When I try to run xmodmap I get an error:
$ xmodmap
Assertion failed: dpy->xcb->reply_data != NULL, file ../../libX11-1.2.99.901/src/xcb_io.c, line 542, function _XRead
Abort(coredump)

I run xmodmap in mdb and got:
$ mdb /usr/X11R7/bin/xmodmap
> :r
mdb: warning: librtld_db failed to initialize; shared library information will not be available
Assertion failed: dpy->xcb->reply_data != NULL, file ../../libX11-1.2.99.901/src/xcb_io.c, line 542
mdb: stop on SIGABRT
mdb: target stopped at:
libc_hwcap1.so.1`_lwp_kill+0x15:jae    +0xc     <libc_hwcap1.so.1`_lwp_kill+0x23>
> ::stack
libc_hwcap1.so.1`_lwp_kill+0x15(1, 6, 8047778, ce7fabde)
libc_hwcap1.so.1`raise+0x22(6, 0, 80477c8, ce7d205a)
libc_hwcap1.so.1`abort+0xf2(65737341, 6f697472, 6166206e, 64656c69, 7064203a, 783e2d79)
libc_hwcap1.so.1`_assert+0x82(ce976312, ce97620c, 21e, ce9dd70c, 0, 804e950)
libX11.so.6.3.0`_XRead+0x120(804e950, 805b1f8, 5c, ce9dd70c, 804e950, 5c)
libX11.so.6.3.0`_XReadPad+0x38(804e950, 805b1f8, 5c, 0, ce700878, 8047ab0)
libX11.so.6.3.0`XGetModifierMapping+0xb1(804e950, ce9f03c8, 8047ad8, 804bfc2, 804e950, 804e940)
initialize_map+0x13(804e950, 804e940, 804e938, cea287a4, ce9f03c8, ce777e71)
main+0xc2(8049170, 1, 8047b00)
_start+0x80(1, 8047bfc, 0, 8047c04, 8047c15, 8047c28)
>


Inside xmodmap (file xmodmap.c in xmodmap.git)
There are three calls to X function before error occurs
1 file xmodmap.c: dpy = XOpenDisplay (displayname);
  after call dpy->xcb->reply_data == 0
2 file xmodmap.c: XDisplayKeycodes (dpy, &min_keycode, &max_keycode);
  after call dpy->xcb->reply_data == 0
3 file handle.c: map = XGetModifierMapping (dpy); <- here I get the error


Inside function XGetModifierMapping (file ModMap.c in libX11)
there is a call
(void) _XReply (dpy, (xReply *)&rep, 0, xFalse);
which returns dpy->xcb->reply_data == 0

Inside function _XReply (file xcb_io.c in libX11)
reply = xcb_wait_for_reply(c, current->sequence, &error);
This call returns 0 when error occurs

Before error there are three successful xcb_wait_for_reply calls.

I added fprintf in files ModMap.c and xcb_io.c
(like this - fprintf (stderr, "Before xcb_wait_for_reply: reply: %p; c: %p; current->sequence: %i; error: %p\n", reply, c, current->sequence, error);)
and got:

$ xmodmap
Before xcb_wait_for_reply: reply: 0; c: 804ff28; current->sequence: 4; error: 80479f0
After xcb_wait_for_reply: reply: 805a738; c: 804ff28; current->sequence: 4; error: 0

Before xcb_wait_for_reply: reply: 0; c: 804ff28; current->sequence: 5; error: 18c1f2e
After xcb_wait_for_reply: reply: 805a738; c: 804ff28; current->sequence: 5; error: 0

Before xcb_wait_for_reply: reply: 0; c: 804ff28; current->sequence: 6; error: 98
After xcb_wait_for_reply: reply: 805a738; c: 804ff28; current->sequence: 6; error: 0

Before _XReply: dpy->xcb->reply_data 0
Before xcb_wait_for_reply: reply: 0; c: 804ff28; current->sequence: 7; error: 5a70837
After xcb_wait_for_reply: reply: 0; c: 804ff28; current->sequence: 7; error: 0
After _XReply: dpy->xcb->reply_data 0
Before _XReadPad: dpy->xcb->reply_data 0
Assertion failed: dpy->xcb->reply_data != NULL, file ../../libX11-1.2.99.901/src/xcb_io.c, line 547, function _XRead
Abort(coredump)



libX11:
commit 19cc5e1fa17a285045662820a8b4de2a0f9a194d
Date:   Fri Sep 18 17:10:04 2009 -0700

xmodmap:
commit 4da097462b0b899e1790fd5a5f5e8ec811623c3d
Date:   Mon May 11 10:16:29 2009 -0700