he library you need is called libsurfaceflinger_client.so. You can pull it from any device running Gingerbread or higher version of Android, with command

adb pull /system/lib/libsurfaceflinger_client.so

On ICS or JB, class ScreenshotClient is part of libgui.so. The makefile for screencap which is the example of using ScreenshotClient, suggests that linker may need other libraries:

libcutils libutils libbinder libskia libui libgui

Also, the code of screencap.cpp goes as follows:

ScreenshotClient screenshot;if(screenshot.update()== NO_ERROR){
     base= screenshot.getPixels();    w = screenshot.getWidth();    h = screenshot.getHeight();    f = screenshot.getFormat();    size = screenshot.getSize();}else{
     constchar* fbpath ="/dev/graphics/fb0";int fb = open(fbpath, O_RDONLY);if(fb >=0){
     struct fb_var_screeninfo vinfo;if(ioctl(fb, FBIOGET_VSCREENINFO,&vinfo)==0){
     ...

This means that at least you must check that update() succeed. Unfortunately, unless teh device is rooted, the apps cannot be granted permission to read from /dev/graphics/fb0 and use the fallback of /system/bin/screencap or /system/bin/screenshot.

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