Efficient Screenshotting under X11 in Linux

/*
    This has been posted in reference to the "Real-Time Neural Style Transfer in Quake 3" article posted on;
    https://james-william-fletcher.medium.com/

    This code is an adaption of;
    https://stackoverflow.com/questions/34176795/any-efficient-way-of-converting-ximage-data-to-pixel-map-e-g-array-of-rgb-quad/38298349#38298349

    Compile using:
    gcc screenshot.c -o screenshot -std=c99 -lX11 -lXext -lpng -Ofast
*/

#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include <stdbool.h>
#include <png.h>
#include <sys/shm.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <X11/extensions/XShm.h>

#define NAME  "screenshot"
#define BPP   4
#define uint unsigned int

struct shmimage
{
    XShmSegmentInfo shminfo;
    XImage *ximage;
    unsigned char *data; // will point to the image's BGRA packed pixels
};

void initimage( struct shmimage * image )
{
    image->ximage = NULL;
    image->shminfo.shmaddr = (char*)-1;
}

void destroyimage( Display * dsp, struct shmimage * image )
{
    if( image->ximage )
    {
        XShmDetach( dsp, &image->shminfo ) ;
        XDestroyImage( image->ximage ) ;
        image->ximage = NULL ;
    }

    if( image->shminfo.shmaddr != ( char * ) -1 )
    {
        shmdt( image->shminfo.shmaddr ) ;
        image->shminfo.shmaddr = ( char * ) -1 ;
    }
}

int createimage( Display * dsp, struct shmimage * image, int width, int height )
{
    // Create a shared memory area
    image->shminfo.shmid = shmget( IPC_PRIVATE, width * height * BPP, IPC_CREAT | 0600 ) ;
    if( image->shminfo.shmid == -1 )
    {
        perror( NAME ) ;
        return false ;
    }

    // Map the shared memory segment into the address space of this process
    image->shminfo.shmaddr = (char *) shmat( image->shminfo.shmid, 0, 0 ) ;
    if( image->shminfo.shmaddr == (char *) -1 )
    {
        perror( NAME ) ;
        return false ;
    }

    image->data = (unsigned char*) image->shminfo.shmaddr ;
    image->shminfo.readOnly = false ;

    // Mark the shared memory segment for removal
    // It will be removed even if this program crashes
    shmctl( image->shminfo.shmid, IPC_RMID, 0 ) ;

    // Allocate the memory needed for the XImage structure
    image->ximage = XShmCreateImage( dsp, XDefaultVisual( dsp, XDefaultScreen( dsp ) ),
                        DefaultDepth( dsp, XDefaultScreen( dsp ) ), ZPixmap, 0,
                        &image->shminfo, 0, 0 ) ;
    if( !image->ximage )
    {
        destroyimage( dsp, image ) ;
        printf( NAME ": could not allocate the XImage structure\n" ) ;
        return false ;
    }

    image->ximage->data = (char *)image->data ;
    image->ximage->width = width ;
    image->ximage->height = height ;

    // Ask the X server to attach the shared memory segment and sync
    XShmAttach( dsp, &image->shminfo ) ;
    XSync( dsp, false ) ;
    return true ;
}

void getrootwindow( Display * dsp, struct shmimage * image )
{
    XShmGetImage( dsp, XDefaultRootWindow( dsp ), image->ximage, 0, 0, AllPlanes ) ;
    // This is how you access the image's BGRA packed pixels
    // Lets set the alpha channel of each pixel to 0xff
    // int x, y ;
    // unsigned int * p = image->data ;
    // for( y = 0 ; y < image->ximage->height; ++y )
    // {
    //     for( x = 0 ; x < image->ximage->width; ++x )
    //     {
    //         *p++ |= 0xff000000 ;
    //     }
    // }
}

void initpngimage( png_image * pi, struct shmimage * image )
{
    bzero( pi, sizeof( png_image ) ) ;
    pi->version = PNG_IMAGE_VERSION ;
    pi->width = image->ximage->width ;
    pi->height = image->ximage->height ;
    pi->format = PNG_FORMAT_BGRA ;
}

int savepng( struct shmimage * image, char * path )
{
    FILE * f = fopen( path, "w" ) ;
    if( !f )
    {
        perror( NAME ) ;
        return false ;
    }
    png_image pi ;
    initpngimage( &pi, image ) ;
    unsigned int scanline = pi.width * BPP ;
    if( !png_image_write_to_stdio( &pi, f, 0, image->data, scanline, NULL) )
    {
        fclose( f ) ;
        printf( NAME ": could not save the png image\n" ) ;
        return false ;
    }
    fclose( f ) ;
    return true ;
}

int main( int argc, char * argv[] )
{
    Display * dsp = XOpenDisplay( NULL ) ;
    if( !dsp )
    {
        printf( NAME ": could not open a connection to the X server\n" ) ;
        return 1 ;
    }

    if( !XShmQueryExtension( dsp ) )
    {
        XCloseDisplay( dsp ) ;
        printf( NAME ": the X server does not support the XSHM extension\n" ) ;
        return 1 ;
    }

    int screen = XDefaultScreen( dsp ) ;
    struct shmimage image ;
    initimage( &image ) ;
    if( !createimage( dsp, &image, XDisplayWidth( dsp, screen ), XDisplayHeight( dsp, screen ) ) )
    {
        XCloseDisplay( dsp ) ;
        return 1 ;
    }

    const int ms = 8;
    int c = 0;
    time_t st = time(0);
    while(time(0)-st <= ms)
    {
        //char name[32];
        //sprintf(name, "%i.png", rand());
        getrootwindow( dsp, &image ) ;
        //savepng( &image, name );
        c++;
    }
    printf("Screenshots Per Second: %i\n", c/ms);

    destroyimage( dsp, &image ) ;
    XCloseDisplay( dsp ) ;
    return 0 ;
}