Debuggers traditionally use the .Fn ptrace interface: .Fd ptrace 4 "process trace ptrace(request, pid, addr, data); int request, pid, *addr, data; This interface provides a means by which a parent process may control the execution of a child process, and examine and change its core image. Its primary use is for the implementation of breakpoint debugging. There are four arguments whose interpretation depends on a request argument. A process being traced behaves normally until it encounters a signal (whether internally generated like ``illegal instruction'' or externally generated like ``interrupt''). Then the traced process enters a stopped state and its parent is notified via .Fn wait . When the child is in the stopped state, its core image can be examined and modified using .Fn ptrace . Another ptrace request can then cause the child either to terminate or to continue, possibly ignoring the signal.

A more general interface is also provided in 4.4BSD; the mount_procfs filesystem attaches an instance of the process name space to the global filesystem name space. The conventional mount point is /proc. The root of the process filesystem contains an entry for each active process. These processes are visible as directories named by the process' ID. In addition, the special entry curproc references the current process. Each directory contains several files, including a ctl file. The debugger finds (or creates) the process that it wants to debug and then issues an attach command via the ctl file. Further interaction can then be done with the process through the other files provided by the /proc filesystem. .Sh 3 "Kernel tracing

Another facility for debugging programs is provided by the .Fn ktrace interface: .Fd ktrace 4 "process tracing ktrace(tracefile, ops, trpoints, pid); char *tracefile; int ops, trpoints, pid; .Fn Ktrace does kernel trace logging for the specified processes. The kernel operations that are traced include system calls, pathname translations, signal processing, and I/O. This facility can be particularly useful to debug programs for which you do not have the source.
We currently do not document the vtrace call
.Fd vtrace 2