Package: libsdl1.2-dev Version: 1.2.13-1 Severity: important Justification: Violation of SYS V ABI specification
Hello,
Christoph Mallon, the driving coder behind ja2 stracciatella [0], has discovered
a serious problem. libsdl is violating the SYS V ABI (x86) [1]. There it is
stated that the "direction flag must be set to the "forward" (that is, zero)
direction before entry and upon exit." [2]. But as the program in the attachment
demonstrates ([3]) libsdl doesn't do that always.
If you use SDL_BlitSurface(), the direction flag is set after exiting the
function. This problem occurs because SDL_BlitSurface() calls SDL_revcpy(), the
sequence is:
SDL_BlitSurface() (#define SDL_Blit_Surface SDL_BlitUpper)
SDL_BlitUpper()
SDL_BlitLower()
src->map->sw_blit() (function pointer to SDL_SoftBlit())
SDL_SoftBlit()
src->map->sw_data->blit() (function pointer to SDL_BlitCopyOverlap())
SDL_BlitCopyOverlap()
SDL_revcpy()
SDL_revcpy() sets but doesn't clear the direction flag. The incriminating code
is to be found in SDL_stdinc.h:
> #if defined(__GNUC__) && defined(i386)
> #define SDL_revcpy(dst, src, len) \
> do { \
> int u0, u1, u2; \
> char *dstp = (char *)(dst); \
> char *srcp = (char *)(src); \
> int n = (len); \
> if ( n >= 4 ) { \
> __asm__ __volatile__ ( \
> "std\n\t" \
> "rep ; movsl\n\t" \
> : "=&c" (u0), "=&D" (u1), "=&S" (u2) \
> : "0" (n >> 2), \
> "1" (dstp+(n-4)), "2" (srcp+(n-4)) \
> : "memory" ); \
> } \
> switch (n & 3) { \
> case 3: dstp[2] = srcp[2]; \
> case 2: dstp[1] = srcp[1]; \
> case 1: dstp[0] = srcp[0]; \
> break; \
> default: \
> break; \
> } \
> } while(0)
> #endif
As a sidenote: in the GCC 4.3 changelog it is stated that: "It is now invalid to
set the flag in asm statement without reseting it afterward.".
Kind regards,
Kai
P.S.: Please CC Christoph always!
[0] http://ja2.dragonriders.de/
[1] http://www.sco.com/developers/devspecs/abi386-4.pdf
[2] [1], page 38, EFLAGS
[3] Used build command: cc -O `sdl-config --cflags --libs` sdl_bug.c
-- System Information:
Debian Release: lenny/sid
APT prefers testing
APT policy: (500, 'testing')
Architecture: i386 (i686)
Kernel: Linux 2.6.22.10-cabal
Locale: LANG=de_DE.UTF-8, LC_CTYPE=de_DE.UTF-8 (charmap=UTF-8) (ignored: LC_ALL
set to de_DE.UTF-8)
Shell: /bin/sh linked to /bin/bash
--
Kai Wasserbäch (Kai Wasserbaech)
E-Mail: [EMAIL PROTECTED]
Jabber (debianforum.de): Drizzt
URL: http://wiki.debianforum.de/Drizzt_Do%27Urden
GnuPG: 0xE1DE59D2 0600 96CE F3C8 E733 E5B6 1587 A309 D76C E1DE 59D2
(http://pgpkeys.pca.dfn.de/pks/lookup?search=0xE1DE59D2&fingerprint=on&hash=on&op=vindex)
#include <SDL.h>
#include <stdio.h>
#include <stdlib.h>
static inline int TestDirectionFlag(void)
{
unsigned eflags;
__asm__ __volatile__("pushf\n\tpop %0" : "=r" (eflags));
return (eflags & 0x400) != 0;
}
int main(void)
{
SDL_Init(SDL_INIT_VIDEO);
SDL_Surface* const s = SDL_SetVideoMode(640, 480, 16, SDL_SWSURFACE);
SDL_Rect src_rect = { 0, 0, 630, 470 };
SDL_Rect dst_rect = { 10, 10, 630, 470 };
if (TestDirectionFlag())
{
fputs("direction flag set BEFORE SDL_BlitSurface()\n", stderr);
abort();
}
SDL_BlitSurface(s, &src_rect, s, &dst_rect);
if (TestDirectionFlag())
{
fputs("direction flag set AFTER SDL_BlitSurface()\n", stderr);
abort();
}
SDL_Quit();
return 0;
}
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