14 Dec
2005
14 Dec
'05
7:31 p.m.
It was thus said that the Great mbbrutman-cctalk at brutman.com once stated:
Quoting Sean 'Captain Napalm' Conner <spc at conman.org>:
The problem may be happening at the linking stage, not the runtime stage
(although that's there the anomalous behavior is seen). A sequence like:
printf("%d %s",23,"hello");
Produces code like:
segment data
text1 db '%d %s',0
text2 db 'hello',0
segment code
pushl offset text2
pushl 23
pushl offset text1
call printf
But the object code produces an output like:
data_blob
text1: db '%d %s',0
text2: db 'hello',0
code_blob
line1: pushl 0
line2: pushl 23
line3: pushl 0
line4: call 0
fixup_data
in code_blob @ line1 byte offset 1 change to address of
data_blob line 2
in code_blob @ line 3 byte offset 1 change to address of
data_blob line 1
in code_blob @ line4 byte offset 1 change to address of
external_code_blob printf
It may be that because the offset of text1 is at offset 0 of the data
segment (as stored in the data_blob) the linker may think it's actually NULL
and do something different. It's not a compiler error, so running the
resulting code under a debugger won't find it---it's a linking error.
-spc (has found compiler errors, but never a linker error ... )
Which isn't vanilla C (but may be vanilla C++) ...
Except for the function name, a C compiler would be pretty happy with it. The
compiler is bad, but not that bad!
Sounds like a fixup problem. If it's the
first constant string, then it
sounds like it's at offset 0 of some segment, and the linker may be thinking
it's a NULL value (easy to see how this could happen actually). And it may
be that that implementation of sprintf() checks for a NULL string, and if
so, "does the right thing" (not print anything, and return 0 characters
written).
I'm guessing this too, but I can't figure out the difference. When the extra
constant is not there a 0x0004 gets loaded into AX. When the extra constant is
there, a 0x0005 gets loaded into AX. I might be looking at the wrong register
before the call, but I never see a case of 0x0 or 0x1 being moved in - I think
the 0x0004 and 0x0005 are the correct offsets.