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diff --git a/REORG.TODO/README.pretty-printers b/REORG.TODO/README.pretty-printers new file mode 100644 index 0000000000..2522cb858d --- /dev/null +++ b/REORG.TODO/README.pretty-printers @@ -0,0 +1,175 @@ +README for the glibc Python pretty printers +=========================================== + +Pretty printers are gdb extensions that allow it to print useful, human-readable +information about a program's variables. For example, for a pthread_mutex_t +gdb would usually output something like this: + +(gdb) print mutex +$1 = { + __data = { + __lock = 22020096, + __count = 0, + __owner = 0, + __nusers = 0, + __kind = 576, + __spins = 0, + __elision = 0, + __list = { + __prev = 0x0, + __next = 0x0 + } + }, + __size = "\000\000P\001", '\000' <repeats 12 times>, "@\002", '\000' <repeats 21 times>, + __align = 22020096 +} + +However, with a pretty printer gdb will output something like this: + +(gdb) print mutex +$1 = pthread_mutex_t = { + Type = Normal, + Status = Not acquired, + Robust = No, + Shared = No, + Protocol = Priority protect, + Priority ceiling = 42 +} + +Before printing a value, gdb will first check if there's a pretty printer +registered for it. If there is, it'll use it, otherwise it'll print the value +as usual. Pretty printers can be registered in various ways; for our purposes +we register them for the current objfile by calling +gdb.printing.register_pretty_printer(). + +Currently our printers are based on gdb.RegexpCollectionPrettyPrinter, which +means they'll be triggered if the type of the variable we're printing matches +a given regular expression. For example, MutexPrinter will be triggered if +our variable's type matches the regexp '^pthread_mutex_t$'. + +Besides the printers themselves, each module may have a constants file which the +printers will import. These constants are generated from C headers during the +build process, and need to be in the Python search path when loading the +printers. + + +Installing and loading +---------------------- + +The pretty printers and their constant files may be installed in different paths +for each distro, though gdb should be able to automatically load them by itself. +When in doubt, you can use the 'info pretty-printer' gdb command to list the +loaded pretty printers. + +If the printers aren't automatically loaded for some reason, you should add the +following to your .gdbinit: + +python +import sys +sys.path.insert(0, '/path/to/constants/file/directory') +end + +source /path/to/printers.py + +If you're building glibc manually, '/path/to/constants/file/directory' should be +'/path/to/glibc-build/submodule', where 'submodule' is e.g. nptl. + + +Testing +------- + +The pretty printers come with a small test suite based on PExpect, which is a +Python module with Expect-like features for spawning and controlling interactive +programs. Each printer has a corresponding C program and a Python script +that uses PExpect to drive gdb through the program and compare its output to +the expected printer's. + +The tests run on the glibc host, which is assumed to have both gdb and PExpect; +if any of those is absent the tests will fail with code 77 (UNSUPPORTED). +Native builds can be tested simply by doing 'make check'; cross builds must use +cross-test-ssh.sh as test-wrapper, like this: + +make test-wrapper='/path/to/scripts/cross-test-ssh.sh user@host' check + +(Remember to share the build system's filesystem with the glibc host's through +NFS or something similar). + +Running 'make check' on a cross build will only compile the test programs, +without running the scripts. + + +Adding new pretty printers +-------------------------- + +Adding new pretty printers to glibc requires following these steps: + +1. Identify which constants must be generated from C headers, and write the +corresponding .pysym file. See scripts/gen-py-const.awk for more information +on how this works. The name of the .pysym file must be added to the +'gen-py-const-headers' variable in your submodule's Makefile (without the .pysym +extension). + +2. Write the pretty printer code itself. For this you can follow the gdb +Python API documentation, and use the existing printers as examples. The printer +code must import the generated constants file (which will have the same name +as your .pysym file). The names of the pretty printer files must be added +to the 'pretty-printers' variable in your submodule's Makefile (without the .py +extension). + +3. Write the unit tests for your pretty printers. The build system calls each +test script passing it the paths to the test program source, the test program +binary, and the printer files you added to 'pretty-printers' in the previous +step. The test scripts, in turn, must import scripts/test_printers_common +and call the init_test function passing it, among other things, the name of the +set of pretty printers to enable (as seen by running 'info pretty-printer'). +You can use the existing unit tests as examples. + +4. Add the names of the pretty printer tests to the 'tests-printers' variable +in your submodule's Makefile (without extensions). In addition, for each test +program you must define a corresponding CFLAGS-* and CPPFLAGS-* variable and +set it to $(CFLAGS-printers-tests) to ensure they're compiled correctly. For +example, test-foo-printer.c requires the following: + +CFLAGS-test-foo-printer.c := $(CFLAGS-printers-tests) +CPPFLAGS-test-foo-printer.c := $(CFLAGS-printers-tests) + +Finally, if your programs need to be linked with a specific library, you can add +its name to the 'tests-printers-libs' variable in your submodule's Makefile. + + +Known issues +------------ + +* Pretty printers are inherently coupled to the code they're targetting, thus +any changes to the target code must also update the corresponding printers. +On the plus side, the printer code itself may serve as a kind of documentation +for the target code. + +* There's no guarantee that the information the pretty printers provide is +complete, i.e. some details might be left off. For example, the pthread_mutex_t +printers won't report whether a thread is spin-waiting in an attempt to acquire +the mutex. + +* Older versions of the gdb Python API have a bug where +gdb.RegexpCollectionPrettyPrinter would not be able to get a value's real type +if it was typedef'd. This would cause gdb to ignore the pretty printers for +types like pthread_mutex_t, which is defined as: + +typedef union +{ + ... +} pthread_mutex_t; + +This was fixed in commit 1b588015839caafc608a6944a78aea170f5fb2f6, and released +as part of gdb 7.8. However, typedef'ing an already typedef'd type may cause +a similar issue, e.g.: + +typedef pthread_mutex_t mutex; +mutex a_mutex; + +Here, trying to print a_mutex won't trigger the pthread_mutex_t printer. + +* The test programs must be compiled without optimizations. This is necessary +because the test scripts rely on the C code structure being preserved when +stepping through the programs. Things like aggressive instruction reordering +or optimizing variables out may make this kind of testing impossible. |