1 This directory holds Python code to support debugging SpiderMonkey with
2 GDB. It includes pretty-printers for common SpiderMonkey types like JS::Value,
3 jsid, and JSObject, and makes GDB "see through" the SpiderMonkey rooting
4 types like js::Rooted and JS::Handle. For example:
7 #0 js::baseops::SetPropertyHelper (cx=0xbf3460,
8 obj=(JSObject * const) 0x7ffff150b060 [object global] delegate,
9 receiver=(JSObject * const) 0x7ffff150b060 [object global] delegate,
10 id=$jsid("x"), defineHow=4, vp=$JS::Int32Value(1), strict=0)
11 at /home/jimb/moz/archer/js/src/jsobj.cpp:4495
12 4495 MOZ_ASSERT((defineHow & ~(DNP_CACHE_RESULT | DNP_UNQUALIFIED)) == 0);
17 - obj, a JS::HandleObject, prints as:
18 obj=(JSObject * const) 0x7ffff150b060 [object global] delegate,
19 This immediately shows the handle's referent, along with a JavaScript-like summary
22 - id, a JS::HandleId, prints as:
24 We show the handle's referent, and print the identifier as a string.
26 - vp, a JS::MutableHandleValue, prints as:
28 We show the handle's referent, using the JS::Value's tag to print it noting
29 its particular internal type and value.
31 You can still see the raw form of a value with 'print/r':
34 $1 = {<js::HandleBase<JSObject*>> = {<No data fields>}, ptr = 0x7fffffffca60}
37 You can also use GDB's 'disable pretty-printer' command to turn off
38 individual pretty-printers; try 'info pretty-printer' first.
40 GDB should pick these extensions up automatically when you debug the shell or
41 the browser, by auto-loading the 'js-gdb.py' file that the build system
42 installs alongside the 'js' executable (or 'libxul.so-gdb.py' for the browser).
43 You may need to add a command like the following to your '$HOME/.gdbinit' file:
45 # Tell GDB to trust auto-load files found under ~/moz.
46 add-auto-load-safe-path ~/moz
48 If you do need this, GDB will tell you.
50 In general, pretty-printers for pointer types include a summary of the
54 Breakpoint 1 at 0x542e0a: file /home/jimb/moz/archer/js/src/jsmath.cpp, line 214.
56 js> Math.atan2('Spleen', 42)
57 Breakpoint 1, math_atan2 (cx=0xbf3440, argc=2, vp=0x7ffff172f0a0)
59 $1 = $JS::Value((JSObject *) 0x7ffff151c0c0 [object Function "atan2"])
61 $2 = $JS::Value((JSObject *) 0x7ffff150d0a0 [object Math])
63 $3 = $JS::Value("Spleen")
65 $4 = $JS::Int32Value(42)
68 We used to also have pretty-printers for the actual contents of a JSString
69 struct, that knew which union branches were live and which were dead. These were
70 more fragile than the summary pretty-printers, and harder to test, so I've
71 removed them until we can see how to do better.
73 There are unit tests; see 'Running the unit tests', below.
75 I'd love for others to pitch in. GDB's Python API is documented in the GDB
78 I've recently rewritten the printers. The new code is simpler, and more
79 robust; unit tests are easier to write; and the new test harness can run
80 the tests in parallel. If a printer you'd contributed to in the past was
81 dropped in the process, I apologize; I felt we should have good test
82 coverage for any printer landed in-tree. You may also be interested in
83 'Personal pretty-printers', below.
88 - js/src/gdb/mozilla: The actual SpiderMonkey support code. GDB auto-loads this
89 when you debug an executable or shared library that contains SpiderMonkey.
90 - js/src/gdb/tests: Unit tests for the above.
91 - Each '.py' file is a unit test, to be run by js/src/gdb/run-tests.py.
92 - Each '.cpp' file contains C++ code fragments for some unit test to use.
93 - js/src/gdb/lib-for-tests: Python modules used by the unit tests.
97 - run-tests.py: test harness for GDB SpiderMonkey support unit tests. See
98 'Running the unit tests', below.
99 - taskpool.py, progressbar.py: Python modules used by run-tests.py.
100 - gdb-tests.cpp, gdb-tests.h: Driver program for C++ code fragments.
101 - gdb-tests-gdb.py.in: Template for GDB autoload file for gdb-tests.
103 Personal pretty-printers
104 ------------------------
106 If you'd like to write your own pretty-printers, you can put them in a
107 module named 'my_mozilla_printers' in a directory somewhere on your Python
108 module search path. Our autoload code tries to import 'my_mozilla_printers'
109 after importing our other SpiderMonkey support modules. For example:
113 $ cat ~/python/my_mozilla_printers.py
115 from mozilla.prettyprinters import ptr_pretty_printer
117 # Simple char16_t * printer. Doesn't show address; chases null pointers.
118 @ptr_pretty_printer('char16_t')
119 class char16Ptr(object):
120 def __init__(self, value, cache): self.value = value
121 def display_hint(self): return 'string'
125 if self.value[i] == 0: break
126 c += unichr(self.value[i])
132 (gdb) print &sample[0]
135 Running the unit tests
136 ----------------------
138 These extensions have unit tests, invoked as follows:
140 $ python run-tests.py [OPTIONS] OBJDIR [TESTS...]
142 where OBJDIR is a directory containing a standalone SpiderMonkey build; TESTS
143 are names of selected tests to run (if omitted, we run them all); and OPTIONS
144 are drawn from the list below.
147 Instead of running whatever 'gdb' we find in our search path, use
148 EXECUTABLE to run the tests.
151 Find the sources corresponding to OBJDIR/dist/bin/libmozjs.so in SRCDIR.
152 Without this option, we use the parent of the directory containing
153 'run-tests.py'. Note that SRCDIR must be a complete SpiderMonkey source
154 directory, as our tests #include internal SpiderMonkey header files (to
155 test pretty-printers for internal types, like parse nodes.)
158 Search for Python scripts and any accompanying C++ source code in
159 TESTDIR. If omitted, we use the 'tests' directory in the directory
160 containing 'run-tests.py'.
163 Build the C++ executable that GDB debugs to run the tests in BUILDDIR.
164 If omitted, create a 'gdb-tests' subdirectory of OBJDIR/js/src.
166 (It is safe to use relative paths for OBJDIR, SRCDIR, and so on. They are
167 always interpreted relative to the directory that was current when
168 run-tests.py was started.)
170 For example, since I build in a subdirectory 'obj~' of the 'js/src'
171 directory, I use this command from 'js/src' to run the pretty-printer unit
174 $ python gdb/run-tests.py obj~
176 Writing new unit tests
177 ----------------------
179 Each unit test consists of a Python script, possibly with some accompanying
180 C++ code. Running tests works like this:
182 - The run-tests.py script calls 'make' in 'BUILDDIR/gdb' to build
185 - Then, for each '.py' test script in js/src/gdb/tests, the harness starts
186 GDB on the 'gdb-tests' executable, and then has GDB run
187 js/src/gdb/lib-for-tests/prologue.py, passing it the test script's path as
188 its first command-line argument.