13.1.10 Tracepoint Restrictions
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
while-steppingmay 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-steppingsequence 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@50can be used to collect the 50 element array pointed to by
- 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
*$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
backtracecommand 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
$pcmust 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-steppingloop. In those cases gdb will warn you that it can't infer
$pc, and default it to zero.