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hacking.md

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Notes for hacking on Magenta

This file contains a random collection of notes for hacking on Magenta.

syscall generation

Syscall support is generated from system/public/magenta/syscalls.sysgen. A host tool called sysgen consumes that file and produces output for both the kernel and userspace in a variety of languages. This output includes C or C++ headers for both the kernel and userspace, syscall entry points, other language bindings, and so on.

This tool is invoked as a part of the build, rather than checking in its output.

Terminal navigation and keyboard shortcuts

  • Alt+Tab switches between VTs
  • Alt+F{1,2,...} switches directly to a VT
  • Alt+Up/Down scrolls up and down by lines
  • Shift+PgUp/PgDown scrolls up and down by half page
  • Ctrl+Alt+Delete reboots

Low level kernel development

For kernel development it's not uncommon to need to monitor or break things before the gfxconsole comes up.

To enable the early console before the graphical console comes up use the gfxconsole.early cmdline option. More information can be found in kernel_cmdline.md. Enabling startup.keep-log-visiblewill ensure that the kernel log stays visible if the gfxconsole comes up after boot. To disable the gfxconsole entirely you can disable the video driver it is binding to via driver.<driver name>.disable. On a skylake system, all these options together would look something like:

$ tools/build-magenta-x86_64/bootserver build-magenta-x86_64/magenta.bin -- gfxconsole.early driver.intel-i915-display.disable

To directly output to the console rather than buffering it (useful in the event of kernel freezes) you can enable ENABLE_KERNEL_LL_DEBUG in your local.mk like so:

EXTERNAL_KERNEL_DEFINES := ENABLE_KERNEL_LL_DEBUG=1

More information on local.mk can be found via make help

Requesting a backtrace from within a program

For debugging purposes, the system crashlogger can print backtraces by request. It requires modifying your source, but in the absence of a debugger, or as a general builtin debug mechanism, this can be useful.

#include <magenta/crashlogger.h>

void my_function() {
  crashlogger_request_backtrace();
}

When crashlogger_request_backtrace is called, it causes an exception used by debuggers for breakpoint handling. If a debugger is not attached, the system crashlogger will process the exception, print a backtrace, and then resume the thread.