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svr4 - System V Release 4 ABI support
To link System V Release 4 (SVR4) ABI support into the kernel:
To load the SVR4 ABI support kernel module:
The svr4 module provides limited System V Release 4 ABI (application
binary interface) compatibility for userland applications. The module
provides the following significant facilities:
· An image activator for correctly branded elf(5) executable images
· Special signal handling for activated images
· SVR4 to native system call translation
· STREAMS network API emulation (via the streams(4) loadable module, or
by means of
in a kernel configuration file)
· Mappings between FreeBSD and SVR4 ioctl(2) calls, or, where no such
mappings exist, reverse-engineered implementations of the SVR4 calls.
It is important to note that the SVR4 ABI support it not provided through
an emulator. Rather, a true (albeit limited) "clean room" reverse-
engineered ABI implementation is provided.
Because the provided ABI has been developed in ignorance of actual SVR4
source code, there are bound to be unforeseen interactions between SVR4
client applications and the emulated ABI which cause applications to
Additionally, some SVR4 operating systems do not adhere to the SVR4 ELF
standard. In particular, Solaris does not set the ELF interpreter field
in the ELF header to a value which would allow the kernel to correctly
identify a client executable as an SVR4 application. Thus, in certain
instances it is necessary to use the brandelf(1) utility to explicitly
brand the executable, or to set the kern.fallback_elf_brand sysctl(8)
variable to define a "default" ABI for unbranded executables. Value
ELFOSABI_SOLARIS represents Solaris; ELFOSABI_SYSV represents other
SysVR4 operating systems. See #include <sys/elf_common.h>
for ELFOSABI branding definitions, and brandelf(1) for information on
The svr4 module can be linked into the kernel statically with the
COMPAT_SVR4 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:
if ! kldstat -v | grep -E ’svr4elf’ > /dev/null; then
kldload svr4 > /dev/null 2>&1
The kernel will check for the presence of the streams(4) module, and load
it if necessary.
Note that dynamically linked SVR4 executables will require a suitable
environment in /compat/svr4.
For information on loading the svr4 kernel loadable module automatically
on system startup, see rc.conf(5). This information applies regardless
of whether the svr4 module is statically linked into the kernel or loaded
as a module.
STREAMS emulation is limited but (largely) functional. Assuming the
streams(4) module is loaded, a STREAMS handle can be obtained by opening
one of the relevant files in /dev or /compat/svr4/dev. Internally, the
streams(4) driver produces a socket descriptor and “tags” it with
additional STREAMS state information before returning it to the client
application. The svr4 environment uses the additional state information
to recognize and manipulate emulated STREAMS handles when STREAMS-
specific ioctl(2) calls are executed.
The subset of STREAMS functionality which is provided is small, probably
little more than what is required to enable programs on the Solaris CD
sets to run.
/compat/svr4 minimal SVR4 run-time environment
/sys/compat/svr4/syscalls.master mappings between SVR4 syscalls and svr4
brandelf(1), streams(4), elf(5)
System V Release 4 ABI support first appeared in FreeBSD 4.0. The ABI
was ported from an equivalent facility present in NetBSD 1.3 written by
Emulation of signal handlers is buggy.
Emulated connectionless STREAMS fail to receive data from the network in
some circumstances (but succeed in others -- probably due to particular
ways of initializing them which the streams(4) module is mishandling, and
interaction between STREAMS and poll(2)). Connection-oriented STREAMS
appear to be functional.
Ironically, this SVR4 emulator does not (yet) support SVR4 semaphores or
ports(7) to automatically create the /compat/svr4 environment do not
exist. tar(1) archives containing pre-populated trees can be obtained
Extensive testing has only really been carried out with Solaris 2.x
binaries, with anecdotal reports of limited success coming from testers
with early-revision SCO media. In theory, the basic SVR4 ABI should be
constant across the set of vendors who produce SVR4 operating systems,
but in practice that is probably not the case. If necessary, future work
can either implement additional kld(4) modules which produce
functionality which contains OS-dependent departures from the behaviour
which has been implemented in this ABI implementation. Alternatively,
sysctl(8) variables could set the “personality” the environment should
present to client applications.