A minimal example for X Input Method.

xim_example.c

#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <X11/Xos.h>
#include <stdlib.h>
#include <stdio.h>
#include <locale.h>
#include <assert.h>

Display *dpy;
Window win;
int scr;

void send_spot(XIC ic, XPoint nspot) {
    XVaNestedList preedit_attr;
    preedit_attr = XVaCreateNestedList(0, XNSpotLocation, &nspot, NULL);
    XSetICValues(ic, XNPreeditAttributes, preedit_attr, NULL);
    XFree(preedit_attr);
}

int main() {
    /* fallback to LC_CTYPE in env */
    setlocale(LC_CTYPE, "");
    /* implementation-dependent behavior, on my machine it defaults to
     * XMODIFIERS in env */
    XSetLocaleModifiers("");

    /* setting up a simple window */
    dpy = XOpenDisplay(NULL);
    scr = DefaultScreen(dpy);
    win = XCreateSimpleWindow(dpy,
            XDefaultRootWindow(dpy),
            0, 0, 100, 100, 5,
            BlackPixel(dpy, scr),
            BlackPixel(dpy, scr));
    XMapWindow(dpy, win);

    /* initialize IM and IC */
    XIM xim = XOpenIM(dpy, NULL, NULL, NULL);
    XIC ic = XCreateIC(xim,
                        /* the following are in attr, val format, terminated by NULL */
                        XNInputStyle, XIMPreeditNothing | XIMStatusNothing,
                        XNClientWindow, win,
                        NULL);
    /* focus on the only IC */
    XSetICFocus(ic);
    /* capture the input */
    XSelectInput(dpy, win, KeyPressMask);
    XPoint spot;
    spot.x = 0;
    spot.y = 0;
    send_spot(ic, spot);

    static char *buff;
    size_t buff_size = 16;
    buff = (char *)malloc(buff_size);
    for (;;)
    {
        KeySym ksym;
        Status status;
        XEvent ev;
        XNextEvent(dpy, &ev);
        if (XFilterEvent(&ev, None))
            continue;
        if (ev.type == KeyPress)
        {
            size_t c = Xutf8LookupString(ic, &ev.xkey,
                                        buff, buff_size - 1,
                                        &ksym, &status);
            if (status == XBufferOverflow)
            {
                printf("reallocate to the size of: %lu\n", c + 1);
                buff = realloc(buff, c + 1);
                c = Xutf8LookupString(ic, &ev.xkey,
                                    buff, c,
                                    &ksym, &status);
            }
            if (c)
            {
                spot.x += 20;
                spot.y += 20;
                send_spot(ic, spot);
                buff[c] = 0;
                printf("delievered string: %s\n", buff);
            }
        }
    }
}