.\" Copyright (c) 2000 Sheldon Hearn
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.Dd February 8, 2010
.Dt LINUX 4
.Os
.Sh NAME
.Nm linux
.Nd Linux ABI support
.Sh SYNOPSIS
To compile support for this ABI into an i386 kernel
place the following line in your
kernel configuration file:
.Bd -ragged -offset indent
.Cd "options COMPAT_LINUX"
.Ed
.Pp
for an amd64 kernel use:
.Bd -ragged -offset indent
.Cd "options COMPAT_LINUX32"
.Ed
.Pp
Alternatively, to load the ABI as a
module at boot time, place the following line in
.Xr loader.conf 5 :
.Bd -literal -offset indent
linux_load="YES"
.Ed
.Sh DESCRIPTION
The
.Nm
module provides limited
Linux ABI (application binary interface) compatibility
for userland applications.
The module provides the following significant facilities:
.Bl -bullet
.It
An image activator
for correctly branded
.Xr elf 5
executable images
.It
Special signal handling for activated images
.It
Linux to native system call translation
.El
.Pp
It is important to note that the Linux ABI support
it not provided through an emulator.
Rather, a true (albeit limited) ABI implementation is provided.
.Pp
The following
.Xr sysctl 8
tunable variables are available:
.Bl -tag -width compat.linux.oss_version
.It compat.linux.osname
Linux kernel operating system name.
.It compat.linux.osrelease
Linux kernel operating system release.
Changing this to something else is discouraged on non-development systems,
because it may change the way Linux programs work.
Recent versions of GNU libc are known to use different syscalls depending
on the value of this sysctl.
.It compat.linux.oss_version
Linux Open Sound System version.
.El
.Pp
The
.Nm
module can be linked into the kernel statically with the
.Dv COMPAT_LINUX
kernel configuration option
or loaded as required.
The following command will load the module
if it is neither linked into the kernel
nor already loaded as a module:
.Bd -literal -offset indent
if ! kldstat -v | grep -E 'linux(aout|elf)' > /dev/null; then
	kldload linux > /dev/null 2>&1
fi
.Ed
.Pp
Note that dynamically linked Linux executables
will require a suitable environment in
.Pa /compat/linux .
Specifically, the Linux run-time linker's hints files
should be correctly initialized.
For this reason, it is common to execute the following commands
to prepare the system to correctly run Linux executables:
.Bd -literal -offset indent
if [ -x /compat/linux/sbin/ldconfig ]; then
	/compat/linux/sbin/ldconfig -r /compat/linux
fi
.Ed
.Pp
For information on loading the
.Nm
kernel loadable module automatically on system startup,
see
.Xr rc.conf 5 .
This information applies
regardless of whether the
.Nm
module is statically linked into the kernel
or loaded as a module.
.Sh FILES
.Bl -tag -width /compat/linux/proc -compact
.It Pa /compat/linux
minimal Linux run-time environment
.It Pa /compat/linux/proc
limited Linux process file system
.It Pa /compat/linux/sys
limited Linux system file system
.El
.Sh SEE ALSO
.Xr brandelf 1 ,
.Xr elf 5 ,
.Xr linprocfs 5 ,
.Xr linsysfs 5
.Sh HISTORY
Linux ABI support first appeared in
.Fx 2.1 .