diff --git a/backend-writing.txt b/backend-writing.txt new file mode 100644 index 00000000..35dc5d68 --- /dev/null +++ b/backend-writing.txt @@ -0,0 +1,296 @@ +2003-09-24 + +Here are a few rules and tips that should help writing a +SANE-conforming backend and including it into the SANE package: + + +GETTING STARTED +--------------- + +* You will need information about the protocol the scanner (or other image + application device) is using. The easiest way is to ask the manufacturer + about it. You should mention that the code will be open-source, however. + +* Read the SANE standard. + +* One approach is to write a stand-alone scanning program first. Debugging + this program is usually easier than using the SANE libraries. However, keep + in mind what you learned from the SANE standard. + +* Once your program works basically, insert its functions into a basically + empty SANE backend. You can get one by removing everything but the SANE + includes and SANE API function definitions from an existing backend (e.g. + test.c). + +* If you have any information about the scanner you want to support that + is not already mentioned in one of the .desc files, please contact the + sane-devel mailing list. Especially if you have written code (e.g. a test + program) or started writing a backend, contact us. + +* Keep other users informed about what you did and want to do. This way no + work is done twice and you may get volunteers for coding or testing. + Set up a website or at least write to sane-devel. + +* When you have a working backend but you don't want to have it included + in the SANE distribution yet, at least the .desc file can be included + (see below for details). So people will find a link to your backend at + the SANE webpage. + + +PROGRAMMING +----------- + +* Please follow the GNU coding standards. It's clear that the style + outlined there is nobody's favorite, but it's much easier to + maintain SANE if everybody follows more or less the same coding + style. It also looks more professional. The GNU standards can be + found at: + + http://www.gnu.org/prep/standards_toc.html + ftp://ftp.gnu.org/pub/gnu/standards/standards.text + + Note that GNU emacs supports automatic indentation according to this + standard. The command "indent -gnu" can be used to reformat + existing sources according to this standard. + +* Please be courteous to programmer's with terminals that are 80 + characters wide. It's not difficult to avoid long lines, so please + do so. Note that in ANSI C you can split long strings into pieces + separated by white space. For example, + "this is an awfully long string" can be written as "this is an " + "awfully long string". + +* Use only ANSI C for your backend. + +* Please do not depend on compiler specific features or, if you do, make + the dependency conditional so other compilers will still be able to + compile the files. In particular: + + - do not use C++ style comments (//-line comments) + + - do not declare dynamically sized automatic arrays; instead, + use alloca() after including "../include/lalloca.h". For example: + + void + func (int n) + { + char buf[n]; + } + + should be re-written as: + + #ifdef _AIX + # include "../include/lalloca.h" /* MUST come first for AIX! */ + #endif + + #include "../include/sane/config.h" + #include "../include/lalloca.h" + : + void + func (int n) + { + char *buf = alloca (n); + } + - Don't use any #pragma directives---they're completely + compiler-dependent. + +* If you use headers or libraries that may not be available on all systems, + write a check for configure.in and include it conditionally. If your backend + depends on these libraries or headers, compile the backend only if they are + available (see pint for an example). + +* Use #include ".../include/sane/..." to include the sane header files + instead of #include . Otherwise problems with different installed + SANE versions may occur. Also this makes clear that the local files are used. + +* Don't forget to #include ".../include/sane/config.h" in your backend before + any other includes. If you use lalloca.h see above for the correct + includes. + +* Include sanei_backend.h after the other includes. + +* It's no longer necessary to #define PATH_MAX (now in sanei_backend.h). + If you define it, do so *after* the system includes. + +* Please use sanei functions whenever possible (e.g. + sanei_config_read()). This makes porting to other os/platforms much + easier. Most of these functions are documented in their respective + header files in include/sane/sanei_*.h. For some of them there is also + documentation in doxygen format: http://sanei.meier-geinitz.de/. These + HTML pages can be generated by calling "doxygen sanei-doxygen.conf" in + the doc/ directory. + +* Initialize global variables explicitly in sane_init. Keep in mind that + sane_init can be called more than once (if sane_exit is called everytime + after sane_init). Therefore don't depend on automatic initialization to + 0 / NULL. + +* Do make sure that your code is byte-order independent. This is + particularly important for networking-related code and when dealing + with non-textual data files. + +* Don't use printf, fprintf or perror to output debug or error messages. + Use the DBG macro instead. If your backend can't detect a scanner for + whatever reason it shouldn't output anything as long as + SANE_DEBUG_BACKENDNAME isn't set. So don't use DBG(0, ...) in this case. + +* Please do not assume that `size_t' is `unsigned int'. On some + systems, it's `unsigned long' and the size of this type may be + bigger than that of an int (this is true for practically any of the + 64-bit systems). To print a variable of type size_t portably, cast + the variable to u_long and print it with the %lu specifier. E.g.: + + size_t len; + + DBG(3, "len=%lu\n", (u_long) len); + +* Please do not assume that `void *' has the same size as `int'. On some + systems, it's `long' and the size of this type may be bigger than that of + an int (this is true for practically any of the 64-bit systems). Where this + comes up is with opaque handles. For example: + + int OpaqueHandle; + MyScanner *s = (MyScanner *)OpaqueHandle; + + will FAIL on most 64 bit systems. Please use `void *' or better + `SANE_Handle'. + +* Don't use exit() in your backend. You will exit the whole program, not only + your backend. + +* If you use wait() or waitpid() in your backend, check its return value. This + is important, if the status value is checked for e.g. WIFEXITED after the + call of wait() or waitpid(). Both functions may fail if the frontend already + did a wait for the children. + +* Please try to avoid compilation warnings. At least with "--disable-warnings" + there shouldn't be warnings when compiling backends. It's not necessary to + fix every "unused parameter" warning but take care that no warnings pointing + to really existing problems or ambiguities are missed. Some programming + techniques generating warnings on gcc may lead to errors on other systems. + +* To support translation of SANE options, please mark the descriptions (desc) + and title of options with SANE_I18N(). See po/README for details. + +* Please check for TL_X < BR_X and TL_Y < BR_Y to avoid segfaults or even + scanner damage. This should NOT be done in sane_control_option, it should + be possible to temporary set TL_X > BR_X or TL_ > BR, otherwise it is hard + for a frontend to set the correct values. + + +TESTING +------- + +* Please test a backend with "scanimage -T" (plus other options, + as appropriate/necessary) as this will make sure that sane_read() + always returns the correct amount of data etc. + +* You can also test a backend with tstbackend. tstbackend is not + compiled nor installed by default. To do that, cd into frontend and + edit the Makefile. Add "tstbackend" to BINPROGS. "tstbackend --help" + gives a short help. + +* Please test a backend not just with scanimage and xscanimage + (+ other frontends), but also with saned. Based on past experience, + it is often the case that remote scanning can uncover subtle bugs. + Note that you don't have to use two different machines to test "remote" + scanning---you can use one and the same machine to test a backend via saned + (just be sure to enable the "net" backend in dll.conf and follow the + steps described in saned(1)). + +* Please test on every available platform/os. Even if no scanner is attached + to this system, test compilation and running scanimage. If you don't have + access to other platforms, ask sane-devel. + +* Please make sure that all global symbols exported from a SANE backend start + with the prefix "sane" or "sanei" to avoid clashes with exported symbols + of other backends. Make sure, the sanei_* symbols are unique, e.g. by using + sanei_backendname_*. Only export symbols that are absolutely necessary. + You can verify this by running GNU "nm" on the static library. For example: + + nm -g backend/.libs/libsane-hp.a + + would list all global symbols in the HP backend. + "./configure --disable-shared; make; make libcheck" in the sane-backends + root directory will name all backend libraries, that contain "illegal" + symbols. + + +DOCUMENTATION +------------- + +* Even if you haven't written a man-page for your backend yet, you *must* + create a .desc file which describes it. Anytime you submit source code for + your backend, you should include an update to the .desc file which reflects + the new state of the backend. The .desc files are used to create the HTML + lists of supported devices. These lists are updated automatically when you + change a .desc file in CVS. See e.g. + http://www.sane-project.org/lists/sane-mfgs-cvs.html for the results. + +* The .desc files are located in the directories "doc/descriptions" and + "doc/descriptions-external" (for included and external backends). + +* "doc/descriptions.txt" describes the format of the ".desc" files. There is + also a template for new .desc files: "template.desc.". The format of the + files in the two directories is very similar. If you'd like to try parsing + your creation to recreate the sane-backends webpages, cd into "tools/" and + enter "make sane-desc". You can either use sane-desc directly (try + "./sane-desc -h") or use "make html-pages" in "doc/". + +* For external backends, you don't need to supply :manpage and :version. The + manpage link won't work anyway and version will be outdated soon. + +* When your backend is included in the SANE distribution, add an entry to + doc/sane.man, AUTHORS and sane-backends.lsm. The sane.man entry should point + to your documentation (man-page, website, readme). Also move your .desc + file from "doc/descriptions-external" to "doc/descriptions" and update + them. + +* In your manual page (backend.man), use @LIBDIR@ and the other macros for + path names to the libraries, config files and documentation. Don't use fixed + paths as they will be wrong if SANE is installed with a different prefix + (e.g. /usr instead of /usr/local). + +* If you want to include READMEs, HTML files or other documentation, please + create your own directory (doc/backendname) and store your files in this + directory. If you only have a manual page a subdirectory isn't necessary. + +* Please keep your manpages and .desc files up-to-date especially regarding + version numbers. + + +CHECKLIST: SUBMITTING A NEW BACKEND +----------------------------------- + +In sane-backends/ +* Add the author(s) name(s) to AUTHORS +* Correct any related entries in the bug-tracking system + +In sane-backends/backend/ +* Use the command "indent -gnu" to reformat your code according to the + standard. +* Add the backend name to dll.conf +* Check that the SANE license is in the backend source files. +* Add the source file names and the .conf file + to DISTFILES in Makefile.in + +In sane-backends/doc/ +* Add an entry for the man page in sane.man +* Add the man page file in doc/Makefile.in +* Move the description file from descriptions-external/ to doc/descriptions/ +* Check that the description file is correct: "cd doc; make html-pages" and check the html pages result with a browser. +* Check that the backend version is the same in the source and in the + description file. + + +INCLUDING INTO CVS +------------------ + +* If you want to include your backend into CVS use the latest CVS to make + patches. Check the mailing list and the bug-tracking system for information + about bugs to avoid. + +* If your backend isn't included yet in the SANE CVS tree, write an email to + the SANE mailing list (sane-devel) and ask for inclusion. Usually one + of the developers will check the backend for common mistakes and test + compilation. If everything is ok the backend will be added to the CVS tree.