enosys(1) — Linux manual page

ENOSYS(1)                     User Commands                     ENOSYS(1)

NAME         top

       enosys - utility to make syscalls fail with ENOSYS

SYNOPSIS         top

       enosys [--syscall|-s syscall] command

DESCRIPTION         top

       enosys is a simple command to execute a child process for which
       certain syscalls fail with errno ENOSYS.

       It can be used to test the behavior of applications in the face of
       missing syscalls as would happen when running on old kernels.

OPTIONS         top

       -s, --syscall
           Syscall to block with ENOSYS. Can be specified multiple times.
           An alternative error number or name can be specified with a
           colon.

       -i, --ioctl
           Ioctl to block with ENOTTY. Can be specified multiple times.
           An alternative error number or name can be specified with a
           colon.

       -l, --list
           List syscalls known to enosys.

       -m, --list-ioctl
           List ioctls known to enosys.

       -d, --dump[=file]
           Dump seccomp bytecode filter to standard output.

           The dump can for example be used by setpriv --seccomp-filter.

       -h, --help
           Display help text and exit.

       -V, --version
           Display version and exit.

EXAMPLES         top

           # fail syscall "fallocate" with ENOSYS
           enosys -s fallocate ...

           # fail syscall "fallocate" with ENOMEM
           enosys -s fallocate:ENOMEM ...

           # fail syscall "fallocate" with value 12/ENOMEM
           enosys -s fallocate:12 ...

           # fail ioctl FIOCLEX with ENOTTY
           enosys -i FIOCLEX ...

           # fail ioctl FIOCLEX with ENOMEM
           enosys -i FIOCLEX:ENOMEM ...

           # fail ioctl FIOCLEX with value 12/ENOMEM
           enosys -i FIOCLEX:12 ...

EXIT STATUS         top

       enosys exits with the status code of the executed process. The
       following values have special meanings:

       1
           internal error

       2
           system does not provide the necessary functionality

AUTHORS         top

       Thomas Weißschuh <[email protected]>

SEE ALSO         top

       syscall(2)

REPORTING BUGS         top

       For bug reports, use the issue tracker
       <https://github.com/util-linux/util-linux/issues>.

AVAILABILITY         top

       The enosys command is part of the util-linux package which can be
       downloaded from Linux Kernel Archive
       <https://www.kernel.org/pub/linux/utils/util-linux/>. This page is
       part of the util-linux (a random collection of Linux utilities)
       project. Information about the project can be found at 
       ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have a
       bug report for this manual page, send it to
       [email protected]. This page was obtained from the
       project's upstream Git repository
       ⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
       2025-08-11. (At that time, the date of the most recent commit that
       was found in the repository was 2025-08-05.) If you discover any
       rendering problems in this HTML version of the page, or you
       believe there is a better or more up-to-date source for the page,
       or you have corrections or improvements to the information in this
       COLOPHON (which is not part of the original manual page), send a
       mail to [email protected]

util-linux 2.42-start-521-ec46  2025-08-09                      ENOSYS(1)

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