mod.std.c Digest V6#11

[Orlando Sotomayor-Diaz]

From: Orlando Sotomayor-Diaz (The Moderator) <cbosgd!std-c>


mod.std.c Digest            Wed, 22 May 85       Volume 6 : Issue  11 

Today's Topics:
                            concatenation
                    Pointer math and ints (2 msgs)
                 signed and unsigned char, same size?
----------------------------------------------------------------------

Date: Tue, 21 May 85 11:27:53 edt
From: decvax!duke!george at idis
Subject: concatenation
To: duke!decvax!ihnp4!sfbc!lr at mcnc

Below is the definition of string literals
from section C.1.4 of X3J11 draft 85-008.

	string-literal:
		unit-string-literal
		string-literal unit-string-literal

The below definition might be more useful.

	string-literal:
		unit-string-literal
		string-literal unit-string-literal
		string-literal character-constant

	
	George Rosenberg
	duke!mcnc!idis!george

------------------------------

Date: 19 May 85 16:26:34 CDT (Sun)
From: plus5!hokey (Hokey)
Subject: Pointer math and ints

>From C.3.6 Additive Operators (semantics), p. 21, draft 85-008:
>  "When two pointers to members of the same array object are subtracted,
>   the result is converted (by division by the size of the member) to
>   a signed integer (the size of which is implementation defined)
>   representing the difference of the subscripts of the two array
>   members.
>
>K&R (p 189, last paragraph of 7.4) states "If two pointers to objects of
>the same type are subtracted, the result is converted (by division by
>the length of the object) to an _int_ representing the number of objects
>separating the pointed-to objects."
>
>Changing the result from "an _int_" to "a signed integer (the size of
>which is implementation defined)" breaks existing code.
>
>[ Someone can prove the above statement? -- Mod --  ]

I proved it in a later statement on the same article, paraphrased again:

 An example of this problem (pointer==long, int==short) occurs when
 one passes the (previously guaranteed to be an int) "p-s" as an actual
 parameter to a subroutine, which is expecting an int.  Under the proposed
 definition, the "p-s" expression pushes a long on the stack, but the
 subroutine expects a short.

[ See more on K&R on article below.  -- Mod -- ]

Jack Jansen: I *never said* pointers had to fit into an int.  I am not
saying it now.  I *am* saying that if the result of (valid) pointer
subtraction is of arbitrary integral size instead of int, existing code
will break and it will be *hard* to find these problems.

Most current Macintosh C compilers have 4 byte pointers and longs, and
2 byte shorts and ints.  Every one of these compilers puts the result
of (valid) pointer subtraction into a *long* instead of an *int*.
This means that when I try and pass the (int) length of a string to a
subroutine via pointer subtraction, 4 bytes are generated instead of 2.

Are there still people who don't see the problem?

------------------------------

Date: Tue, 21 May 85 13:07:30 edt
From: ima!haddock!ism780!ico!spence
Subject: Pointer math and ints
To: ima!cbosgd!std-c

   On the subject of pointer subtraction, K&R is misleading.
   ((&a[1] - &a[0]) / sizeof( a[0] )) would compute one object "separating
   the pointed-to objects" when in fact the two objects are adjacent.
   It also has the problem of potentially indicating a negative number of
   intervening objects.  The proposed definition provides more information
   and is more precise.

Spencer Rugaber (ima!spence)

------------------------------

Date: Tue, 21 May 85 13:07:30 edt
From: ima!haddock!ism780!ico!spence
Subject: signed and unsigned char, same size?
To: ima!cbosgd!std-c

   I was asked by a friend the other day whether it was safe to store an
   unsigned char into a char.  The general problem is whether signed and
   unsigned objects (of the same variety char/short/int/long) are guaranteed
   to be the same size.  I could find no discussion of the question in K&R
   and can recall none from the proposed standard.  It seems like a
   question that the standard could reasonable speak to.

Spencer Rugaber (ima!spence)

[ Hope this helps.
	"For signed char and each type of int, there is a corresponding
	unsigned type (declared with the keyword unsigned) that
	occupies the same amount of storage."

Section C.1.2.5 Types, p. 16, Draft 85-045 (4/30/85) --
			-- Mod --	]

------------------------------

End of mod.std.c Digest - Wed, 22 May 85 21:25:42 EDT
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