gdb: Tracepoint Conditions
13.1.4 Tracepoint Conditions
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The simplest sort of tracepoint collects data every time your program
reaches a specified place. You can also specify a "condition" for a
tracepoint. A condition is just a Boolean expression in your
programming language (
Expressions Expressions.). A tracepoint
with a condition evaluates the expression each time your program reaches
it, and data collection happens only if the condition is true.
Tracepoint conditions can be specified when a tracepoint is set, by
using 'if' in the arguments to the 'trace' command. Setting
Tracepoints Create and Delete Tracepoints. They can also be set or
changed at any time with the 'condition' command, just as with
breakpoints.
Unlike breakpoint conditions, GDB does not actually evaluate the
conditional expression itself. Instead, GDB encodes the expression into
an agent expression (Agent Expressions) suitable for execution
on the target, independently of GDB. Global variables become raw memory
locations, locals become stack accesses, and so forth.
For instance, suppose you have a function that is usually called
frequently, but should not be called after an error has occurred. You
could use the following tracepoint command to collect data about calls
of that function that happen while the error code is propagating through
the program; an unconditional tracepoint could end up collecting
thousands of useless trace frames that you would have to search through.
(gdb) trace normal_operation if errcode > 0
Info Catalog
gdb: Tracepoint Passcounts
gdb: Set Tracepoints
gdb: Trace State Variables