Debugging with GDB: Create and Delete Tracepoints

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13.1.1 Create and Delete Tracepoints

trace location

The trace command is very similar to the break command. Its argument location can be any valid location. 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 takes effect immediately if the remote stub supports the ‘InstallInTrace’ feature (see install tracepoint in tracing). If remote stub doesn’t support the ‘InstallInTrace’ feature, all these changes don’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. In addition, GDB supports pending tracepoints—tracepoints whose address is not yet resolved. (This is similar to pending breakpoints.) Pending tracepoints are not downloaded to the target and not installed until they are resolved. The resolution of pending tracepoints requires GDB support—when debugging with the remote target, and GDB disconnects from the remote stub (see disconnected tracing), pending tracepoints can not be resolved (and downloaded to the remote stub) while GDB is disconnected.

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 cond

Set a tracepoint with condition cond; evaluate the expression cond each time the tracepoint is reached, and collect data only if the value is nonzero—that is, if cond evaluates as true. See Tracepoint Conditions, for more information on tracepoint conditions.

ftrace location [ if cond ]

The 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.

On 32-bit x86-architecture systems, fast tracepoints normally need to be placed at an instruction that is 5 bytes or longer, but can be placed at 4-byte instructions if the low 64K of memory of the target program is available to install trampolines. Some Unix-type systems, such as GNU/Linux, exclude low addresses from the program’s address space; but for instance with the Linux kernel it is possible to let GDB use this area by doing a sysctl command to set the mmap_min_addr kernel parameter, as in

sudo sysctl -w vm.mmap_min_addr=32768

which sets the low address to 32K, which leaves plenty of room for trampolines. The minimum address should be set to a page boundary.

strace location [ if cond ]

The 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 ]

Permanently delete one or more tracepoints. With no argument, the default is to delete all tracepoints. Note that the regular 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.

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