5 \___|\___/|_| \_\_____|
9 The project is split in two. The library and the client. The client part uses
10 the library, but the library is designed to allow other applications to use
13 The largest amount of code and complexity is in the library part.
17 All changes to the sources are committed to the git repository as soon as
18 they're somewhat verified to work. Changes shall be committed as independently
19 as possible so that individual changes can be easier spotted and tracked
22 Tagging shall be used extensively, and by the time we release new archives we
23 should tag the sources with a name similar to the released version number.
28 We write curl and libcurl to compile with C89 compilers. On 32bit and up
29 machines. Most of libcurl assumes more or less POSIX compliance but that's
32 We write libcurl to build and work with lots of third party tools, and we
33 want it to remain functional and buildable with these and later versions
34 (older versions may still work but is not what we work hard to maintain):
50 On systems where configure runs, we aim at working on them all - if they have
51 a suitable C compiler. On systems that don't run configure, we strive to keep
60 When writing code (mostly for generating stuff included in release tarballs)
61 we use a few "build tools" and we make sure that we remain functional with
66 GNU Automake 1.7 (we currently avoid 1.10 due to Solaris-related bugs)
70 groff ? (any version that supports "groff -Tps -man [in] [out]")
76 There are a few differences in how to program curl the unix way compared to
77 the Windows way. The four perhaps most notable details are:
79 1. Different function names for socket operations.
81 In curl, this is solved with defines and macros, so that the source looks
82 the same at all places except for the header file that defines them. The
83 macros in use are sclose(), sread() and swrite().
85 2. Windows requires a couple of init calls for the socket stuff.
87 That's taken care of by the curl_global_init() call, but if other libs also
88 do it etc there might be reasons for applications to alter that behaviour.
90 3. The file descriptors for network communication and file operations are
91 not easily interchangeable as in unix.
93 We avoid this by not trying any funny tricks on file descriptors.
95 4. When writing data to stdout, Windows makes end-of-lines the DOS way, thus
96 destroying binary data, although you do want that conversion if it is
97 text coming through... (sigh)
99 We set stdout to binary under windows
101 Inside the source code, We make an effort to avoid '#ifdef [Your OS]'. All
102 conditionals that deal with features *should* instead be in the format
103 '#ifdef HAVE_THAT_WEIRD_FUNCTION'. Since Windows can't run configure scripts,
104 we maintain a curl_config-win32.h file in lib directory that is supposed to
105 look exactly as a curl_config.h file would have looked like on a Windows
108 Generally speaking: always remember that this will be compiled on dozens of
109 operating systems. Don't walk on the edge.
114 (See LIBCURL-STRUCTS for a separate document describing all major internal
115 structs and their purposes.)
117 There are plenty of entry points to the library, namely each publicly defined
118 function that libcurl offers to applications. All of those functions are
119 rather small and easy-to-follow. All the ones prefixed with 'curl_easy' are
120 put in the lib/easy.c file.
122 curl_global_init_() and curl_global_cleanup() should be called by the
123 application to initialize and clean up global stuff in the library. As of
124 today, it can handle the global SSL initing if SSL is enabled and it can init
125 the socket layer on windows machines. libcurl itself has no "global" scope.
127 All printf()-style functions use the supplied clones in lib/mprintf.c. This
128 makes sure we stay absolutely platform independent.
130 curl_easy_init() allocates an internal struct and makes some initializations.
131 The returned handle does not reveal internals. This is the 'SessionHandle'
132 struct which works as an "anchor" struct for all curl_easy functions. All
133 connections performed will get connect-specific data allocated that should be
134 used for things related to particular connections/requests.
136 curl_easy_setopt() takes three arguments, where the option stuff must be
137 passed in pairs: the parameter-ID and the parameter-value. The list of
138 options is documented in the man page. This function mainly sets things in
139 the 'SessionHandle' struct.
141 curl_easy_perform() is just a wrapper function that makes use of the multi
142 API. It basically curl_multi_init(), curl_multi_add_handle(),
143 curl_multi_wait(), and curl_multi_perform() until the transfer is done and
146 Some of the most important key functions in url.c are called from multi.c
147 when certain key steps are to be made in the transfer operation.
151 Analyzes the URL, it separates the different components and connects to the
152 remote host. This may involve using a proxy and/or using SSL. The
153 Curl_resolv() function in lib/hostip.c is used for looking up host names
154 (it does then use the proper underlying method, which may vary between
155 platforms and builds).
157 When Curl_connect is done, we are connected to the remote site. Then it is
158 time to tell the server to get a document/file. Curl_do() arranges this.
160 This function makes sure there's an allocated and initiated 'connectdata'
161 struct that is used for this particular connection only (although there may
162 be several requests performed on the same connect). A bunch of things are
163 inited/inherited from the SessionHandle struct.
167 Curl_do() makes sure the proper protocol-specific function is called. The
168 functions are named after the protocols they handle.
170 The protocol-specific functions of course deal with protocol-specific
171 negotiations and setup. They have access to the Curl_sendf() (from
172 lib/sendf.c) function to send printf-style formatted data to the remote
173 host and when they're ready to make the actual file transfer they call the
174 Curl_Transfer() function (in lib/transfer.c) to setup the transfer and
177 If this DO function fails and the connection is being re-used, libcurl will
178 then close this connection, setup a new connection and re-issue the DO
179 request on that. This is because there is no way to be perfectly sure that
180 we have discovered a dead connection before the DO function and thus we
181 might wrongly be re-using a connection that was closed by the remote peer.
183 Some time during the DO function, the Curl_setup_transfer() function must
184 be called with some basic info about the upcoming transfer: what socket(s)
185 to read/write and the expected file transfer sizes (if known).
189 Called during the transfer of the actual protocol payload.
191 During transfer, the progress functions in lib/progress.c are called at a
192 frequent interval (or at the user's choice, a specified callback might get
193 called). The speedcheck functions in lib/speedcheck.c are also used to
194 verify that the transfer is as fast as required.
198 Called after a transfer is done. This function takes care of everything
199 that has to be done after a transfer. This function attempts to leave
200 matters in a state so that Curl_do() should be possible to call again on
201 the same connection (in a persistent connection case). It might also soon
202 be closed with Curl_disconnect().
206 When doing normal connections and transfers, no one ever tries to close any
207 connections so this is not normally called when curl_easy_perform() is
208 used. This function is only used when we are certain that no more transfers
209 is going to be made on the connection. It can be also closed by force, or
210 it can be called to make sure that libcurl doesn't keep too many
211 connections alive at the same time.
213 This function cleans up all resources that are associated with a single
219 HTTP offers a lot and is the protocol in curl that uses the most lines of
220 code. There is a special file (lib/formdata.c) that offers all the multipart
223 base64-functions for user+password stuff (and more) is in (lib/base64.c) and
224 all functions for parsing and sending cookies are found in (lib/cookie.c).
226 HTTPS uses in almost every means the same procedure as HTTP, with only two
227 exceptions: the connect procedure is different and the function used to read
228 or write from the socket is different, although the latter fact is hidden in
229 the source by the use of Curl_read() for reading and Curl_write() for writing
230 data to the remote server.
232 http_chunks.c contains functions that understands HTTP 1.1 chunked transfer
235 An interesting detail with the HTTP(S) request, is the Curl_add_buffer()
236 series of functions we use. They append data to one single buffer, and when
237 the building is done the entire request is sent off in one single write. This
238 is done this way to overcome problems with flawed firewalls and lame servers.
242 The Curl_if2ip() function can be used for getting the IP number of a
243 specified network interface, and it resides in lib/if2ip.c.
245 Curl_ftpsendf() is used for sending FTP commands to the remote server. It was
246 made a separate function to prevent us programmers from forgetting that they
247 must be CRLF terminated. They must also be sent in one single write() to make
248 firewalls and similar happy.
252 The kerberos support is mainly in lib/krb4.c and lib/security.c.
256 Telnet is implemented in lib/telnet.c.
260 The file:// protocol is dealt with in lib/file.c.
264 Everything LDAP is in lib/ldap.c and lib/openldap.c
268 URL encoding and decoding, called escaping and unescaping in the source code,
269 is found in lib/escape.c.
271 While transferring data in Transfer() a few functions might get used.
272 curl_getdate() in lib/parsedate.c is for HTTP date comparisons (and more).
274 lib/getenv.c offers curl_getenv() which is for reading environment variables
275 in a neat platform independent way. That's used in the client, but also in
276 lib/url.c when checking the proxy environment variables. Note that contrary
277 to the normal unix getenv(), this returns an allocated buffer that must be
280 lib/netrc.c holds the .netrc parser
282 lib/timeval.c features replacement functions for systems that don't have
283 gettimeofday() and a few support functions for timeval conversions.
285 A function named curl_version() that returns the full curl version string is
286 found in lib/version.c.
288 Persistent Connections
289 ======================
291 The persistent connection support in libcurl requires some considerations on
292 how to do things inside of the library.
294 o The 'SessionHandle' struct returned in the curl_easy_init() call must never
295 hold connection-oriented data. It is meant to hold the root data as well as
296 all the options etc that the library-user may choose.
297 o The 'SessionHandle' struct holds the "connection cache" (an array of
298 pointers to 'connectdata' structs).
299 o This enables the 'curl handle' to be reused on subsequent transfers.
300 o When libcurl is told to perform a transfer, it first checks for an already
301 existing connection in the cache that we can use. Otherwise it creates a
302 new one and adds that the cache. If the cache is full already when a new
303 conncetion is added added, it will first close the oldest unused one.
304 o When the transfer operation is complete, the connection is left
305 open. Particular options may tell libcurl not to, and protocols may signal
306 closure on connections and then they won't be kept open of course.
307 o When curl_easy_cleanup() is called, we close all still opened connections,
308 unless of course the multi interface "owns" the connections.
310 The curl handle must be re-used in order for the persistent connections to
313 multi interface/non-blocking
314 ============================
316 The multi interface is a non-blocking interface to the library. To make that
317 interface work as good as possible, no low-level functions within libcurl
318 must be written to work in a blocking manner. (There are still a few spots
319 violating this rule.)
321 One of the primary reasons we introduced c-ares support was to allow the name
322 resolve phase to be perfectly non-blocking as well.
324 The FTP and the SFTP/SCP protocols are examples of how we adapt and adjust
325 the code to allow non-blocking operations even on multi-stage command-
326 response protocols. They are built around state machines that return when
327 they would otherwise block waiting for data. The DICT, LDAP and TELNET
328 protocols are crappy examples and they are subject for rewrite in the future
329 to better fit the libcurl protocol family.
334 Originally libcurl supported SSLeay for SSL/TLS transports, but that was then
335 extended to its successor OpenSSL but has since also been extended to several
336 other SSL/TLS libraries and we expect and hope to further extend the support
337 in future libcurl versions.
339 To deal with this internally in the best way possible, we have a generic SSL
340 function API as provided by the sslgen.[ch] system, and they are the only SSL
341 functions we must use from within libcurl. sslgen is then crafted to use the
342 appropriate lower-level function calls to whatever SSL library that is in
348 All symbols used internally in libcurl must use a 'Curl_' prefix if they're
349 used in more than a single file. Single-file symbols must be made static.
350 Public ("exported") symbols must use a 'curl_' prefix. (There are exceptions,
351 but they are to be changed to follow this pattern in future versions.) Public
352 API functions are marked with CURL_EXTERN in the public header files so that
353 all others can be hidden on platforms where this is possible.
355 Return Codes and Informationals
356 ===============================
358 I've made things simple. Almost every function in libcurl returns a CURLcode,
359 that must be CURLE_OK if everything is OK or otherwise a suitable error code
360 as the curl/curl.h include file defines. The very spot that detects an error
361 must use the Curl_failf() function to set the human-readable error
364 In aiding the user to understand what's happening and to debug curl usage, we
365 must supply a fair amount of informational messages by using the Curl_infof()
366 function. Those messages are only displayed when the user explicitly asks for
367 them. They are best used when revealing information that isn't otherwise
373 We make an effort to not export or show internals or how internals work, as
374 that makes it easier to keep a solid API/ABI over time. See docs/libcurl/ABI
375 for our promise to users.
380 main() resides in src/tool_main.c.
382 src/tool_hugehelp.c is automatically generated by the mkhelp.pl perl script
383 to display the complete "manual" and the src/tool_urlglob.c file holds the
384 functions used for the URL-"globbing" support. Globbing in the sense that the
385 {} and [] expansion stuff is there.
387 The client mostly messes around to setup its 'config' struct properly, then
388 it calls the curl_easy_*() functions of the library and when it gets back
389 control after the curl_easy_perform() it cleans up the library, checks status
392 When the operation is done, the ourWriteOut() function in src/writeout.c may
393 be called to report about the operation. That function is using the
394 curl_easy_getinfo() function to extract useful information from the curl
397 It may loop and do all this several times if many URLs were specified on the
398 command line or config file.
403 The file lib/memdebug.c contains debug-versions of a few functions. Functions
404 such as malloc, free, fopen, fclose, etc that somehow deal with resources
405 that might give us problems if we "leak" them. The functions in the memdebug
406 system do nothing fancy, they do their normal function and then log
407 information about what they just did. The logged data can then be analyzed
408 after a complete session,
410 memanalyze.pl is the perl script present in tests/ that analyzes a log file
411 generated by the memory tracking system. It detects if resources are
412 allocated but never freed and other kinds of errors related to resource
415 Internally, definition of preprocessor symbol DEBUGBUILD restricts code which
416 is only compiled for debug enabled builds. And symbol CURLDEBUG is used to
417 differentiate code which is _only_ used for memory tracking/debugging.
419 Use -DCURLDEBUG when compiling to enable memory debugging, this is also
420 switched on by running configure with --enable-curldebug. Use -DDEBUGBUILD
421 when compiling to enable a debug build or run configure with --enable-debug.
423 curl --version will list 'Debug' feature for debug enabled builds, and
424 will list 'TrackMemory' feature for curl debug memory tracking capable
425 builds. These features are independent and can be controlled when running
426 the configure script. When --enable-debug is given both features will be
427 enabled, unless some restriction prevents memory tracking from being used.
432 The test suite is placed in its own subdirectory directly off the root in the
433 curl archive tree, and it contains a bunch of scripts and a lot of test case
436 The main test script is runtests.pl that will invoke test servers like
437 httpserver.pl and ftpserver.pl before all the test cases are performed. The
438 test suite currently only runs on unix-like platforms.
440 You'll find a description of the test suite in the tests/README file, and the
441 test case data files in the tests/FILEFORMAT file.
443 The test suite automatically detects if curl was built with the memory
444 debugging enabled, and if it was it will detect memory leaks, too.
449 There's no magic to this. When you consider everything stable enough to be
452 1. Tag the source code accordingly.
454 2. run the 'maketgz' script (using 'make distcheck' will give you a pretty
455 good view on the status of the current sources). maketgz requires a
456 version number and creates the release archive. maketgz uses 'make dist'
457 for the actual archive building, why you need to fill in the Makefile.am
458 files properly for which files that should be included in the release
461 3. When that's complete, sign the output files.
465 5. Update web site and changelog on site
467 6. Send announcement to the mailing lists
469 NOTE: you must have curl checked out from git to be able to do a proper
470 release build. The release tarballs do not have everything setup in order to
471 do releases properly.