trace
locationtrace
command is very similar to the break
command.
Its argument location can be a source line, a function name, or
an address in the target program. See Specify Location. The
trace
command defines a tracepoint, which is a point in the
target program where the debugger will briefly stop, collect some
data, and then allow the program to continue. Setting a tracepoint or
changing its actions doesn't take effect until the next tstart
command, and once a trace experiment is running, further changes will
not have any effect until the next trace experiment starts.
Here are some examples of using the trace
command:
(gdb) trace foo.c:121 // a source file and line number (gdb) trace +2 // 2 lines forward (gdb) trace my_function // first source line of function (gdb) trace *my_function // EXACT start address of function (gdb) trace *0x2117c4 // an address
You can abbreviate trace
as tr
.
trace
location if
condftrace
location [ if
cond ]
ftrace
command sets a fast tracepoint. For targets that
support them, fast tracepoints will use a more efficient but possibly
less general technique to trigger data collection, such as a jump
instruction instead of a trap, or some sort of hardware support. It
may not be possible to create a fast tracepoint at the desired
location, in which case the command will exit with an explanatory
message.
gdb handles arguments to ftrace
exactly as for
trace
.
strace
location [ if
cond ]
strace
command sets a static tracepoint. For targets that
support it, setting a static tracepoint probes a static
instrumentation point, or marker, found at location. It may not
be possible to set a static tracepoint at the desired location, in
which case the command will exit with an explanatory message.
gdb handles arguments to strace
exactly as for
trace
, with the addition that the user can also specify
-m
marker as location. This probes the marker
identified by the marker string identifier. This identifier
depends on the static tracepoint backend library your program is
using. You can find all the marker identifiers in the ‘ID’ field
of the info static-tracepoint-markers
command output.
See Listing Static Tracepoint Markers. For example, in the following small program using the UST
tracing engine:
main () { trace_mark(ust, bar33, "str %s", "FOOBAZ"); }
the marker id is composed of joining the first two arguments to the
trace_mark
call with a slash, which translates to:
(gdb) info static-tracepoint-markers Cnt Enb ID Address What 1 n ust/bar33 0x0000000000400ddc in main at stexample.c:22 Data: "str %s" [etc...]
so you may probe the marker above with:
(gdb) strace -m ust/bar33
Static tracepoints accept an extra collect action — collect
$_sdata
. This collects arbitrary user data passed in the probe point
call to the tracing library. In the UST example above, you'll see
that the third argument to trace_mark
is a printf-like format
string. The user data is then the result of running that formating
string against the following arguments. Note that info
static-tracepoint-markers
command output lists that format string in
the ‘Data:’ field.
You can inspect this data when analyzing the trace buffer, by printing the $_sdata variable like any other variable available to gdb. See Tracepoint Action Lists.
The convenience variable $tpnum
records the tracepoint number
of the most recently set tracepoint.
delete tracepoint
[num]delete
command can remove tracepoints also.
Examples:
(gdb) delete trace 1 2 3 // remove three tracepoints (gdb) delete trace // remove all tracepoints
You can abbreviate this command as del tr
.