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Tracepoints

In some applications, it is not feasible for the debugger to interrupt the program's execution long enough for the developer to learn anything helpful about its behavior. If the program's correctness depends on its real-time behavior, delays introduced by a debugger might cause the program to change its behavior drastically, or perhaps fail, even when the code itself is correct. It is useful to be able to observe the program's behavior without interrupting it.

Using GDB's trace and collect commands, you can specify locations in the program, called tracepoints, and arbitrary expressions to evaluate when those tracepoints are reached. Later, using the tfind command, you can examine the values those expressions had when the program hit the tracepoints. The expressions may also denote objects in memory--structures or arrays, for example--whose values GDB should record; while visiting a particular tracepoint, you may inspect those objects as if they were in memory at that moment. However, because GDB records these values without interacting with you, it can do so quickly and unobtrusively, hopefully not disturbing the program's behavior.

The tracepoint facility is currently available only for remote targets. See section Specifying a Debugging Target. In addition, your remote target must know how to collect trace data. This functionality is implemented in the remote stub; however, none of the stubs distributed with GDB support tracepoints as of this writing.

This chapter describes the tracepoint commands and features.

Commands to Set Tracepoints

Before running such a trace experiment, an arbitrary number of tracepoints can be set. Like a breakpoint (see section Setting breakpoints), a tracepoint has a number assigned to it by GDB. Like with breakpoints, tracepoint numbers are successive integers starting from one. Many of the commands associated with tracepoints take the tracepoint number as their argument, to identify which tracepoint to work on.

For each tracepoint, you can specify, in advance, some arbitrary set of data that you want the target to collect in the trace buffer when it hits that tracepoint. The collected data can include registers, local variables, or global data. Later, you can use GDB commands to examine the values these data had at the time the tracepoint was hit.

This section describes commands to set tracepoints and associated conditions and actions.

Create and Delete Tracepoints

trace
The trace command is very similar to the break command. Its argument can be a source line, a function name, or an address in the target program. See section Setting breakpoints. 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 commands doesn't take effect until the next tstart command; thus, you cannot change the tracepoint attributes once a trace experiment is running. 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. The convenience variable $tpnum records the tracepoint number of the most recently set tracepoint.
delete tracepoint [num]
Permanently delete one or more tracepoints. With no argument, the default is to delete all tracepoints. Examples:
(gdb) delete trace 1 2 3 // remove three tracepoints

(gdb) delete trace       // remove all tracepoints
You can abbreviate this command as del tr.

Enable and Disable Tracepoints

disable tracepoint [num]
Disable tracepoint num, or all tracepoints if no argument num is given. A disabled tracepoint will have no effect during the next trace experiment, but it is not forgotten. You can re-enable a disabled tracepoint using the enable tracepoint command.
enable tracepoint [num]
Enable tracepoint num, or all tracepoints. The enabled tracepoints will become effective the next time a trace experiment is run.

Tracepoint Passcounts

passcount [n [num]]
Set the passcount of a tracepoint. The passcount is a way to automatically stop a trace experiment. If a tracepoint's passcount is n, then the trace experiment will be automatically stopped on the n'th time that tracepoint is hit. If the tracepoint number num is not specified, the passcount command sets the passcount of the most recently defined tracepoint. If no passcount is given, the trace experiment will run until stopped explicitly by the user. Examples:
(gdb) passcount 5 2 // Stop on the 5th execution of
                                   // tracepoint 2

(gdb) passcount 12  // Stop on the 12th execution of the
                                   // most recently defined tracepoint.
(gdb) trace foo
(gdb) pass 3
(gdb) trace bar
(gdb) pass 2
(gdb) trace baz
(gdb) pass 1        // Stop tracing when foo has been
                                    // executed 3 times OR when bar has
                                    // been executed 2 times
                                    // OR when baz has been executed 1 time.

Tracepoint Action Lists

actions [num]
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 end. So far, the only defined actions are collect and while-stepping. 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 while stepping. 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 $fp, $sp
  > end
end
collect 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:
$regs
collect all registers
$args
collect all function arguments
$locals
collect all local variables.
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 command info scope (see section Examining the Symbol Table) is particularly useful for figuring out what data to collect.
while-stepping n
Perform n single-step traces after the tracepoint, collecting new data at each step. The while-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
>
You may abbreviate while-stepping as ws or stepping.

Listing Tracepoints

info tracepoints [num]
Display information about the tracepoint num. If you don't specify a tracepoint number, displays information about all the tracepoints defined so far. For each tracepoint, the following information is shown:
  • its number
  • whether it is enabled or disabled
  • its address
  • its passcount as given by the passcount n command
  • its step count as given by the while-stepping n command
  • where in the source files is the tracepoint set
  • its action list as given by the actions command
(gdb) info trace
Num Enb Address    PassC StepC What
1   y   0x002117c4 0     0     <gdb_asm>
2   y   0x0020dc64 0     0     in g_test at g_test.c:1375
3   y   0x0020b1f4 0     0     in get_data at ../foo.c:41
(gdb)
This command can be abbreviated info tp.

Starting and Stopping Trace Experiment

tstart
This command takes no arguments. It starts the trace experiment, and begins collecting data. This has the side effect of discarding all the data collected in the trace buffer during the previous trace experiment.
tstop
This command takes no arguments. It ends the trace experiment, and stops collecting data. Note: a trace experiment and data collection may stop automatically if any tracepoint's passcount is reached (see section Tracepoint Passcounts), or if the trace buffer becomes full.
tstatus
This command displays the status of the current trace data collection.

Here is an example of the commands we described so far:

(gdb) trace gdb_c_test
(gdb) actions
Enter actions for tracepoint #1, one per line.
> collect $regs,$locals,$args
> while-stepping 11
  > collect $regs
  > end
> end
(gdb) tstart
	[time passes ...]
(gdb) tstop

Using the collected data

After the tracepoint experiment ends, you use GDB commands for examining the trace data. The basic idea is that each tracepoint collects a trace snapshot every time it is hit and another snapshot every time it single-steps. All these snapshots are consecutively numbered from zero and go into a buffer, and you can examine them later. The way you examine them is to focus on a specific trace snapshot. When the remote stub is focused on a trace snapshot, it will respond to all GDB requests for memory and registers by reading from the buffer which belongs to that snapshot, rather than from real memory or registers of the program being debugged. This means that all GDB commands (print, info registers, backtrace, etc.) will behave as if we were currently debugging the program state as it was when the tracepoint occurred. Any requests for data that are not in the buffer will fail.

tfind n

The basic command for selecting a trace snapshot from the buffer is tfind n, which finds trace snapshot number n, counting from zero. If no argument n is given, the next snapshot is selected.

Here are the various forms of using the tfind command.

tfind start
Find the first snapshot in the buffer. This is a synonym for tfind 0 (since 0 is the number of the first snapshot).
tfind none
Stop debugging trace snapshots, resume live debugging.
tfind end
Same as `tfind none'.
tfind
No argument means find the next trace snapshot.
tfind -
Find the previous trace snapshot before the current one. This permits retracing earlier steps.
tfind tracepoint num
Find the next snapshot associated with tracepoint num. Search proceeds forward from the last examined trace snapshot. If no argument num is given, it means find the next snapshot collected for the same tracepoint as the current snapshot.
tfind pc addr
Find the next snapshot associated with the value addr of the program counter. Search proceeds forward from the last examined trace snapshot. If no argument addr is given, it means find the next snapshot with the same value of PC as the current snapshot.
tfind outside addr1, addr2
Find the next snapshot whose PC is outside the given range of addresses.
tfind range addr1, addr2
Find the next snapshot whose PC is between addr1 and addr2.
tfind line [file:]n
Find the next snapshot associated with the source line n. If the optional argument file is given, refer to line n in that source file. Search proceeds forward from the last examined trace snapshot. If no argument n is given, it means find the next line other than the one currently being examined; thus saying tfind line repeatedly can appear to have the same effect as stepping from line to line in a live debugging session.

The default arguments for the tfind commands are specifically designed to make it easy to scan through the trace buffer. For instance, tfind with no argument selects the next trace snapshot, and tfind - with no argument selects the previous trace snapshot. So, by giving one tfind command, and then simply hitting RET repeatedly you can examine all the trace snapshots in order. Or, by saying tfind - and then hitting RET repeatedly you can examine the snapshots in reverse order. The tfind line command with no argument selects the snapshot for the next source line executed. The tfind pc command with no argument selects the next snapshot with the same program counter (PC) as the current frame. The tfind tracepoint command with no argument selects the next trace snapshot collected by the same tracepoint as the current one.

In addition to letting you scan through the trace buffer manually, these commands make it easy to construct GDB scripts that scan through the trace buffer and print out whatever collected data you are interested in. Thus, if we want to examine the PC, FP, and SP registers from each trace frame in the buffer, we can say this:

(gdb) tfind start
(gdb) while ($trace_frame != -1)
> printf "Frame %d, PC = %08X, SP = %08X, FP = %08X\n", \
          $trace_frame, $pc, $sp, $fp
> tfind
> end

Frame 0, PC = 0020DC64, SP = 0030BF3C, FP = 0030BF44
Frame 1, PC = 0020DC6C, SP = 0030BF38, FP = 0030BF44
Frame 2, PC = 0020DC70, SP = 0030BF34, FP = 0030BF44
Frame 3, PC = 0020DC74, SP = 0030BF30, FP = 0030BF44
Frame 4, PC = 0020DC78, SP = 0030BF2C, FP = 0030BF44
Frame 5, PC = 0020DC7C, SP = 0030BF28, FP = 0030BF44
Frame 6, PC = 0020DC80, SP = 0030BF24, FP = 0030BF44
Frame 7, PC = 0020DC84, SP = 0030BF20, FP = 0030BF44
Frame 8, PC = 0020DC88, SP = 0030BF1C, FP = 0030BF44
Frame 9, PC = 0020DC8E, SP = 0030BF18, FP = 0030BF44
Frame 10, PC = 00203F6C, SP = 0030BE3C, FP = 0030BF14

Or, if we want to examine the variable X at each source line in the buffer:

(gdb) tfind start
(gdb) while ($trace_frame != -1)
> printf "Frame %d, X == %d\n", $trace_frame, X
> tfind line
> end

Frame 0, X = 1
Frame 7, X = 2
Frame 13, X = 255

tdump

This command takes no arguments. It prints all the data collected at the current trace snapshot.

(gdb) trace 444
(gdb) actions
Enter actions for tracepoint #2, one per line:
> collect $regs, $locals, $args, gdb_long_test
> end

(gdb) tstart

(gdb) tfind line 444
#0  gdb_test (p1=0x11, p2=0x22, p3=0x33, p4=0x44, p5=0x55, p6=0x66)
at gdb_test.c:444
444        printp( "%s: arguments = 0x%X 0x%X 0x%X 0x%X 0x%X 0x%X\n", )

(gdb) tdump
Data collected at tracepoint 2, trace frame 1:
d0             0xc4aa0085       -995491707
d1             0x18     24
d2             0x80     128
d3             0x33     51
d4             0x71aea3d        119204413
d5             0x22     34
d6             0xe0     224
d7             0x380035 3670069
a0             0x19e24a 1696330
a1             0x3000668        50333288
a2             0x100    256
a3             0x322000 3284992
a4             0x3000698        50333336
a5             0x1ad3cc 1758156
fp             0x30bf3c 0x30bf3c
sp             0x30bf34 0x30bf34
ps             0x0      0
pc             0x20b2c8 0x20b2c8
fpcontrol      0x0      0
fpstatus       0x0      0
fpiaddr        0x0      0
p = 0x20e5b4 "gdb-test"
p1 = (void *) 0x11
p2 = (void *) 0x22
p3 = (void *) 0x33
p4 = (void *) 0x44
p5 = (void *) 0x55
p6 = (void *) 0x66
gdb_long_test = 17 '\021'

(gdb)

save-tracepoints filename

This command saves all current tracepoint definitions together with their actions and passcounts, into a file `filename' suitable for use in a later debugging session. To read the saved tracepoint definitions, use the source command (see section Command files).

Convenience Variables for Tracepoints

(int) $trace_frame
The current trace snapshot (a.k.a. frame) number, or -1 if no snapshot is selected.
(int) $tracepoint
The tracepoint for the current trace snapshot.
(int) $trace_line
The line number for the current trace snapshot.
(char []) $trace_file
The source file for the current trace snapshot.
(char []) $trace_func
The name of the function containing $tracepoint.

Note: $trace_file is not suitable for use in printf, use output instead.

Here's a simple example of using these convenience variables for stepping through all the trace snapshots and printing some of their data.

(gdb) tfind start

(gdb) while $trace_frame != -1
> output $trace_file
> printf ", line %d (tracepoint #%d)\n", $trace_line, $tracepoint
> tfind
> end


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