Atom defined as c_ulong

[mickvangelderen]

I am confused with the definition of Atom (and friends) in xlib.rs. The X protocol specification says it should be a 32 bit value. That is how it is defined in Xproto.h for me:

#define Atom CARD32

On my 64 bit machine, c_ulong ends up as a u64. Can anyone shed some light on this?

[ghost]

It's because CARD32 itself is defined as unsigned long. The API has been around since long before the C standard had uint32_t, and int was only 16 bit in many implementations at the time. This crate has to do things exactly as the C libraries do for binary compatibility.

[mickvangelderen]

Ah I see.

I'm trying to figure out how those basic X types are defined now but it isn't simple. The type definitions depend mostly on LONG64 and _XSERVER64 being defined or not. Also they're spread over multiple files like X.h, Xmd.h, Xproto.h, Xdefs.h and maybe more.

In Xproto.h the defintions are said to be overwritten and restored:

/* For the purpose of the structure definitions in this file,
we must redefine the following types in terms of Xmd.h's types, which may
include bit fields.  All of these are #undef'd at the end of this file,
restoring the definitions in X.h.  */

Then there's exceptions like this in XI.h

/*
 * Make XEventClass be a CARD32 for 64 bit servers.  Don't affect client
 * definition of XEventClass since that would be a library interface change.
 * See the top of X.h for more _XSERVER64 magic.
 *
 * But, don't actually use the CARD32 type.  We can't get it defined here
 * without polluting the namespace.
 */
#ifdef _XSERVER64
typedef	unsigned int	XEventClass;
#else
typedef	unsigned long	XEventClass;
#endif

Found this comment https://gitlab.freedesktop.org/xorg/xserver/issues/553#note_94062

For the second, it appears you've missed the _XSERVER64 definition that's required when building X server code on 64-bit systems - this is normally
defined in xorg-config.h, xorg-server.h, or dix-config.h depending on which
portion of the X server source code you're working in. The difference in types between clients & servers in 64-bit builds is a very sad mistake from the distant past we're stuck with to avoid breaking compatibility.

To me it looks like X clients use 64 bit integers for some types, but the data structures communicating with the X server (Xproto.h) use the right (as per the X protocol specification) integer widths. Approximately, probably.