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_ _ ____ _ ___| | | | _ \| | / __| | | | |_) | | | (__| |_| | _ <| |___ \___|\___/|_| \_\_____| INTERNALS 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 it. The largest amount of code and complexity is in the library part. GIT === All changes to the sources are committed to the git repository as soon as they're somewhat verified to work. Changes shall be committed as independently as possible so that individual changes can be easier spotted and tracked afterwards. 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. Portability =========== We write curl and libcurl to compile with C89 compilers. On 32bit and up machines. Most of libcurl assumes more or less POSIX compliance but that's not a requirement. We write libcurl to build and work with lots of third party tools, and we want it to remain functional and buildable with these and later versions (older versions may still work but is not what we work hard to maintain): OpenSSL 0.9.6 GnuTLS 1.2 zlib 1.1.4 libssh2 0.16 c-ares 1.6.0 libidn 0.4.1 cyassl 2.0.0 openldap 2.0 MIT krb5 lib 1.2.4 qsossl V5R2M0 NSS 3.14.x axTLS 1.2.7 Heimdal ? On systems where configure runs, we aim at working on them all - if they have a suitable C compiler. On systems that don't run configure, we strive to keep curl running fine on: Windows 98 AS/400 V5R2M0 Symbian 9.1 Windows CE ? TPF ? When writing code (mostly for generating stuff included in release tarballs) we use a few "build tools" and we make sure that we remain functional with these versions: GNU Libtool 1.4.2 GNU Autoconf 2.57 GNU Automake 1.7 (we currently avoid 1.10 due to Solaris-related bugs) GNU M4 1.4 perl 5.004 roffit 0.5 groff ? (any version that supports "groff -Tps -man [in] [out]") ps2pdf (gs) ? 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. That's taken care of by the curl_global_init() call, but if other libs also do it etc there might be reasons for applications to alter that behaviour. 3. The file descriptors for network communication and file operations are not easily interchangeable 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 a curl_config-win32.h file in lib directory that is supposed to look exactly as a curl_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. Library ======= 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_resolv() function in lib/hostip.c is used for looking up host names (it does then use the proper underlying method, which may vary between platforms and builds). 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 Curl_ftp(). 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. 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. Some time during the DO function, the Curl_setup_transfer() function must be called with some basic info about the upcoming transfer: what socket(s) to read/write and the expected file transfer sizes (if known). 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 connection. 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 connections. 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_handler::do_it() - 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(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 (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 encoding. An interesting detail with the HTTP(S) request, is the Curl_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. FTP 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. Kerberos The kerberos support is mainly in lib/krb4.c and lib/security.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 and lib/openldap.c GENERAL URL encoding and decoding, called escaping and unescaping in the source code, is found in lib/escape.c. While transferring data in Transfer() a few functions might get used. curl_getdate() in lib/parsedate.c is for HTTP date comparisons (and more). 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 conversions. A function named curl_version() that returns the full curl version string is found in lib/version.c. 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. Note that when you use the multi interface, the multi handle will hold the connection cache and not the particular easy handle. This of course to allow all easy handles in a multi stack to be able to share and re-use connections. o This enables the 'curl handle' to be reused on subsequent transfers. 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, unless of course the multi interface "owns" the connections. You do realize that the curl handle must be re-used in order for the persistent connections to work. multi interface/non-blocking ============================ We make an effort to provide a non-blocking interface to the library, the multi interface. To make that interface work as good as possible, no low-level functions within libcurl must be written to work in a blocking manner. One of the primary reasons we introduced c-ares support was to allow the name resolve phase to be perfectly non-blocking as well. The ultimate goal is to provide the easy interface simply by wrapping the multi interface functions and thus treat everything internally as the multi interface is the single interface we have. The FTP and the SFTP/SCP protocols are thus perfect examples of how we adapt and adjust the code to allow non-blocking operations even on multi-stage protocols. They are built around state machines that return when they could block waiting for data. The DICT, LDAP and TELNET protocols are crappy examples and they are subject for rewrite in the future to better fit the libcurl protocol family. SSL libraries ============= Originally libcurl supported SSLeay for SSL/TLS transports, but that was then extended to its successor OpenSSL but has since also been extended to several other SSL/TLS libraries and we expect and hope to further extend the support in future libcurl versions. To deal with this internally in the best way possible, we have a generic SSL function API as provided by the sslgen.[ch] system, and they are the only SSL functions we must use from within libcurl. sslgen is then crafted to use the appropriate lower-level function calls to whatever SSL library that is in use. 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.) Public API functions are marked with CURL_EXTERN in the public header files so that all others can be hidden on platforms where this is possible. 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 description. 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 obvious. API/ABI ======= We make an effort to not export or show internals or how internals work, as that makes it easier to keep a solid API/ABI over time. See docs/libcurl/ABI for our promise to users. Client ====== main() resides in src/main.c together with most of the client code. src/tool_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 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 session. 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, memanalyze.pl is the perl script present in tests/ that analyzes a log file generated by the memory tracking system. It detects if resources are allocated but never freed and other kinds of errors related to resource management. Internally, definition of preprocessor symbol DEBUGBUILD restricts code which is only compiled for debug enabled builds. And symbol CURLDEBUG is used to differentiate code which is _only_ used for memory tracking/debugging. Use -DCURLDEBUG when compiling to enable memory debugging, this is also switched on by running configure with --enable-curldebug. Use -DDEBUGBUILD when compiling to enable a debug build or run configure with --enable-debug. curl --version will list 'Debug' feature for debug enabled builds, and will list 'TrackMemory' feature for curl debug memory tracking capable builds. These features are independent and can be controlled when running the configure script. When --enable-debug is given both features will be enabled, unless some restriction prevents memory tracking from being used. Test Suite ========== The test suite 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 runtests.pl that will invoke test servers like httpserver.pl and ftpserver.pl before all the test cases are performed. The test suite currently only runs on unix-like platforms. You'll find a description of the test suite in the tests/README file, and the test case data files in the tests/FILEFORMAT 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, do this: 1. Tag the source code accordingly. 2. run the 'maketgz' script (using 'make distcheck' will give you a pretty good view on the status of the current sources). maketgz requires a version number and creates the release archive. maketgz uses 'make dist' for the actual archive building, why you need to fill in the Makefile.am files properly for which files that should be included in the release archives. 3. When that's complete, sign the output files. 4. Upload 5. Update web site and changelog on site 6. Send announcement to the mailing lists NOTE: you must have curl checked out from git to be able to do a proper release build. The release tarballs do not have everything setup in order to do releases properly.