Signals are used for both clock profiling and hardware counter profiling. SIGPROF is used in data collection for all experiments. The period for generating the signal depends on the data being collected. SIGEMT on Solaris or SIGIO on Linux is used for hardware counter profiling. The overflow interval depends on the user parameter for profiling. Any user code that uses or manipulates the profiling signals may potentially interfere with data collection. When the Collector installs its signal handler for a profile signal, it sets a flag that ensures that system calls are not interrupted to deliver signals. This setting could change the behavior of a target program that uses the profiling signals for other purposes.
When the Collector installs its signal handler for a profile signal, it remembers whether or not the target had installed its own signal handler. The Collector also interposes on some signal-handling routines and does not allow the user to install a signal handler for these signals; it saves the user's handler, just as it does when the Collector replaces a user handler on starting the experiment.
Profiling signals are delivered by from the profiling timer or hardware-counter-overflow handling code in the kernel, or in response to: the kill(2), sigsend(2) tkill(2), tgkill(2) or _lwp_kill(2) system calls, the raise(3C) and sigqueue(3C) library calls or the kill command. A signal code is delivered with the signal so that the Collector can distinguish the origin. If it is delivered for profiling, it is processed by the Collector; If it is not delivered for profiling, it is delivered to the target signal handler.
When the Collector is running under dbx, the profiling signal delivered occasionally has its signal code corrupted, and a profile signal may be treated as if it were generated from a system or library call or a command. In that case, it will be incorrectly delivered to the user's handler. If the user handler was set to SIG_DFL, it will cause the process to fail core dump.
When the Collector is invoked after attaching to a target process, it will install its signal handler, but it cannot interpose on the signal-handling routines. If those user code installs a signal handler after the attach, it will override the Collector's signal handler, and data will be lost.
Note that any signal including either of the profiling signals might cause premature termination of a system call. The program must be prepared to handle that behavior. When libcollector installs the signal handlers for data collection it specifies to restart system calls that can be restarted. However, some system calls such as sleep(3C) return early without reporting an error.
Signals may be specified by the user as a sample signal (-l) or a pause-resume signal (-y). SIGUSR1 or SIGUSR2 are recommended for this use, but any signal that is not used by the target, may be used.
The profiling signals may be used if the process does not otherwise use them, but they should be used only if no other signal is available. The Collector interposes on some signal-handling routines and does not allow the user to install a signal handler for these signals; it saves the user's handler, just as it does when the Collector replaces a user handler on starting the experiment.
If the Collector is invoked after attaching to a target process, and the user code installs a signal handler for the sample or pause-resume signal, those signals will not longer operate as specified.