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download PDF document: aout.4.pdf

AOUT(4) FreeBSD Kernel Interfaces Manual AOUT(4)
NAME aout - kernel support for executing binary files in legacy a.out format
SYNOPSIS kldload a.out
DESCRIPTION The a.out(5) executable format was used before the release of FreeBSD 3.0. Since i386 was the only supported architecture at that time, a.out(5) executables can only be activated on platforms that support execution of i386 code, such as i386 and amd64.
To add kernel support for old syscalls and old syscall invocation methods, place the following options in the kernel configuration file:
options COMPAT_43 options COMPAT_FREEBSD32
The COMPAT_FREEBSD32 option is only required on 64-bit CPU architectures.
The aout.ko module needs to be loaded with the kldload(8) utility in order to support the a.out(5) image activator:
kldload aout
Alternatively, to load the module at boot time, place the following line in loader.conf(5):
aout_load="YES"
The a.out(5) format was mainstream quite a long time ago. Reasonable default settings and security requirements of modern operating systems today contradict the default environment of that time and require adjustments of the system to mimic natural environment for old binaries.
The following sysctl(8) tunables are useful for this:
security.bsd.map_at_zero Set to 1 to allow mapping of process pages at address 0. Some very old ZMAGIC executable images require text mapping at address 0.
kern.pid_max Old versions of FreeBSD used signed 16-bit type for pid_t. Current kernels use 32-bit type for pid_t, and allow process id's up to 99999. Such values cannot be represented by old pid_t, mostly causing issues for processes using wait(2) syscalls, for example shells. Set the sysctl to 30000 to work around the problem.
kern.elf32.read_exec Set to 1 to force any accessible memory mapping performed by 32-bit process to allow execution, see mmap(2). Old i386 CPUs did not have a bit in PTE which disallowed execution from the page, so
SEE ALSO execve(2), a.out(5), elf(5), sysctl(8)
HISTORY The a.out(5) executable format was used on ancient AT&T UNIX and served as the main executable format for FreeBSD from the beginning up to FreeBSD 2.2.9. In FreeBSD 3.0 it was superseded by elf(5).
AUTHORS The aout manual page was written by Konstantin Belousov <kib@FreeBSD.org>.
BUGS On 64bit architectures, not all wrappers for older syscalls are implemented.
FreeBSD 14.0-RELEASE-p6 August 14, 2012 FreeBSD 14.0-RELEASE-p6