MIT-SHM example https://www.x.org/releases/X11R7.7/doc/xextproto/shm.html

mitshm.c

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

#include <X11/Xlib.h>              /* of course */
#include <X11/Xutil.h>             /* duuuh     */
#include <sys/ipc.h>
#include <sys/shm.h>
#include <X11/extensions/XShm.h>   /* obviously */

void save_image_as_ppm(XImage *image, const char *filepath)
{
    FILE *f = fopen(filepath, "wb");
    fprintf(f, "P6\n");
    fprintf(f, "%d %d\n", image->width, image->height);
    fprintf(f, "255\n");
    for (int i = 0; i < image->width * image->height; ++i) {
        putc(image->data[i * 4 + 2], f);
        putc(image->data[i * 4 + 1], f);
        putc(image->data[i * 4 + 0], f);
    }
    fclose(f);
}

int main(int argc, char *argv[])
{
    Display *display = XOpenDisplay(NULL);
    if (display == NULL) {
        fprintf(stderr, "Could not open Display\n");
        abort();
    }

    int major = 0, minor = 0;
    Bool pixmaps = 0;
    XShmQueryVersion(display, &major, &minor, &pixmaps);
    printf("SHM Version %d.%d, Pixmaps supported: %s\n",
           major, minor,
           pixmaps ? "yesu" : "nah");


    XVisualInfo vinfo;
    XMatchVisualInfo(
        display,
        XDefaultScreen(display),
        24,
        TrueColor,
        &vinfo);

    XWindowAttributes attributes;
    XGetWindowAttributes(
        display,
        DefaultRootWindow(display),
        &attributes);

    XShmSegmentInfo shminfo;
    XImage *image = XShmCreateImage(
        display, vinfo.visual, 24, ZPixmap, NULL,
        &shminfo,
        attributes.width,
        attributes.height);

    shminfo.shmid = shmget(
        IPC_PRIVATE,
        image->bytes_per_line * image->height,
        IPC_CREAT | 0777);

    shminfo.shmaddr = image->data = shmat(shminfo.shmid, 0, 0);
    shminfo.readOnly = False;

    Status err = XShmAttach(display, &shminfo);
    assert(err != 0);
    XSync(display, False);

    XShmGetImage(display, DefaultRootWindow(display), image, 0, 0, AllPlanes);
    XSync(display, False);

    save_image_as_ppm(image, "screenshot.ppm");

    XShmDetach(display, &shminfo);
    XDestroyImage(image);
    shmdt(shminfo.shmaddr);
    shmctl(shminfo.shmid, IPC_RMID, 0);
    XCloseDisplay(display);

    return 0;
}