how can i generate warnings for implicit casts that lose bits?

[robert bristow-johnson]

here is a post i put out (using Google Groups) that got dropped by
google:

i am using gcc as so:
$ gcc -v
Using built-in specs.
Target: i386-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --
infodir=/usr/share/info --enable-shared --enable-threads=posix --
enable-checking=releas e --with-system-zlib --enable-__cxa_atexit --
disable-libunwind-exceptions --enable-libgcj-multifile --enable-
languages=c,c++ ,objc,obj-c++,java,fortra n,ada --enable-java-awt=gtk --
disable-dssi --with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre
--with-cpu=generic --host=i386-redhat-linux
Thread model: posix
gcc version 4.1.1 20060525 (Red Hat 4.1.1-1)

and have compiled a simple test program (FILE: hello.c):

//
// $ gcc -Wconversion -o hello hello.c
// $ hello
//

#include <stdio.h>
main()
{
unsigned long a_ulong = 0; // 32 bit
short a_short_array[128]; // 16 bit each

a_ulong = 1234567;

a_short_array[26] = a_ulong;

printf("%d, %hx, %x, %lx \n", sizeof(a_short_ array),
a_short_array[26], a_short_array[26], a_ulong );
//
// printf output is:
//
// 256, d687, ffffd687, 12d687
//
}

and ran it as so:

$ gcc -Wconversion -o hello hello.c
$ hello

getting output:

256, d687, ffffd687, 12d687

now, i have confirmed that a short is 16 bits and an unsigned long is
32 bits. why does not this line of code:
a_short_array[26] = a_ulong;
generate a warning when i have the -Wconversion or -Wall flags set on
the gcc invocation line?

there is clearly a loss of bits (or a changing of value).

here is what the manual says about it:
from http://gcc.gnu.org/onlinedocs/gcc/Wa...arning-Options
:

-Wconversion
Warn for implicit conversions that may alter a value. This
includes conversions between real and integer, like abs (x) when x is
double; conversions between signed and unsigned, like unsigned ui =
-1; and conversions to smaller types, like sqrtf (M_PI). Do not warn
for explicit casts like abs ((int) x) and ui = (unsigned) -1, or if
the value is not changed by the conversion like in abs (2.0). Warnings
about conversions between signed and unsigned integers can be disabled
by using -Wno-sign-conversion.

For C++, also warn for conversions between NULL and non-pointer
types; confusing overload resolution for user-defined conversions; and
conversions that will never use a type conversion operator:
conversions to void, the same type, a base class or a reference to
them. Warnings about conversions between signed and unsigned integers
are disabled by default in C++ unless -Wsign-conversion is explicitly
enabled.

is there some other compiler flag i need to hit? i don't get why this
doesn't generate a warning.
finally, please reply to both newsgroups as i don't hang around
comp.lang.c very much.

thank you,

r b-j

[glen herrmannsfeldt]

Jean-Marc Bourguet wrote:
(snip)

The historical meaning of byte is for sure this one:

"A group of bits sufficient to represent one character is called a _byte_
-- a term coined in 1958 by Werner Buchholz."
Computer Architecture, Concepts and Evolution
Gerrit A. Blaauw and Frederick P. Brooks, Jr.

(They give as reference a paper of 1959 which they co-authored with
Buchholz).

The IBM 7030 (aka Stretch) had instructions to manipulate bytes (the
architecture description use the term byte) whose width was anything
between 1 and 8 bits. BTW, Buchholz, Brooks and Blaauw where all three
part of the architecture team of Stretch. The character set designed for
stretch was the first 8 bits one I know of.

I have the book, I will have to look at it. I always thought that
EBCDIC was designed for S/360.

The DEC PDP-10 had instructions to manipulate bytes (again the architecture
description use the term byte) whose width was anything between 1 and 36
bits and was commonly used with ASCII 7 bits character (yes, there was a
lost bit per word).

The bit isn't lost if you have files with line numbers.

How much of the PDP-10 has heritage in the IBM 36 bit machines?

-- glen

[Richard Bos]

Jerry Avins <jy*@ieee.orgwr ote:

David Thompson wrote:

An IMNVHO underappreciate d benefit of Ada is that its terminology and
particularly language keywords was carefully chosen -- admittedly
starting from a clean slate -- to not only be precise _and_ clear, but
to fit together well. IME it's the only serious language other than
COBOL in which one can write reasonably good code that also reads as
tolerable prose. ...

Forth?

Inform 7.

Richard

[Jean-Marc Bourguet]

glen herrmannsfeldt <ga*@ugcs.calte ch.eduwrites:

Jean-Marc Bourguet wrote:
(snip)

The historical meaning of byte is for sure this one:

"A group of bits sufficient to represent one character is called a _byte_
-- a term coined in 1958 by Werner Buchholz."
Computer Architecture, Concepts and Evolution Gerrit A. Blaauw and
Frederick P. Brooks, Jr.

(They give as reference a paper of 1959 which they co-authored with
Buchholz).

The IBM 7030 (aka Stretch) had instructions to manipulate bytes (the
architecture description use the term byte) whose width was anything
between 1 and 8 bits. BTW, Buchholz, Brooks and Blaauw where all three
part of the architecture team of Stretch. The character set designed for
stretch was the first 8 bits one I know of.

I have the book, I will have to look at it. I always thought that EBCDIC
was designed for S/360.

EBCDIC was designed for S/360. The character set designed for Stretch was
pecular. For example, it is with Baudot the only character set I know of
for which the digits are not consecutive.

The DEC PDP-10 had instructions to manipulate bytes (again the architecture
description use the term byte) whose width was anything between 1 and 36
bits and was commonly used with ASCII 7 bits character (yes, there was a
lost bit per word).

The bit isn't lost if you have files with line numbers.

Thanks for reminding me that.

How much of the PDP-10 has heritage in the IBM 36 bit machines?

I don't know. I've copied comp.arch and alt.folklore.co mputers and set the
follow up there, people there probably know that.

Yours,

--
Jean-Marc