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From: aiv@euraiv1.UUCP (Eelco van Asperen)
Newsgroups: comp.sys.ibm.pc
Subject: Turbo C bug
Message-ID: <582@euraiv1.UUCP>
Date: Mon, 3-Aug-87 09:21:41 EDT
Article-I.D.: euraiv1.582
Posted: Mon Aug  3 09:21:41 1987
Date-Received: Tue, 4-Aug-87 02:08:33 EDT
Organization: EUR/EF/AIV,PObox 1738,Rotterdam,Neth.
Lines: 94
Keywords: Turbo C;bugs

I think I've found 2 more bugs in Turbo C; the first one is about 
the use of hexadecimal character constants. According to the manual
(p131, User's Guide): "..., modern C allows you to declare character
constants in hex notation. The general format is '\xDD', where DD 
represents one or two hexadecimal digits (0..9, A..F). These escape
sequences can be directly assigned to char variables, or they can be
embedded in strings".
So, the following test program should run without problems;

main()
{
	char wrong[] = "test:\x40abcd";
	char right[] = "test:\x040abcd";

	printf("wrong: '%s'\n\nright: '%s'\n",wrong,right);
}

Alas, the wrong-string is printed as 'test:\nbcd' so that 'bcd' is 
printed on the next line. Apparently a 3 digit hex-number is expected
of which the first digit is not used hence the '\n' (=0x0A).

The second bug is about interrupt-procedures; Turbo C enables you to
prefix a procedure with the interrupt-keyword and the result will be
that all registers are saved upon entry and restored at exit.
Now, if you compile such a procedure with 'Test Stack Overflow On',
then Turbo C will happily add a check for stack-overflow just after
saving the registers; naturally, this will usually crash your program
and most likely your PC as well. 

Example C-program:

	void interrupt handler(void)
	{
		/* nop */
	}

	main()
	{
		printf("demo demo demo\n");
	}

Assembler code produced by 'tcc -c -S -N bug.c':

	; Line 3
	_handler	proc	far
		push	ax
		push	bx
		push	cx
		push	dx
		push	es
		push	ds
		push	si
		push	di
		push	bp
		mov	bp,dgroup
		mov	ds,bp
		cmp	word ptr dgroup:___brklvl,sp	;!! ERROR !!
		jb	@2
		call	far ptr overflow@
	@2:
	; Line 4
	; Line 5
	@1:
		pop	bp
		pop	di
		pop	si
		pop	ds
		pop	es
		pop	dx
		pop	cx
		pop	bx
		pop	ax
		iret	
	_handler	endp


Of course, you can avoid this by putting the interrupt-handler in
a separate file that you compile without the check; however, this 
approach will not work with the integrated environment (tc) I like 
to use for development. 

I'ld like to emphasize that I like Turbo C a *lot* and that any
product that sells like Turbo C (>100K copies), is bound to 
generate a lot more bug reports that other, less successfull,
ones.

Eelco van Asperen.

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