The LLDB Debugger

# Stack Frame and Thread Format

LLDB was recently modified to allow users to define the format of the information that generates the descriptions for threads and stack frames. Typically when your program stops at a breakpoint you will get a line that describes why your thread stopped:

`* thread #1: tid = 0x2e03, 0x0000000100000e85 a.out`main + 4, stop reason = breakpoint 1.1`

Stack backtraces frames also have a similar information line:

`(lldb) thread backtrace
thread #1: tid = 0x2e03, stop reason = breakpoint 1.1
frame #0: 0x0000000100000e85 a.out`main + 4 at test.c:19
frame #1: 0x0000000100000e40 a.out`start + 52`

The two format strings can currently be set using the settings set command:

`(lldb) settings set frame-prompt STRING
(lldb) settings set thread-prompt STRING`

# Format Strings

So what is the format of the format strings? Format strings can contain plain text, control characters and variables that have access to the current program state.

Normal characters are any text that doesn't contain a `'{'`, `'}'`, `'$'`, or `'\'` character.

Variable names are found in between a `"${"` prefix, and end with a `"}"` suffix. In other words, a variable looks like `"${frame.pc}"`.

# Variables

A complete list of currently supported format string variables is listed below:

Variable Name | Description
file.basename | The current compile unit file basename for the current frame.
file.fullpath | The current compile unit file fullpath for the current frame.
frame.index | The frame index (0, 1, 2, 3...)
frame.pc | The generic frame register for the program counter.
frame.sp | The generic frame register for the stack pointer.
frame.fp | The generic frame register for the frame pointer.
frame.flags | The generic frame register for the flags register.
frame.reg.NAME | Access to any platform specific register by name (replace NAME with the name of the desired register).
function.name | The name of the current function or symbol
function.pc-offset | The program counter offset within the current function or symbol
line.file.basename | The line table entry basename to the file for the current line entry in the current frame.
line.file.fullpath | The line table entry fullpath to the file for the current line entry in the current frame.
line.number | The line table entry line number for the current line entry in the current frame.
line.start-addr | The line table entry start address for the current line entry in the current frame.
line.end-addr | The line table entry end address for the current line entry in the current frame.
module.file.basename | The basename of the current module (shared library or executable)
module.file.fullpath | The basename of the current module (shared library or executable)
process.file.basename | The basename of the file for the process
process.file.fullpath | The fullname of the file for the process
process.id | The process ID native the the system on which the inferior runs.
process.name | The name of the process at runtime
thread.id | The thread identifier for the current thread
thread.index | The unique one based thread index ID which is guaranteed to be unique as threads come and go.
thread.name | The name of the thread if the target OS supports naming threads
thread.queue | The queue name of the thread if the target OS supports dispatch queues
thread.stop-reason | A textual reason each thread stopped
target.arch | The architecture of the current target

# Control Characters

Control characters include `'{'`, `'}'`, and `'\'`.

The '{' and '}' are used for scoping blocks, and the '\' character allows you to desensitize control characters and also emit non-printable characters.

# Desensitizing Characters in the format string

The backslash control character allows your to enter the typical `"\a"`, `"\b"`, `"\f"`, `"\n"`, `"\r"`, `"\t"`, `"\v"`, `"\\"`, characters and along with the standard octal representation `"\0123"` and hex `"\xAB"` characters. This allows you to enter escape characters into your format strings and will allow colorized output for terminals that support color.

# Scoping

Many times the information that you might have in your prompt might not be available and you won't want it to print out if it isn't valid. To take care of this you can enclose everything that must resolve into a scope. A scope is starts with `'{'` and ends with `'}'`. For example in order to only display the current frame line table entry basename and line number when the information is available for the current frame:

`"{ at {$line.file.basename}:${line.number}}"`

Broken down this is:

- The start the scope

  `"{"`

- format whose content will only be displayed if all information is available:

  `"at {$line.file.basename}:${line.number}"`

- end the scope:

  `"}"`

# Making the Frame Format

The information that we see when stopped in a frame:

`frame #0: 0x0000000100000e85 a.out`main + 4 at test.c:19`

can be displayed with the following format:

`"frame #${frame.index}: ${frame.pc}{ ${module.file.basename}`${function.name}{${function.pc-offset}}}{ at ${line.file.basename}:${line.number}}\n"`

This breaks down to:

- Always print the frame index and frame PC:

  `"frame #${frame.index}: ${frame.pc}"`

- only print the module followed by a tick if there is a valid module for the current frame:

  `"{ ${module.file.basename}`}"`

- print the function name with optional offset:

  `"{${function.name}{${function.pc-offset}}}"`

- print the line info if it is available:

  `"{ at ${line.file.basename}:${line.number}}"`

- then finish off with a newline:

  `"\n"`

# Making Your Own Formats

When modifying your own format strings, it is useful to start with the default values for the frame and thread format strings. These can be accessed with the `"settings show"` command:

`(lldb)` settings show thread-format
thread-format (string) = 'thread #${thread.index}: tid = ${thread.id}{, ${frame.pc}}{ ${module.file.basename}`${function.name}{${function.pc-offset}}}{, stop reason = ${thread.stop-reason}}{, name = ${thread.name}}{, queue = ${thread.queue}}\n'
(lldb) settings show frame-format
frame-format (string) = 'frame #${frame.index}: ${frame.pc}{ ${module.file.basename}`${function.name}{${function.pc-offset}}}{ at ${line.file.basename}:${line.number}}\n'

When making thread formats, you will need surround any of the information that comes from a stack frame with scopes ({ frame-content }) as the thread format doesn't always want to show frame information. When displaying the backtrace for a thread, we don't need to duplicate the information for frame zero in the thread information:

`(lldb) thread backtrace
thread #1: tid = 0x2e03, stop reason = breakpoint 1.1 2.1
frame #0: 0x0000000100000e85 a.out`main + 4 at test.c:19
frame #1: 0x0000000100000e40 a.out`start + 52`

The frame related variables are:

- `${file.*}`
- `${frame.*}`
- `${function.*}`
- `${line.*}`
- `${module.*}`

Looking at the default format for the thread, and underlining the frame information:

`'thread #${thread.index}: tid = ${thread.id}{, ${frame.pc}}{ ${module.file.basename}`${function.name}{${function.pc-offset}}}{, stop reason = ${thread.stop-reason}}{, name = ${thread.name}}{, queue = ${thread.queue}}\n'`

We can see that all frame information is contained in scopes so that when the thread information is displayed in a context where we only want to show thread information, we can do so.