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:
•
The boot loader may prompt for the name or location of the kernel to be booted.
•
The kernel will prompt for the root file system device.
•
The kernel will prompt for the root file system type.
•
The kernel will prompt for the location of the dump device.
•
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:
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This flag is obsolete.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_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_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 system 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.