Using and understanding the Valgrind core: Advanced Topics
• GDB version needed for ARM and PPC32/64.
You must use a GDB version which is able to read XML target description sent by a gdbserver. This is the standard
setup if GDB was configured and built with the "expat" library. If your GDB was not configured with XML support,
it will report an error message when using the "target" command. Debugging will not work because GDB will then
not be able to fetch the registers from the Valgrind gdbserver. For ARM programs using the Thumb instruction set,
you must use a GDB version of 7.1 or later, as earlier versions have problems with next/step/breakpoints in Thumb
code.
• Stack unwinding on PPC32/PPC64.
On PPC32/PPC64, stack unwinding for leaf functions (functions that do not call any other functions) works
properly only when you give the option
--vex-iropt-register-updates=allregs-at-mem-access
or
--vex-iropt-register-updates=allregs-at-each-insn
. You must also pass this option in
order to get a precise stack when a signal is trapped by GDB.
• Breakpoints encountered multiple times.
Some instructions (e.g. x86 "rep movsb") are translated by Valgrind using a loop.
If a breakpoint is placed on
such an instruction, the breakpoint will be encountered multiple times -- once for each step of the "implicit" loop
implementing the instruction.
• Execution of Inferior function calls by the Valgrind gdbserver.
GDB allows the user to "call" functions inside the process being debugged. Such calls are named "inferior calls" in
the GDB terminology. A typical use of an inferior call is to execute a function that prints a human-readable version
of a complex data structure. To make an inferior call, use the GDB "print" command followed by the function to
call and its arguments.
As an example, the following GDB command causes an inferior call to the libc "printf"
function to be executed by the process being debugged:
(gdb) p printf("process being debugged has pid %d\n", getpid())
$5 = 36
(gdb)
The Valgrind gdbserver supports inferior function calls. Whilst an inferior call is running, the Valgrind tool will
report errors as usual.
If you do not want to have such errors stop the execution of the inferior call, you can use
v.set vgdb-error
to set a big value before the call, then manually reset it to its original value when the call is
complete.
To execute inferior calls, GDB changes registers such as the program counter, and then continues the execution
of the program. In a multithreaded program, all threads are continued, not just the thread instructed to make the
inferior call.
If another thread reports an error or encounters a breakpoint, the evaluation of the inferior call is
abandoned.
Note that inferior function calls are a powerful GDB feature, but should be used with caution. For example, if the
program being debugged is stopped inside the function "printf", forcing a recursive call to printf via an inferior call
will very probably create problems. The Valgrind tool might also add another level of complexity to inferior calls,
e.g. by reporting tool errors during the Inferior call or due to the instrumentation done.
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