3 Finding your way around

Here is a brief map of the GRUB code base.

GRUB uses Autoconf and Automake, with most of the Automake input generated by a Python script. The top-level build rules are in configure.ac, grub-core/Makefile.core.def, and Makefile.util.def. Each block in a *.def file represents a build target, and specifies the source files used to build it on various platforms. The *.def files are processed into Automake input by gentpl.py (which you only need to look at if you are extending the build system). If you are adding a new module which follows an existing pattern, such as a new command or a new filesystem implementation, it is usually easiest to grep grub-core/Makefile.core.def and Makefile.util.def for an existing example of that pattern to find out where it should be added.

In general, code that may be run at boot time is in a subdirectory of grub-core, while code that is only run from within a full operating system is in a subdirectory of the top level.

Low-level boot code, such as the MBR implementation on PC BIOS systems, is in the grub-core/boot/ directory.

The GRUB kernel is in grub-core/kern/. This contains core facilities such as the device, disk, and file frameworks, environment variable handling, list processing, and so on. The kernel should contain enough to get up to a rescue prompt. Header files for kernel facilities, among others, are in include/.

Terminal implementations are in grub-core/term/.

Disk access code is spread across grub-core/disk/ (for accessing the disk devices themselves), grub-core/partmap/ (for interpreting partition table data), and grub-core/fs/ (for accessing filesystems). Note that, with the odd specialised exception, GRUB only contains code to read from filesystems and tries to avoid containing any code to write to filesystems; this lets us confidently assure users that GRUB cannot be responsible for filesystem corruption.

PCI and USB bus handling is in grub-core/bus/.

Video handling code is in grub-core/video/. The graphical menu system uses this heavily, but is in a separate directory, grub-core/gfxmenu/.

Most commands are implemented by files in grub-core/commands/, with the following exceptions:

• A few core commands live in grub-core/kern/corecmd.c.
• Commands related to normal mode live under grub-core/normal/.
• Commands that load and boot kernels live under grub-core/loader/.
• The ‘loopback’ command is really a disk device, and so lives in grub-core/disk/loopback.c.
• The ‘gettext’ command lives under grub-core/gettext/.
• The ‘loadfont’ and ‘lsfonts’ commands live under grub-core/font/.
• The ‘serial’, ‘terminfo’, and ‘background_image’ commands live under grub-core/term/.
• The ‘efiemu_*’ commands live under grub-core/efiemu/.
• OS-dependent code should be under grub-core/osdep/
• Utility programs meant to be run from a full operating system (except OS-dependent code mentioned previously) are in util/.

There are a few other special-purpose exceptions; grep for them if they matter to you.