What? ctypes and Bash, these two together? Yes, you read it right.

Someone — namely Tavis Ormandy — is crazy enough to make a foreign function interface for Bash, behold, the ctypes.sh! It’s a Bash’s C extension with a helper Bash script that set up the environment for you to go nuts.

Don’t like Bash’s printf, want the real deal, the standard C printf(3)? You can have it:


% dlopen libc.so.6
0x7feec65e09a0
% dlcall $RTLD_DEFAULT printf $'Hello, World\n'
Hello, World

Although, I don’t see the point since Bash’s version is quite the same. Or do some mathematics?


% dlopen libm.so
0x8584b0
% dlcall -n RAND -r int32 ${DLHANDLES[libm.so]} rand
int32:719885386
% dlcall -r double ${DLHANDLES[libm.so]} sqrt ${RAND/int32/double}
double:26830.679939

When I saw this example, I was wondering how I could do basic operations with floating numbers since Bash can only do integer, it would be great if I can figure this one out.

I also thought about an actual example, and I decided to port xcinfo, a C project gathers X cursor information for use in shell scripting, to Bash, xcinfo.sh. Well just a simple script for this post, not gonna put this into that repository.


#!/bin/bash
# Written by Yu-Jie Lin @ 2015-07-28T23:48:04Z
# Public Domain

source ctypes.sh

 >/dev/null dlopen libX11.so
 >/dev/null dlopen libXfixes.so

2>/dev/null dlcall -n dpy -r pointer ${DLHANDLES[libX11.so]} XOpenDisplay 'pointer:(nil)'
2>/dev/null dlcall -n scr -r int     ${DLHANDLES[libX11.so]} XDefaultScreen $dpy
2>/dev/null dlcall -n rtw -r int     ${DLHANDLES[libX11.so]} XRootWindow    $dpy $scr
2>/dev/null dlcall -n sw  -r int     ${DLHANDLES[libX11.so]} XDisplayWidth  $dpy $scr
2>/dev/null dlcall -n sh  -r int     ${DLHANDLES[libX11.so]} XDisplayHeight $dpy $scr

declare -a ci
{
  unset n
  ci[ci_x       = n++]="int16"
  ci[ci_y       = n++]="int16"
  ci[ci_w       = n++]="uint16"
  ci[ci_h       = n++]="uint16"
  ci[ci_xh      = n++]="uint16"
  ci[ci_yh      = n++]="uint16"
}

2>/dev/null dlcall -n cibuf -r pointer $RTLD_DEFAULT malloc 1024
2>/dev/null dlcall -n cibuf -r pointer ${DLHANDLES[libXfixes.so]} XFixesGetCursorImage $dpy
unpack $cibuf ci

printf "\
Display   : %s
Screen    : %8d
RootWindow: %8d
x         : %8d
y         : %8d
Width     : %8d
Height    : %8d
w         : %8d
h         : %8d
xh        : %8d
yh        : %8d

${ci[ci_x]##*:} ${ci[ci_y]##*:} ${sw##*:} ${sh##*:} \
${ci[ci_w]##*:} ${ci[ci_h]##*:} ${ci[ci_xh]##*:} ${ci[ci_yh]##*:}\n" \
${dpy##*:} ${scr##*:} ${rtw##*:} ${ci[ci_x]##*:} ${ci[ci_y]##*:} ${sw##*:} ${sh##*:} ${ci[ci_w]##*:} ${ci[ci_h]##*:} ${ci[ci_xh]##*:} ${ci[ci_yh]##*:}

dlcall $RTLD_DEFAULT free $cibuf

An output example is:


% ./xcinfo.sh
Display   : 0x6da450
Screen    :        0
RootWindow:      645
x         :     1206
y         :      860
Width     :     1680
Height    :     1050
w         :        9
h         :       16
xh        :        4
yh        :        8

1206 860 1680 1050 9 16 4 8

ctypes.sh is written in C and Bash, under the MIT License. Be sure to read what people said about ctypes.sh in README. This is probably my favorite: “you’ve gone too far with this.” Too far? Nah, it’s because he can.