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. The table below gives several examples of signals:
|SIGINT||You type an interrupt, Ctrl C|
|SIGSEGV||The program references a place in memory far away from all the areas in use.|
|SIGALRM||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 (killing your program immediately) if the program hasn't specified in advance some other way to handle the signal. SIGINT doesn't indicate an error in your program, but it's 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, it's set up to:
- Ignore signals like SIGALRM that don't indicate an error so as not to interfere with their role in the functioning of your program.
- Stop your program immediately whenever an error signal happens.
You can change these settings with the handle command.
- info signals or 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.
- handle signal keywords...
- Change the way GDB handles signal signal. The signal can be the number of a signal or its name (with or without the SIG at the beginning). The keywords say what change to make.
The keywords allowed by the handle command can be abbreviated. Their full names are:
- GDB shouldn't stop your program when this signal happens. It may still print a message telling you that the signal has come in.
- GDB should stop your program when this signal happens. This implies the print keyword as well.
- GDB should print a message when this signal happens.
- GDB shouldn't mention the occurrence of the signal at all. This implies the nostop keyword as well.
- 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.
- GDB shouldn't allow your program to see this signal.
When a signal stops your program, the signal isn't 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 use the handle command with pass or nopass to control whether your program sees that signal when you continue.
You can also use the signal command to prevent your program from seeing a signal, or cause it to see a signal it normally doesn't 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 .