Aids for Debugging


This repository contains a .gdbinit file that contains helper functions to aid debugging of data structures. GDB will load this file automatically if you have added the directory which contains the .gdbinit file to GDB’s auto-load safe-path, and you start GDB from the directory which holds the .gdbinit file.

You can see the safe-path with show auto-load safe-path on a GDB prompt. You can configure it by setting it in ~/.gdbinit with:

add-auto-load-safe-path /path/to/mongo-c-driver

If you haven’t added the path to your auto-load safe-path, or start GDB in another directory, load the file with:

source path/to/mongo-c-driver/.gdbinit

The .gdbinit file defines the printbson function, which shows the contents of a bson_t * variable. If you have a local bson_t, then you must prefix the variable with a &.

An example GDB session looks like:

(gdb) printbson bson
ALLOC [0x555556cd7310 + 0] (len=475)
    'bool' : true,
    'int32' : NumberInt("42"),
    'int64' : NumberLong("3000000042"),
    'string' : "Stŕìñg",
    'objectId' : ObjectID("5A1442F3122D331C3C6757E1"),
    'utcDateTime' : UTCDateTime(1511277299031),
    'arrayOfInts' : [
        '0' : NumberInt("1"),
        '1' : NumberInt("2")
    'embeddedDocument' : {
        'arrayOfStrings' : [
            '0' : "one",
            '1' : "two"
        'double' : 2.718280,
        'notherDoc' : {
            'true' : NumberInt("1"),
            'false' : false
    'binary' : Binary("02", "3031343532333637"),
    'regex' : Regex("@[a-z]+@", "im"),
    'null' : null,
    'js' : JavaScript("print foo"),
    'jsws' : JavaScript("print foo") with scope: {
        'f' : NumberInt("42"),
        'a' : [
            '0' : 3.141593,
            '1' : 2.718282
    'timestamp' : Timestamp(4294967295, 4294967295),
    'double' : 3.141593


This repository also includes a script that customizes LLDB’s standard print command to print a bson_t or bson_t * as JSON:

(lldb) print b
(bson_t) $0 = {"x": 1, "y": 2}

The custom bson command provides more options:

(lldb) bson --verbose b
  "x": 1,
  "y": 2
(lldb) bson --raw b

Type help bson for a list of options.

The script requires a build of libbson with debug symbols, and an installation of PyMongo. Install PyMongo with:

python -m pip install pymongo

If you see “No module named pip” then you must install pip, then run the previous command again.

Create a file ~/.lldbinit containing:

command script import /path/to/mongo-c-driver/

If you see “bson command installed by lldb_bson” at the beginning of your LLDB session, you’ve installed the script correctly.