How to upload 32 bit image to server-side pixmap

Well, your code works for 32 bits images if you just create a GC passing a drawable on argument which is 32 bits. XCreateGC(dpy, drawable, 0, 0), where drawable can be a pixmap with 32 bits depth. It works perfect with me.

The problem is with DefaultGC() which return a GC with bit depth of system default screen. If you look at line 53 of your gist paste you see that this is 24:

depth of root window: 24 planes

On line 63 you see that it uses 0x22 as default which is shown in more detail in line 64 to 70:

  visual:
    visual id: 0x22
    class: TrueColor
    depth: 24 planes
    available colormap entries: 256 per subfield
    red, green, blue masks: 0xff0000, 0xff00, 0xff
    significant bits in color specification: 8 bits

You could probably do this a bit nicer but as a start you can try this:

Note: This uses system visuals so most probably only support depth of 24 or 32.

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


#ifdef DEBUG
int dbg = 1;
#else
int dbg = 0;
#endif

/* Return a GC based on depth */
int gc_depth(int depth, Display *dpy, Window scr, Window root, GC *gc)
{
        Window win;
        Visual *visual;
        XVisualInfo vis_info;
        XSetWindowAttributes win_attr;
        unsigned long win_mask;

        if(!XMatchVisualInfo(dpy, scr, depth, TrueColor, &vis_info)) {
                fprintf(stderr,
                        " * ERR: %d depth not supported\n",
                        depth
                );
                return 1;
        }

        visual = vis_info.visual;

        win_attr.colormap = XCreateColormap(dpy, root, visual, AllocNone);
        win_attr.background_pixel = 0;
        win_attr.border_pixel = 0;

        win_mask = CWBackPixel | CWColormap | CWBorderPixel;

        win = XCreateWindow(
                        dpy, root,
                        0, 0,
                        100, 100,        /* dummy size */
                        0, depth,
                        InputOutput, visual,
                        win_mask, &win_attr);
        /* To flush out any errors */
        if (dbg) XSync(dpy, True);

        *gc = XCreateGC(dpy, win, 0, 0);
        if (dbg) XSync(dpy, True);

        XDestroyWindow(dpy, win);
        if (dbg) XSync(dpy, True);

        return 0;
}

int main(void)
{
        int w = 100;
        int h = 100;
        int depth = 32;
        int bitmap_pad = 32;
        int bpl = 0;

        Display *dpy;
        Window root;
        Window scr;
        GC gc;
        int root_depth;

        Pixmap pm;
        XImage *img;
        unsigned char *buf_img;

        if(!(dpy = XOpenDisplay(NULL))) {
                fprintf(stderr,
                        " * ERR: Failed to open display.\n");
                return 1;
        }

#ifdef DEBUG
        /* To get errors in order, slows down
         * One can also define int _Xdebug = 1;
         * */
        XSynchronize(dpy, True);
#endif

        root = XDefaultRootWindow(dpy);
        scr  = XDefaultScreen(dpy);

        if ((buf_img = malloc(w * h * 4)) == NULL) {
                fprintf(stderr,
                        " * ERR: Unable to alloacte %d bytes\n",
                        w * h * 4);
                return 1;
        }

        root_depth = DefaultDepth(dpy, scr);

        fprintf(stderr,
                "Default depth: %d\n",
                root_depth);

        /* This should be doen more nice */
        if (depth != root_depth) {
               if (gc_depth(depth, dpy, scr, root, &gc) != 0)
                        return 1;
        } else {
                gc = DefaultGC(dpy, 0);
        }

        img = XCreateImage(
                        dpy, CopyFromParent,
                        depth, ZPixmap,
                        0, (char *)buf_img,
                        w, h,
                        bitmap_pad, bpl);
        /* To flush out any errors */
        if (dbg) XSync(dpy, True);

        pm = XCreatePixmap(
                        dpy, root,
                        w, h,
                        depth);
        if (dbg) XSync(dpy, True);

        XPutImage(
                dpy, pm,
                gc, img,
                0, 0,
                0, 0,
                w, h);
        if (dbg) XSync(dpy, True);

        XFreePixmap(dpy, pm);
        XDestroyImage(img);
        XFreeGC(dpy, gc);
        if (dbg) XSync(dpy, True);

        fprintf(stderr,
                "OK!\n");

        return 0;
}