RE: query related to semantic behaviourial comparision in C thru gcc

[lrtaylor at micron dot com]

Hi,

If you're using the same compiler text book that I used (we implemented
a Tiger compiler, too), then I pity you... :-)

I believe this is expected behavior in C.  "If" statements expect a
"boolean" expression, which is interpreted as meaning a non-zero result
is true, zero is false.  Basically, it's really an integer expression.
Symtab cannot be converted to an integer (or, if it can, the compiler
doesn't realize it) and cannot therefore be used in the "if" statement.
The pointer to Symtab, however, can be converted to an integer, and can
therefore be used in the expression.  The net result of using a pointer
is that if the statement is true, then it means the pointer (not the
location being pointed to) has a value.  Otherwise, it's NULL (zero).

Cheers,
Lyle


-----Original Message-----
From: gcc-help-owner@gcc.gnu.org [mailto:gcc-help-owner@gcc.gnu.org] On
Behalf Of Silver Chandrakant L.
Sent: Wednesday, March 10, 2004 12:50 PM
To: gcc-help@gcc.gnu.org
Subject: query related to semantic behaviourial comparision in C thru
gcc

Hi,
 I want to ask you something related to the behaviour i found in C, 
actually compiled thru GCC 3.2.2

The problem is or could be in the semantic behaviour.
            ~~                    ~~~~~~~~

Please note the following excerpt from the attached file program
symdef.c
~~~~~~~~~~~~~~~~~~~
~~~~~~~~
void printSymtab(void)
{
  Symtab *tptr;
  Symtab t;


  puts("**************\n\t\tSymTable**************");
  puts("    Name              PIB    PIH    BlkNo ");

  printf("%u", t);
  if (t) ;

  if (tptr);

  if (*tptr);

  return;
}
``````````````````````````````````````````````
Compilation
~~~~~~~~~~~
[u02139@cs123]$ gcc -Wall symdef.c
symdef.c: In function `printSymtab':
symdef.c:84: invalid operands to binary !=
symdef.c:88: invalid operands to binary !=
[u02139@cs123]$


``````````````````````````````````````````````
The error/semantic behaviour to be pointed out is, when we print the 
variable "t" it is allowed, but when comparison is done implicit or 
explicit it gives an error.


void printSymtab(void)
{
  Symtab *tptr;
  Symtab t;


  puts("**************\n\t\tSymTable**************");
  puts("    Name              PIB    PIH    BlkNo ");

  printf("%u", t);      	<-------------ALLOWED
  if (t) ;			<-------------NOT ALLOWED (line 84)

  if (tptr);			<-------------This is OKAY

  if (*tptr);			<---------NOT ALLOWED/possibly it is
error
								(line
88)	
  return;
}


Why so ? could please explain me this semantic behaviour. When allowed
in 
one case can also be implemented/used in other case. 
Actually it shd be allowed in either cases.

I'm doing CC(compiler construction) course in our department and was 
working out for Tiger language. I just started writing code for the 
semantic analysis phase, and during compilation such error was 
encountered. Actually, line 88 was done by mistake, but helped get into 
these other things. 

The files used for compilation are attached.

Pleaze help me, 
Waiting for your response.

Bye & take care.
-- 
-SilverZ.
--
Always forgive your enemies - Nothing annoys them so much.
--------------------------------------
Silver Chandrakant L
   MCA (Batch 2002)
   IInd year
   University of Pune.
   India.

chandrakant_silver@hotmail.com