluff
This also checks for an existing connection to the requested site and uses
that one if it is possible.
curl_do - starts a transfer
curl_transfer() - transfers data
curl_done - ends a transfer
curl_disconnect - disconnects from a remote site. This is called when the
disconnect is really requested, which doesn't necessarily have to be
exactly after curl_done in case we want to keep the connection open for
a while.
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.
http_chunks.c contains functions that understands HTTP 1.1 chunked transfer
An interesting detail with the HTTP(S) request, is the add_buffer() series of
functions we use. They append data to one single buffer, and when the
building is done the entire request is sent off in one single write. This is
done this way to overcome problems with flawed firewalls and lame servers.
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.
Persistent Connections
The persistent connection support in libcurl requires some considerations on
how to do things inside of the library.
o The 'SessionHandle' struct returned in the curl_easy_init() call must never
hold connection-oriented data. It is meant to hold the root data as well as
all the options etc that the library-user may choose.
o The 'SessionHandle' struct holds the "connection cache" (an array of
pointers to 'connectdata' structs). There's one connectdata struct
allocated for each connection that libcurl knows about.
o This also enables the 'curl handle' to be reused on subsequent transfers,
something that was illegal before libcurl 7.7.
o When we are about to perform a transfer with curl_easy_perform(), we first
check for an already existing connection in the cache that we can use,
otherwise we create a new one and add to the cache. If the cache is full
already when we add a new connection, we close one of the present ones. We
select which one to close dependent on the close policy that may have been
previously set.
o When the transfer operation is complete, we try to leave the connection
open. Particular options may tell us not to, and protocols may signal
closure on connections and then we don't keep it open of course.
o When curl_easy_cleanup() is called, we close all still opened connections.
You do realize that the curl handle must be re-used in order for the
persistent connections to work.
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 to be changed to follow this pattern in future versions.)
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 this several times if many URLs were
specified on the command line or config file.
Memory Debugging
The file 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 can then be analyzed
after a complete session, is the perl script present only present 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, this is also
switched on by running configure with --enable-debug.
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
tests/README file.
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
archive. maketgz uses 'make dist' for the actual archive building, why you
need to fill in the files properly for which files that should
be included in the release archives.