The Collector uses two signals to collect profiling data: SIGPROF for all experiments and SIGEMT for hardware counter experiments only. The Collector installs a signal handler for each of these signals. The signal handler intercepts and processes its own signal, but passes other signals on to any other signal handlers that are installed. If a program installs its own signal handler for these signals, the Collector reinstalls its signal handler as the primary handler to guarantee the integrity of the performance data.
The collect command can also use user-specified signals for pausing and resuming data collection and for recording samples. These signals are not protected by the Collector although a warning is written to the experiment if a user handler is installed. It is your responsibility to ensure that there is no conflict between use of the specified signals by the Collector and any use made by the application of the same signals.
The signal handlers installed by the Collector set a flag that ensures that system calls are not interrupted for signal delivery. This flag setting could change the behavior of the program if the program’s signal handler sets the flag to permit interruption of system calls. One important example of a change in behavior occurs for the asynchronous I/O library, libaio.so, which uses SIGPROF for asynchronous cancel operations, and which does interrupt system calls. If the collector library, libcollector.so, is installed, the cancel signal invariably arrives too late to cancel the asynchronous I/O operation.
If you attach dbx to a process without preloading the collector library and enable performance data collection, and the program subsequently installs its own signal handler, the Collector does not reinstall its own signal handler. In this case, the program’s signal handler must ensure that the SIGPROF and SIGEMT signals are passed on so that performance data is not lost. If the program’s signal handler interrupts system calls, both the program behavior and the profiling behavior are different from when the collector library is preloaded.