sizeof stuff and declarations

[Kevin]

I have two questions. I hope they are about C. I chose to post
here instead of comp.unix.progr ammer (or some other) because I do
think this is a C question.

Question 1:

%cat float.c
main() {
float **m = malloc(5 * sizeof(float*)) ;
m[0] = malloc( sizeof(float) );
m[0][0] = 1.2;
printf("%f\n",m[0][0]);
}

Is this legal? I never really declare and assign so fast, but I
was wondering if it works because I'm lucky or if it really is
legal. Gcc doesn't like it much.

%gcc -c float.c
float.c: In function `main':
float.c:2: warning: initialization makes pointer from integer
without a cast float.c:3: warning: assignment makes pointer from
integer without a cast

Question 2:

(gdb) p sizeof(int)
$1 = 4
(gdb) p sizeof(long)
$2 = 4

I was expecting to see long a little bigger.
Why are they the same size?

Thank you.

[Kevin]

On 18 Mar 2005 01:24:10 GMT,
Chris Torek <no****@torek.n et> wrote:

[on machine X, "sizeof(int )" and "sizeof(lon g)" are the same]

On Thu, 17 Mar 2005 15:03:23 -0800,
Andrey Tarasevich <an************ **@hotmail.com> wrote:

Why not? This is perfectly legal in C. 'long' is notrequired> to be bigger than 'int'.

In article <20************ *************** @hotmail.com>
Kevin <ke***@hotmail. com> wrote:

But if they take the same amount of bytes, then how is it
possible that long can take bigger numbers?

Maybe it can't -- on machine X anyway.

Right.

An analogy might help. Suppose the motor-vehicle
administration( DMV or MVA or whatever it is called where you
are) says that you need a different kind of license to drive a
motorcycle vs a car vs a tractor-trailer. You only have a
"regular car" license. Suppose also that they say "a car must
hold two or more people side by side". If your vehicle holds
eight people, two in front seats and three in second and third
row seats, is it a car? (What if it holds two or so people but
can pull a triple trailer? :-) )
To be honest, I'm not sure how to answer, but here's my attempt:

Question 1: Is it a car? Yes. Because to be a car, must hold
two or more people side by side. My vehicle holds eight people
such that:

(a) two in front seats (respects the rule)
(b) three in second row (respect the rule: ``or more'')
(c) three in third row (respect the rule: ``or more'')

Therefore it's a car. I assume that the seats in each row are
really ``side by side''.

Question 2: What if it can pull a triple trailer?

I don't know what a triple trailer is... but as long as your car
respects the rule above, then it's a car. If a car couldn't pull
something, then it should be specified what it can't do. Is isn't
specified, so we are not breaking the rule.

I hope I got it :-)

The C standard says that an "int" variable must hold, at the
least, values in the range -32767 to +32767, and "long" has to
cover at least -2147483647 to [phone]. If your "int" and
"long" both happen to hold values in the range -2147483648 to
[phone], is that sufficient?
Yes, it's sufficient. More than sufficient actually. It's
necessary for an int to allow -32767 to +32767, if you allow
more, than it's all good. Right?

It is not unusual for a standard (or a law) to set some sort of
"least quality required" limit, and for people to exceed that,
for whatever reason.

Yeah, I think I understand it now... I hope I do :-)

Thank you.

[Keith Thompson]

On modern systems, it's very common either for short and int to be the
same size, or for int and long to be the same size.

Roughly speaking:
char has to be at least 8 bits
short has to be at least 16 bits (and at least as wide as char)
int has to be at least 16 bits (and at least as wide as short)
long has to be at least 32 bits (and at least as wide as int)

There's often an advantage in having predefined integer types of sizes
8, 16, and 32 bits. Without defining additional types, there are
several ways to do this (assuming all sizes are powers of 2):
char 8 bits
short 16 bits
int 16 bits
long 32 bits
or
char 8 bits
short 16 bits
int 32 bits
long 32 bits
or
char 8 bits
short 16 bits
int 32 bits
long 64 bits

You're likely to see the latter only on 64-bit systems.

Even on 64-bit systems, int is likely to be only 32 bits, because
making it 64 bits leaves a hole in the type system (either there's no
16-bit type, or there's no 32-bit type); Cray systems, however, have
64-bit ints anyway.

None of this is guaranteed, and there's seldom any great need to
assume that it is. You can write code that doesn't make any
assumptions beyond what's guaranteed by the standard.

C99 introduces typedefs for exact-width types: int16_t, int32_t,
int64_t, and so forth.

(Above, I wrote "Roughly speaking" because what the standard
guarantees isn't these sizes. Instead, it guarantees certain
representable ranges; either -127..+127 or 0..255 for char,
-32767..+32767 for short and int, -2147483647..+21 47483647 for long.
These usually translate to the specified number of bits, but extra
padding bits are permitted. With padding bits, it's even possible to
have sizeof(short) > sizeof(int), but I don't think any real
implementations do this. Also, for a 16-bit type, the range is more
likely to be -32728..+32767 rather than -32767..+32767; the standard's
minimum ranges allow for sign-magnitude and one's-complement
representations , but two's-complement is much more common these days.)

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Chris Torek]

This is off-topic but what the heck... :-)

On 18 Mar 2005 01:24:10 GMT,
Chris Torek <no****@torek.n et> wrote:

An analogy might help. ... motorcycle vs a car vs a tractor-trailer. ...
... is it a car? (What if it holds two or so people but
can pull a triple trailer? :-) )

In article <20************ *************** @hotmail.com>
Kevin <ke***@hotmail. com> wrote [in part]:I don't know what a triple trailer is... but as long as your car
respects the rule above, then it's a car.
If you are familiar with "18-wheelers" at all, you know what a
tractor cab is (e.g., a Peterbilt or a Mack). These tow trailers.
Trailers can be doubled-up or even tripled-up, so that one cab is
pulling three trailers. Some states put limits on the train-length;
I believe triples are outlawed in all of Caliornia, for instance.
If a car couldn't pull something, then it should be specified what
it can't do. Is isn't specified, so we are not breaking the rule.

I hope I got it :-)

Pretty much, yes. Sometimes the specifications are a bit lacking.
There is an enormous pickup truck (that does qualify as a "pickup
truck", despite being 21.5 feet long and 9 feet high) that I suspect
could pull trailers, and hence cross the somewhat fuzzy line between
"ordinary car" drivers' licenses and commercial truck drivers'
licenses. Of course, Hummers are already so heavy (over 6000 pounds
"gross vehicle weight") that they are illegal on most California
suburban streets -- but good luck finding anyone to enforce that
when Arnold Schwarzenegger is out for a drive. :-)

(The truck -- the International CXT -- has a GVWR of 25,999 lbs,
which is just under the line for requiring a Class B license.)
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) [phone]
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.