BOOTHOWTO(9)						   BSD Kernel Developer's Manual					      BOOTHOWTO(9)

NAME

     boothowto, BOOT_FLAG -- flags passed to kernel during boot or shutdown

SYNOPSIS

     #include <sys/reboot.h>
     #include <sys/systm.h>

     extern int boothowto;

     #include <sys/boot_flag.h>

     #define BOOT_FLAG(arg, retval) ...

DESCRIPTION

     The boothowto variable contains flags passed to the kernel by the boot loader (see boot(8)), or the reboot(2) system call.  The value is
     interpreted as a bit mask, with bits defined by the ``RB_*'' and ``AB_*'' symbols in <sys/reboot.h>.  The value is made available via the
     sysctl(7) variable kern.boothowto.

     The BOOT_FLAG macro defined in <sys/boot_flag.h> is used by many boot loaders to convert command line options into boothowto flags.  Note
     that not all boot loaders use this macro, and some boot loaders may have incompatible options.

     Where possible, flags set by the reboot(2) system call will be passed to the new kernel after a reboot; the extent to which this is possible
     is machine dependent.

     In the following tables, each flag is listed with its symbolic name, the corresponding numeric value defined in <sys/reboot.h>, and the
     option letter (if any) understood by the BOOT_FLAG macro.

   Flags that affect booting or shutting down
     The following flags affect actions taken during system boot or shutdown.

     RB_AUTOBOOT     0			 The default if no other flags are set.  Causes the system to boot in the normal way.
     RB_ASKNAME      0x00000001    -a	 This flag causes various parts of the system to prompt:
					 o   The boot loader may prompt for the name or location of the kernel to be booted.
					 o   The kernel will prompt for the root file system device.
					 o   The kernel will prompt for the root file system type.
					 o   The kernel will prompt for the location of the dump device.
					 o   The kernel will prompt for the path to the init(8) program.
					 Some subsystems set this flag when they are unable to automatically make a decision.
     RB_SINGLE	     0x00000002    -s	 Boot in single-user mode.  If this flag is set, the kernel passes the -s option to init(8).
     RB_NOSYNC	     0x00000004 	 If this flag is set, then some parts of the shutdown process will be less graceful than usual:
					 o   Disks will not be synced (see sync(2) and cpu_reboot(9)).
					 o   Devices will not be detached (see autoconf(9)).
					 o   File systems will not be unmounted (see cpu_reboot(9), and vfs_shutdown(9)).
					 o   The time of day clock will not be set (see resettodr(9)).
     RB_HALT	     0x00000008    -b	 If this flag is set, then reboot(2) will cause the system to halt instead of rebooting.  This flag may be
					 set at boot time, and cannot be cleared by reboot(2).
     RB_INITNAME     0x00000010 	 This flag is obsolete.  It was previously used to cause the kernel to prompt for the name of the init(8)
					 program, but that function is now controlled by the RB_ASKNAME flag.
     RB_KDB	     0x00000040    -d	 Gives control to a kernel debugger early in the boot sequence.  See ddb(4), ``options KGDB'' in
					 options(4), and ipkdb(9).
     RB_RDONLY	     0x00000080 	 This flag is deprecated.  It previously caused the kernel to mount the root file system in read-only
					 mode, but now that is the default, and this flag has no effect.
     RB_DUMP	     0x00000100 	 Causes the kernel to dump memory to the dump device during shutdown.  See savecore(8), cpu_reboot(9), and
					 dumpsys(9).
     RB_MINIROOT     0x00000200    -m	 This flag informs the kernel that a mini-root file system is present in memory.  See md(4), and
					 mdsetimage(8).
     RB_STRING	     0x00000400 	 This flag indicates that a boot string is present.  The string may be provided by reboot(2) and will be
					 passed to the boot loader if possible.
     RB_POWERDOWN    (RB_HALT|0x800)	 This flag is used in conjunction with RB_HALT.  If this flag is set, then then system will be powered
					 down if possible.  If powerdown is not supported, then the system will halt.
     RB_USERCONF     0x00001000    -c	 This flag causes the kernel to invoke the userconf(4) device configuration manager early in the boot
					 sequence.

   Flags that affect verbosity
     The following flags affect the verbosity of messages printed by the kernel.  These flags are used by several functions described in
     kprintf(9) to control whether output is sent to the console, the system log, both, or neither.  The use of flags that increase verbosity
     simultaneously with the use of flags that decrease verbosity, is not well defined.

     AB_NORMAL	     0			 The default, if none of the other AB_* flags is set, is that ordinary kernel messages are sent both to
					 the console and to the system log.
     AB_QUIET	     0x00010000    -q	 Boot quietly.	Ordinary kernel messages are sent to the system log, but not to the console.  Messages
					 printed with aprint_naive(9) are sent to the console, but not to the system log.
     AB_VERBOSE      0x00020000    -v	 Boot verbosely.  Some messages will be printed that would otherwise not be printed.  Both ordinary kernel
					 messages, and messages printed with aprint_verbose(9), will be sent both to the console and to the system
					 log.  If this flag is not set, then messages printed with aprint_verbose(9) will be sent only to the sys-
					 tem log.
     AB_SILENT	     0x00040000    -z	 Boot silently.  Most kernel messages will be sent only to the system log, not to the console.	The
					 aprint_*() functions display a spinning symbol on the console.
     AB_DEBUG	     0x00080000    -x	 Boot with debug messages.

   Machine-dependent flags
     The following flags have machine-dependent meanings.

     RB_MD1	     0x10000000    -1	 Some ports use this flag to disable multiprocessor mode, making them use only a single CPU.  The zaurus
					 port uses this flag to enable the serial console.
     RB_MD2	     0x20000000    -2	 The i386 and amd64 ports use this flag to disable acpi(4).
     RB_MD3	     0x40000000    -3	 This flag is currently not used by any ports.
     RB_MD4	     0x80000000    -4	 This flag is currently not used by any ports.

SEE ALSO

     reboot(2), ddb(4), userconf(4), sysctl(7), boot(8), crash(8), init(8), reboot(8), savecore(8), sync(8), cpu_reboot(9), kprintf(9)

HISTORY

     The boothowto variable appeared in 4.0BSD.  The BOOT_FLAG macro appeared in NetBSD 1.6.

     The RB_DFLTROOT option is now obsolete.

BSD
								September 11, 2009							       BSD