gdb: Tracepoint Restrictions
13.1.10 Tracepoint Restrictions
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There are a number of restrictions on the use of tracepoints. As
described above, tracepoint data gathering occurs on the target without
interaction from GDB. Thus the full capabilities of the debugger are
not available during data gathering, and then at data examination time,
you will be limited by only having what was collected. The following
items describe some common problems, but it is not exhaustive, and you
may run into additional difficulties not mentioned here.
* Tracepoint expressions are intended to gather objects (lvalues).
Thus the full flexibility of GDB's expression evaluator is not
available. You cannot call functions, cast objects to aggregate
types, access convenience variables or modify values (except by
assignment to trace state variables). Some language features may
implicitly call functions (for instance Objective-C fields with
accessors), and therefore cannot be collected either.
* Collection of local variables, either individually or in bulk with
'$locals' or '$args', during 'while-stepping' may behave
erratically. The stepping action may enter a new scope (for
instance by stepping into a function), or the location of the
variable may change (for instance it is loaded into a register).
The tracepoint data recorded uses the location information for the
variables that is correct for the tracepoint location. When the
tracepoint is created, it is not possible, in general, to determine
where the steps of a 'while-stepping' sequence will advance the
program--particularly if a conditional branch is stepped.
* Collection of an incompletely-initialized or partially-destroyed
object may result in something that GDB cannot display, or displays
in a misleading way.
* When GDB displays a pointer to character it automatically
dereferences the pointer to also display characters of the string
being pointed to. However, collecting the pointer during tracing
does not automatically collect the string. You need to explicitly
dereference the pointer and provide size information if you want to
collect not only the pointer, but the memory pointed to. For
example, '*ptr@50' can be used to collect the 50 element array
pointed to by 'ptr'.
* It is not possible to collect a complete stack backtrace at a
tracepoint. Instead, you may collect the registers and a few
hundred bytes from the stack pointer with something like
'*(unsigned char *)$esp@300' (adjust to use the name of the actual
stack pointer register on your target architecture, and the amount
of stack you wish to capture). Then the 'backtrace' command will
show a partial backtrace when using a trace frame. The number of
stack frames that can be examined depends on the sizes of the
frames in the collected stack. Note that if you ask for a block so
large that it goes past the bottom of the stack, the target agent
may report an error trying to read from an invalid address.
* If you do not collect registers at a tracepoint, GDB can infer that
the value of '$pc' must be the same as the address of the
tracepoint and use that when you are looking at a trace frame for
that tracepoint. However, this cannot work if the tracepoint has
multiple locations (for instance if it was set in a function that
was inlined), or if it has a 'while-stepping' loop. In those cases
GDB will warn you that it can't infer '$pc', and default it to
zero.
Info Catalog
gdb: Starting and Stopping Trace Experiments
gdb: Set Tracepoints