kastiglione / swift_po

Substitute po for Swift

swift_po aka spo

Substitute po command for Swift, with fewer corner cases to watch out for.

• ✓ Avoids memory leaks caused by lldb
• ✓ Automatically uses frame variable (vo) if possible
• ✓ Supports object addresses, even in Swift

Avoids Leaks

There's a bug in lldb that creates strong references to any object printed with po. The result is leaked objects, and missing calls to deinit.

(lldb) po self

After running this, self will never be released. This is not specific to self, it happens to any object. A leak will also happen when using p or expression.

There are two work arounds:

1. frame variable -O
2. call print(...)

These are what swift_po does for you. If the input can be resolved as a variable, then frame variable is used, otherwise it performs call print(...).

frame variable

LLDB has added a vo command, an alias for frame variable -O. This much like po, but is for variable expressions only, not arbitrary code evaluation. Printing via vo is faster than po. When using swift_po, the first option is perform a vo operaton. If that fails, call print(...) is performed instead.

Object Pointers

From Swift, po 0x76543210 will not work. This does work from objc though. swift_po works with memory addresses too, same as objc.

Installation

First clone swift_po:

git clone https://github.com/kastiglione/swift_po.git

Next, edit your ~/.lldbinit to import the spo command:

command script import path/to/swift_po/spo.py

Usage

Call spo anywhere you'd normally call po.

If you would like to override po to be spo, add these to your ~/.lldbinit:

command unalias po
command alias po spo

What swift_po is doing

A summary of what swift_po does for you.

spo command	equivalent lldb
spo 0x76543210	expression -l objc -O -- 0x76543210
spo self	frame variable -O self
spo self.objects	frame variable -O self.objects
spo self.view	expression print(self.view)
spo some.function()	expression print(some.function())

This table show that some properties cannot be accessed using frame variable, in those cases print() is used instead.