• Debugging
• Pretty_Printers

GDB pretty-printers for GLIBC

Pretty printers are gdb extensions, implemented in Python, 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)

   • Really? This doesn't seem right. You shouldn't have to set CFLAGS and CPPFLAGS both to the same thing.

   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.