actions [num]actions without bothering about its number). You specify the
actions themselves on the following lines, one action at a time, and
terminate the actions list with a line containing just end. So
far, the only defined actions are collect, teval, and
while-stepping.
actions is actually equivalent to commands (see Breakpoint Command Lists), except that only the defined
actions are allowed; any other gdb command is rejected.
To remove all actions from a tracepoint, type ‘actions num’ and follow it immediately with ‘end’.
(gdb) collect data // collect some data
(gdb) while-stepping 5 // single-step 5 times, collect data
(gdb) end // signals the end of actions.
In the following example, the action list begins with collect
commands indicating the things to be collected when the tracepoint is
hit. Then, in order to single-step and collect additional data
following the tracepoint, a while-stepping command is used,
followed by the list of things to be collected after each step in a
sequence of single steps. The while-stepping command is
terminated by its own separate end command. Lastly, the action
list is terminated by an end command.
(gdb) trace foo
(gdb) actions
Enter actions for tracepoint 1, one per line:
> collect bar,baz
> collect $regs
> while-stepping 12
> collect $pc, arr[i]
> end
end
collect[/mods] expr1, expr2, ...$regs$args$locals$_retNote: The return address location can not always be reliably
determined up front, and the wrong address / registers may end up
collected instead. On some architectures the reliability is higher
for tracepoints at function entry, while on others it's the opposite.
When this happens, backtracing will stop because the return address is
found unavailable (unless another collect rule happened to match it).
$_probe_argc$_probe_argn$_sdataprintf function call. The
tracing library is able to collect user specified data formatted to a
character string using the format provided by the programmer that
instrumented the program. Other backends have similar mechanisms.
Here's an example of a UST marker call:
const char master_name[] = "$your_name";
trace_mark(channel1, marker1, "hello %s", master_name)
In this case, collecting $_sdata collects the string
‘hello $yourname’. When analyzing the trace buffer, you can
inspect ‘$_sdata’ like any other variable available to
gdb.
You can give several consecutive collect commands, each one
with a single argument, or one collect command with several
arguments separated by commas; the effect is the same.
The optional mods changes the usual handling of the arguments.
s requests that pointers to chars be handled as strings, in
particular collecting the contents of the memory being pointed at, up
to the first zero. The upper bound is by default the value of the
print elements variable; if s is followed by a decimal
number, that is the upper bound instead. So for instance
‘collect/s25 mystr’ collects as many as 25 characters at
‘mystr’.
The command info scope (see info scope) is
particularly useful for figuring out what data to collect.
teval expr1, expr2, ...collect
action were used.
while-stepping nwhile-stepping
command is followed by the list of what to collect while stepping
(followed by its own end command):
> while-stepping 12
> collect $regs, myglobal
> end
>
Note that $pc is not automatically collected by
while-stepping; you need to explicitly collect that register if
you need it. You may abbreviate while-stepping as ws or
stepping.
set default-collect expr1, expr2, ...collect action prepended
to every tracepoint action list. The expressions are parsed
individually for each tracepoint, so for instance a variable named
xyz may be interpreted as a global for one tracepoint, and a
local for another, as appropriate to the tracepoint's location.
show default-collect