Debugging with GDB (February 2008)

46 Debugging with GDB
even though the test in a C for-loop is written before the body of the loop.
The until command appeared to step back to the beginning of the loop when
it advanced to this expression; however, it has not really gone to an earlier
statement—not in terms of the actual machine code.
until with no argument works by means of single instruction stepping, and
hence is slower than until with an argument.
until location
u location
Continue running your program until either the specified location is reached,
or the current stack frame returns. location is any of the forms of argument
acceptable to break (see Section 5.1.1 [Setting breakpoints], page 33). This form
of the command uses breakpoints, and hence is quicker than until without an
argument.
stepi
stepi arg
si Execute one machine instruction, then stop and return to the debugger.
It is often useful to do ‘display/i $pc’ when stepping by machine instructions.
This makes GDB automatically display the next instruction to be executed,
each time your program stops. See Section 8.6 [Automatic display], page 68.
An argument is a repeat count, as in step.
nexti
nexti arg
ni Execute one machine instruction, but if it is a function call, proceed until the
function returns.
An argument is a repeat count, as in next.
5.3 Signals
A signal is an asynchronous event that can happen in a program. The operating system
defines the possible kinds of signals, and gives each kind a name and a number. For example,
in Unix SIGINT is the signal a program gets when you type an interrupt character (often C-
c); SIGSEGV is the signal a program gets from referencing a place in memory far away from
all the areas in use; SIGALRM occurs when the alarm clock timer goes off (which happens
only if your program has requested an alarm).
Some signals, including SIGALRM, are a normal part of the functioning of your program.
Others, such as SIGSEGV, indicate errors; these signals are fatal (they kill your program
immediately) if the program has not specified in advance some other way to handle the
signal. SIGINT does not indicate an error in your program, but it is normally fatal so it can
carry out the purpose of the interrupt: to kill the program.
GDB has the ability to detect any occurrence of a signal in your program. You can tell
GDB in advance what to do for each kind of signal.
Normally, GDB is set up to ignore non-erroneous signals like SIGALRM (so as not to
interfere with their role in the functioning of your program) but to stop your program