x11 - XShmGetImage fails without any error displayed -


i have tried below simple program use xshmgetimage desktop image. 

#include <x11/xlib.h> #include <x11/xatom.h> #include <x11/xutil.h> #include <x11/extensions/xshm.h> #include <x11/extensions/xfixes.h> #include <stdio.h> #include <stdlib.h> #include <sys/ipc.h> #include <sys/shm.h>  int main(int argc, const char *argv[]) {     int screen;     window root;     display* display;     ximage* img,      int shm=0;     xshmsegmentinfo shminfo;      /* desktop screen resolution */      int width=1360;     int height=768;      display = xopendisplay(getenv("display"));      shm = xshmqueryextension(display);      if ( shm) {         printf ("ha... queryextension successful..\n");         int scr = xdefaultscreen (display);         printf ("\n screen number %d ", scr);         img = xshmcreateimage (display, defaultvisual(display, scr),                             defaultdepth ( display, scr),                         zpixmap,                         null,                        &shminfo,                         width,                         height);         printf ("\n bytes per line %d ", img->bytes_per_line);          shminfo.shmid = shmget (ipc_private, img->bytes_per_line * img->height, ipc_creat | 0777);          if ( shminfo.shmid == -1 ) {             printf ("\n can not shared memory ...");         } else {             printf ("\n greate able shared memory..");         }         shminfo.shmaddr = img->data =shmat (shminfo.shmid, 0,0);        shminfo.readonly = false;         if (!xshmattach (display, &shminfo)) {            printf ("\n unable attach now..");        } else {            printf ("\n super.. able attach shared memory extension ");        }          if ( !xshmgetimage (display, rootwindow(display, defaultscreen(display)), img, 0,0, allplanes)){            printf ("\n should have image in ximage");        } else {            printf ("\n ooops.. wrong.");         }     }

output:

 ha... queryextension successful..  screen number 0   bytes per line 5440   greate able shared memory..  super.. able attach shared memory extension   ooops.. wrong.

unfortunately, xshmgetimage fails, , no information displayed. please help.

there blunder side. actually, works properly, , misinterpreted return value of xshmgetimage api().

the correct 1 is

if ( !xshmgetimage (display, rootwindow(display, defaultscreen(display)), img, 0,0, allplanes)){            printf ("\n ooops.. wrong.");         } else {            printf ("\n should have image in ximage");    }

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