Handling 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; 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 (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, DDD 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 immediately whenever an error signal happens. In DDD, you can view and edit these settings via Status => Signals.

Status => Signals pops up a panel showing all the kinds of signals and how GDB has been told to handle each one. The settings available for each signal are:

• Stop
• If set, GDB should stop your program when this signal happens. This also implies Print being set.
• Print

• If set, GDB should print a message when this signal happens.

  If unset, GDB should not mention the occurrence of the signal at all. This also implies Stop being unset.

• Pass

• If set, GDB 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.

  If unset, GDB should not allow your program to see this signal.

The entry All Signals is special. Changing a setting here affects all signals at once--except those used by the debugger, typically SIGTRAP and SIGINT.

To undo any changes, use Edit => Undo. The Reset button restores the saved settings.

When a signal stops your program, the signal is not visible 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 change the Pass setting in Status => Signals to control whether your program sees that signal when you continue.

You can also cause your program to see a signal it normally would not see, or to give it any signal at any time. The Send button will resume execution where your program stopped, but immediately give it the signal shown.

On the other hand, you can also prevent your program from seeing a signal. 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 resume execution using Commands => Continue Without Signal.

Signal settings are not saved across DDD invocations, since changed signal settings are normally useful within specific projects only. Instead, signal settings are saved with the current session, using File => Save Session As.

Node:Killing the Program, Previous:Signals, Up:Running