7.3. 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 let the non-erroneous signals like
SIGALRM be silently passed to your program
(so as not to interfere with their role in the program's functioning)
but to stop your program immediately whenever an error signal happens.
You can change these settings with the handle command.
- info signals, info handle
Print a table of all the kinds of signals and how gdb has been told to
handle each one. You can use this to see the signal numbers of all
the defined types of signals.
info handle is an alias for info signals.
- handle signal keywords…
Change the way gdb handles signal signal. signal
can be the number of a signal or its name (with or without the
SIG at the beginning); a list of signal numbers of the form
low-high; or the word all, meaning all the
known signals. The keywords say what change to make.
The keywords allowed by the handle command can be abbreviated.
Their full names are:
- nostop
{No value for `
<listitem>GDBN'} should not stop your program when this signal happens. It may
still print a message telling you that the signal has come in.
- stop
{No value for `
<listitem>GDBN'} should stop your program when this signal happens. This implies
the print keyword as well.
- print
{No value for `
<listitem>GDBN'} should print a message when this signal happens.
- noprint
{No value for `
<listitem>GDBN'} should not mention the occurrence of the signal at all. This
implies the nostop keyword as well.
- pass, noignore
{No value for `
<listitem>GDBN'} should allow your program to see this signal; your program
can handle the signal, or else it may terminate if the signal is fatal
and not handled. pass and noignore are synonyms.
- nopass, ignore
{No value for `
<listitem>GDBN'} should not allow your program to see this signal.
nopass and ignore are synonyms.
When a signal stops your program, the signal is not visible to the
program until you
continue. Your program sees the signal then, if pass is in
effect for the signal in question at that time. In other words,
after gdb reports a signal, you can use the handle
command with pass or nopass to control whether your
program sees that signal when you continue.
The default is set to nostop, noprint, pass for
non-erroneous signals such as SIGALRM, SIGWINCH and
SIGCHLD, and to stop, print, pass for the
erroneous signals.
You can also use the signal command to prevent your program from
seeing a signal, or cause it to see a signal it normally would not see,
or to give it any signal at any time. For example, if your program stopped
due to some sort of memory reference error, you might store correct
values into the erroneous variables and continue, hoping to see more
execution; but your program would probably terminate immediately as
a result of the fatal signal once it saw the signal. To prevent this,
you can continue with signal 0.
Refer to Section 16.3 Giving your program a signal.
