Q: My v2 program crashes, but only under CWSDPMI; it runs OK under
other DPMI hosts like Windows, OS/2 or QDPMI. Is this a bug in CWSDPMI?
A: No, it probably is a bug in your program which just goes
unnoticed on Windows. Unlike other DPMI hosts, CWSDPMI supports some
DPMI 1.0 extensions which allow DJGPP to capture and disallow
dereference of pointers which point to addresses less than 1000h
(a.k.a. NULL pointer protection). The tell-tale sign of these
problems is a message "Page fault at ..." that is printed when a
program crashes, and an error code of 4 or 6. The NULL pointer
protection feature can be disabled by setting the
_CRT0_FLAG_NULLOK bit in _crt0_startup_flags and
recompiling the program; if this makes SIGSEGV crashes go
away, your program is using such invalid pointers; the stack trace
printed when the program crashes should be a starting point to debug
this. See how to debug SIGSEGV, for more details about debugging these
problems.
To make spotting uninitialized memory simpler, you can set
_crt0_startup_flags to _CRT0_FLAG_FILL_DEADBEEF (don't
laugh!); this will cause the sbrk()'ed memory to be filled with the
value 0xdeadbeef (-559038737 in signed decimal or
3735928559 in unsigned decimal) which should be easy to spot with
a debugger. Any pointer variable which has this value was used without
initializing it first.
An insufficient stack size can also be a cause of your program's demise, see setting the stack size, below.