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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 Ctrl-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 signalsinfo handleinfo signals siginfo handle is an alias for info signals.
catch signal [signal... | ‘all’]handle signal [keywords...]The keywords allowed by the handle command can be abbreviated.
Their full names are:
nostopstopprint keyword as well.
printnoprintnostop keyword as well.
passnoignorepass and noignore are synonyms.
nopassignorenopass 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’. See Giving your Program a Signal.
gdb optimizes for stepping the mainline code. If a signal
that has handle nostop and handle pass set arrives while
a stepping command (e.g., stepi, step, next) is
in progress, gdb lets the signal handler run and then resumes
stepping the mainline code once the signal handler returns. In other
words, gdb steps over the signal handler. This prevents
signals that you've specified as not interesting (with handle
nostop) from changing the focus of debugging unexpectedly. Note that
the signal handler itself may still hit a breakpoint, stop for another
signal that has handle stop in effect, or for any other event
that normally results in stopping the stepping command sooner. Also
note that gdb still informs you that the program received a
signal if handle print is set.
If you set handle pass for a signal, and your program sets up a
handler for it, then issuing a stepping command, such as step
or stepi, when your program is stopped due to the signal will
step into the signal handler (if the target supports that).
Likewise, if you use the queue-signal command to queue a signal
to be delivered to the current thread when execution of the thread
resumes (see Giving your Program a Signal), then a
stepping command will step into the signal handler.
Here's an example, using stepi to step to the first instruction
of SIGUSR1's handler:
(gdb) handle SIGUSR1
Signal Stop Print Pass to program Description
SIGUSR1 Yes Yes Yes User defined signal 1
(gdb) c
Continuing.
Program received signal SIGUSR1, User defined signal 1.
main () sigusr1.c:28
28 p = 0;
(gdb) si
sigusr1_handler () at sigusr1.c:9
9 {
The same, but using queue-signal instead of waiting for the
program to receive the signal first:
(gdb) n
28 p = 0;
(gdb) queue-signal SIGUSR1
(gdb) si
sigusr1_handler () at sigusr1.c:9
9 {
(gdb)
On some targets, gdb can inspect extra signal information
associated with the intercepted signal, before it is actually
delivered to the program being debugged. This information is exported
by the convenience variable $_siginfo, and consists of data
that is passed by the kernel to the signal handler at the time of the
receipt of a signal. The data type of the information itself is
target dependent. You can see the data type using the ptype
$_siginfo command. On Unix systems, it typically corresponds to the
standard siginfo_t type, as defined in the signal.h
system header.
Here's an example, on a gnu/Linux system, printing the stray referenced address that raised a segmentation fault.
(gdb) continue
Program received signal SIGSEGV, Segmentation fault.
0x0000000000400766 in main ()
69 *(int *)p = 0;
(gdb) ptype $_siginfo
type = struct {
int si_signo;
int si_errno;
int si_code;
union {
int _pad[28];
struct {...} _kill;
struct {...} _timer;
struct {...} _rt;
struct {...} _sigchld;
struct {...} _sigfault;
struct {...} _sigpoll;
} _sifields;
}
(gdb) ptype $_siginfo._sifields._sigfault
type = struct {
void *si_addr;
}
(gdb) p $_siginfo._sifields._sigfault.si_addr
$1 = (void *) 0x7ffff7ff7000
Depending on target support, $_siginfo may also be writable.