[9fans] debugging

[jmk@plan9.bell-labs.com jmk@plan9.bell-labs.com]

The original usage of consdebug was just to give a hook into specialised
functions, usually during driver devlopment. That facility is still there,
but the default behaviour now is to drop into the remote debugger:

     RDBFS(4)                                                 RDBFS(4)

     NAME
          rdbfs - remote kernel debugging file system

     SYNOPSIS
          rdbfs [ -d ] [ -p pid ] [ -t text ] [ device ]

     DESCRIPTION
          Rdbfs presents in /proc/pid (default /proc/1) a set of pro-
          cess files for debugging a kernel over the serial line
          device (default /dev/eia0).

          The text file presented is just a copy of text (default
          /386/9pc).  It can usually be ignored, since the debuggers
          open kernel files directly rather than using /proc/n/text.

          Kernels can be remotely debugged  only when they are sus-
          pended and serving a textual debugging protocol over their
          serial lines.  Typing ``^t^td'' (control-t, control-t, d) at
          the console will cause the kernel to enter this mode when it
          `panics'.  Typing ``^t^tD'' causes the kernel to enter this
          mode immediately.

          Because the debugging protocol is textual, a console pro-
          vided by consolefs(4) may be substituted for the serial
          device.

     SOURCE
          /sys/src/cmd/rdbfs.c
          /sys/src/9/port/rdb.c

     SEE ALSO
          acid(1), db(1), consolefs(4)

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