How to use shm pixmap with xcb?

Question

I try to learn how to use shared memory pixmaps in the xcb library. Did any of you have experience with this and want to share example codes and/or information? This would be very helpful.

Thanks

Answer 1

After some research I found out how to use shared memory pixmaps in xcb.

Here is my testcode:

#include <stdlib.h>
#include <stdio.h>

#include <sys/ipc.h>
#include <sys/shm.h>

#include <xcb/xcb.h>
#include <xcb/shm.h>
#include <xcb/xcb_image.h>


#define WID 512
#define HEI 512



int main(){
    xcb_connection_t*       connection;
    xcb_window_t            window;
    xcb_screen_t*           screen;
    xcb_gcontext_t          gcontext;
    xcb_generic_event_t*    event;

    uint32_t value_mask;
    uint32_t value_list[2];

    //connect to the X server and get screen

    connection = xcb_connect(NULL, NULL);
    screen = xcb_setup_roots_iterator(xcb_get_setup(connection)).data;

    //create a window

    value_mask = XCB_CW_BACK_PIXEL | XCB_CW_EVENT_MASK;
    value_list[0] = screen->black_pixel;
    value_list[1] = XCB_EVENT_MASK_EXPOSURE;

    window = xcb_generate_id(connection);

    xcb_create_window(
        connection,            
        screen->root_depth,  
        window,    
        screen->root,    
        0, 0,         
        WID, HEI,    
        0,              
        XCB_WINDOW_CLASS_INPUT_OUTPUT,   
        screen->root_visual,    
        value_mask, value_list  
    );

    //create a graphic context

    value_mask = XCB_GC_FOREGROUND | XCB_GC_GRAPHICS_EXPOSURES;
    value_list[0] = screen->black_pixel;
    value_list[1] = 0;

    gcontext = xcb_generate_id(connection);
    xcb_create_gc(connection, gcontext, window, value_mask, value_list);

    //map the window onto the screen

    xcb_map_window(connection, window);
    xcb_flush(connection);


    //Shm test
    xcb_shm_query_version_reply_t*  reply;
    xcb_shm_segment_info_t          info;

    reply = xcb_shm_query_version_reply(
        connection,
        xcb_shm_query_version(connection),
        NULL
    );

    if(!reply || !reply->shared_pixmaps){
        printf("Shm error...\n");
        exit(0);
    }

    info.shmid   = shmget(IPC_PRIVATE, WID*HEI*4, IPC_CREAT | 0777);
    info.shmaddr = shmat(info.shmid, 0, 0);

    info.shmseg = xcb_generate_id(connection);
    xcb_shm_attach(connection, info.shmseg, info.shmid, 0);
    shmctl(info.shmid, IPC_RMID, 0);

    uint32_t* data = info.shmaddr;

    xcb_pixmap_t pix = xcb_generate_id(connection);
    xcb_shm_create_pixmap(
        connection,
        pix,
        window,
        WID, HEI,
        screen->root_depth,
        info.shmseg,
        0
    );

    int i = 0;
    while(1){
        usleep(10000);

        data[i] = 0xFFFFFF;
        i++;

        xcb_copy_area(
            connection,
            pix,
            window,
            gcontext,
            0, 0, 0, 0,
            WID, HEI
        );

        xcb_flush(connection);
    }

    xcb_shm_detach(connection, info.shmseg);
    shmdt(info.shmaddr);

    xcb_free_pixmap(connection, pix);

    xcb_destroy_window(connection, window);
    xcb_disconnect(connection);

    return 0;
}