Previous: Interrupted System Calls, Up: Thread Stops

#### 5.5.6 Observer Mode

If you want to build on non-stop mode and observe program behavior without any chance of disruption by gdb, you can set variables to disable all of the debugger's attempts to modify state, whether by writing memory, inserting breakpoints, etc. These operate at a low level, intercepting operations from all commands.

When all of these are set to `off`, then gdb is said to be observer mode. As a convenience, the variable `observer` can be set to disable these, plus enable non-stop mode.

Note that gdb will not prevent you from making nonsensical combinations of these settings. For instance, if you have enabled `may-insert-breakpoints` but disabled `may-write-memory`, then breakpoints that work by writing trap instructions into the code stream will still not be able to be placed.

`set observer on`
`set observer off`
  When set to `on`, this disables all the permission variables below (except for `insert-fast-tracepoints`), plus enables non-stop debugging. Setting this to `off` switches back to normal debugging, though remaining in non-stop mode.
`show observer`
  Show whether observer mode is on or off.

`set may-write-registers on`
`set may-write-registers off`
  This controls whether gdb will attempt to alter the values of registers, such as with assignment expressions in `print`, or the `jump` command. It defaults to `on`.
`show may-write-registers`
  Show the current permission to write registers.

`set may-write-memory on`
`set may-write-memory off`
  This controls whether gdb will attempt to alter the contents of memory, such as with assignment expressions in `print`. It defaults to `on`.
`show may-write-memory`
  Show the current permission to write memory.

`set may-insert-breakpoints on`
`set may-insert-breakpoints off`
  This controls whether gdb will attempt to insert breakpoints. This affects all breakpoints, including internal breakpoints defined by gdb. It defaults to `on`.
`show may-insert-breakpoints`
  Show the current permission to insert breakpoints.

`set may-insert-tracepoints on`
`set may-insert-tracepoints off`
  This controls whether gdb will attempt to insert (regular) tracepoints at the beginning of a tracing experiment. It affects only non-fast tracepoints, fast tracepoints being under the control of `may-insert-fast-tracepoints`. It defaults to `on`.
`show may-insert-tracepoints`
  Show the current permission to insert tracepoints.

`set may-insert-fast-tracepoints on`
`set may-insert-fast-tracepoints off`
  This controls whether gdb will attempt to insert fast tracepoints at the beginning of a tracing experiment. It affects only fast tracepoints, regular (non-fast) tracepoints being under the control of `may-insert-tracepoints`. It defaults to `on`.
`show may-insert-fast-tracepoints`
  Show the current permission to insert fast tracepoints.

`set may-interrupt on`
`set may-interrupt off`
  This controls whether gdb will attempt to interrupt or stop program execution. When this variable is `off`, the `interrupt` command will have no effect, nor will `Ctrl-c`. It defaults to `on`.
`show may-interrupt`
  Show the current permission to interrupt or stop the program.