|Debugging with GDB: Tracepoint Actions|
This command will prompt for a list of actions to be taken when the
tracepoint is hit. If the tracepoint number num is not
specified, this command sets the actions for the one that was most
recently defined (so that you can define a tracepoint and then say
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
far, the only defined actions are
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
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
(gdb) trace foo (gdb) actions Enter actions for tracepoint 1, one per line: > collect bar,baz > collect $regs > while-stepping 12 > collect $pc, arr[i] > end end
collect[/mods] expr1, expr2, …
Collect values of the given expressions when the tracepoint is hit. This command accepts a comma-separated list of any valid expressions. In addition to global, static, or local variables, the following special arguments are supported:
Collect all registers.
Collect all function arguments.
Collect all local variables.
Collect the return address. This is helpful if you want to see more of a backtrace.
Note: The return address location can not always be reliably determined up front, and the wrong address / registers may end up collected instead. On some architectures the reliability is higher for tracepoints at function entry, while on others it’s the opposite. When this happens, backtracing will stop because the return address is found unavailable (unless another collect rule happened to match it).
Collects the number of arguments from the static probe at which the tracepoint is located. See Static Probe Points.
n is an integer between 0 and 11. Collects the nth argument from the static probe at which the tracepoint is located. See Static Probe Points.
Collect static tracepoint marker specific data. Only available for
static tracepoints. See Tracepoint Action
Lists. On the UST static tracepoints library backend, an
instrumentation point resembles a
printf function call. The
tracing library is able to collect user specified data formatted to a
character string using the format provided by the programmer that
instrumented the program. Other backends have similar mechanisms.
Here’s an example of a UST marker call:
const char master_name = "$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
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
info scope (see info scope) is
particularly useful for figuring out what data to collect.
teval expr1, expr2, …
Evaluate the given expressions when the tracepoint is hit. This
command accepts a comma-separated list of expressions. The results
are discarded, so this is mainly useful for assigning values to trace
state variables (see Trace State Variables) without adding those
values to the trace buffer, as would be the case if the
action were used.
Perform n single-step instruction traces after the tracepoint,
collecting new data after each step. The
command is followed by the list of what to collect while stepping
(followed by its own
> while-stepping 12 > collect $regs, myglobal > end >
$pc is not automatically collected by
while-stepping; you need to explicitly collect that register if
you need it. You may abbreviate
set default-collect expr1, expr2, …
This variable is a list of expressions to collect at each tracepoint
hit. It is effectively an additional
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 the list of expressions that are collected by default at each tracepoint hit.