Attaching to a running process

ptrace(S) allows a process to take control of another process while it is running. This is subject to various security controls: you must own the process you are attaching to, you must be able to read its code file, and the "attacked" process must not have different effective and saved UIDs or GIDs.

dbXtra/dbxtra (and dbx) uses these features to allow you to debug a running process. This is done by specifying the PID of the target process on the command line instead of the name of a core file. Remember that you still have to specify the name of the object file.

The process will be stopped and the debugger will take control of the process. Note:

• Remember that signals sent to the process will be intercepted by the debugger. If you execute cont (continue), any signals that arrived while you were setting up your debugging session will be lost. You can set dbXtra/dbxtra to ignore certain signals, but signals that arrive before you set that up and issue the first cont will still be lost. Recall that you can deliver a signal to a process with a cont signal-number debugger command.

  In particular, this applies to alarm(S). If the process you attach to is sleeping in alarm(S) when you take control, and the alarm timer expires before the process is continued, the alarm signal will be lost. You should use the cont command to issue the alarm signal in this case.

  Also remember that a ptrace(S) restriction is that only one signal can be delivered when a process is continued. If more than one signal was pending, the others will be lost.

• If you want the process to continue to run after you stop the debugger, you must issue the detach command. dbXtra/dbxtra will remove all breakpoints and issue the ptrace detach function to tell the operating system to turn the process loose. You will have no further control of the process.

  Any changes you made (like assigning to variables) will remain in effect.

  If you do not issue a detach command, the process will be killed when the debugger terminates. This prevents a possibly modified process from continuing when it should not.

• If dbXtra/dbxtra cannot attach to the desired process, it issues a warning message and starts an ordinary debugging session.

• Make sure you are in the source directory, or use -I arguments to specify the location of the source for the program.