Howto print off_t

Howto print off_t

[Holger Kiehl]

Hello

What is the best way to print an off_t variable. On a 32 bit machine
you can print it with %ld and on a 64 bit machine you need to print
it with %lld. One could code this as follows:

    if (sizeof(off_t) == 4)
       printf("%ld\n", off_t_var);
    else
       printf("%lld\n", off_t_var);

But is this portable? Do all implementations know about %lld? Don't
some of them use %Ld or some other syntax.

Or is there a better way of doing this? I don't like the above code
since it makes it ugly to read when you have to print lots of off_t's.

Thanks,
Holger

Re: Howto print off_t

[Glynn Clements]

Holger Kiehl wrote:

> What is the best way to print an off_t variable. On a 32 bit machine
> you can print it with %ld and on a 64 bit machine you need to print
> it with %lld.

On a 64-bit machine, "long" is probably 64 bits.

> One could code this as follows:
> 
>     if (sizeof(off_t) == 4)
>        printf("%ld\n", off_t_var);
>     else
>        printf("%lld\n", off_t_var);

It's probably better to use the preprocessor, i.e.

	#if sizeof(off_t) == 4

> But is this portable?

No. This should be portable:

	#if sizeof(off_t) == sizeof(int)
		printf("%d\n", off_t_var);
	#elif sizeof(off_t) == sizeof(long)
		printf("%ld\n", off_t_var);
	#else
	#error cannot print off_t
	#endif

You could also include tests for specific platforms which support "ll"
or "q".

> Do all implementations know about %lld?

No.

> Don't some of them use %Ld or some other syntax.

"%Ld" is invalid; the "L" modifier is only valid when preceding a, A,
e, E, f, g, or G, and indicates a "long double" argument.

> Or is there a better way of doing this? I don't like the above code
> since it makes it ugly to read when you have to print lots of off_t's.

Assuming that off_t is no larger than "long", you could just cast it,
e.g.:

	printf("%ld", (long) off_t_var);

If off_t is larger than "long", then you have to deal with platform
specifics, as ANSI C doesn't have anything bigger than long.

-- 
Glynn Clements <glynn.clements@virgin.net>

Re: Howto print off_t

[Andrew Edmondson]

On Thu, 27 Jun 2002, Glynn Clements wrote:
> Holger Kiehl wrote:
> > What is the best way to print an off_t variable. On a 32 bit machine
> > you can print it with %ld and on a 64 bit machine you need to print
> > it with %lld.
>
> On a 64-bit machine, "long" is probably 64 bits.
>
> > Do all implementations know about %lld?
>
> No.


Technically no, but c99 defines long long with the modifier ll in conjunction 
with d, i, o, u, x, X so %lld is valid and should be portable.



-- 
Andrew Edmondson
Test Development Engineer
<-------------------------->
The discerning heart seeks knowledge, 
but the mouth of a fool feeds on folly.

Re: Howto print off_t

[Glynn Clements]

Andrew Edmondson wrote:

> > > What is the best way to print an off_t variable. On a 32 bit machine
> > > you can print it with %ld and on a 64 bit machine you need to print
> > > it with %lld.
> >
> > On a 64-bit machine, "long" is probably 64 bits.
> >
> > > Do all implementations know about %lld?
> >
> > No.
> 
> 
> Technically no, but c99 defines long long with the modifier ll in conjunction 
> with d, i, o, u, x, X so %lld is valid and should be portable.

Well, portable to all C99-conforming implementations, which is
basically the same thing as "not portable".

-- 
Glynn Clements <glynn.clements@virgin.net>

Re: Howto print off_t

[Holger Kiehl]

On Thu, 27 Jun 2002, Glynn Clements wrote:

>
> Holger Kiehl wrote:
>
> > What is the best way to print an off_t variable. On a 32 bit machine
> > you can print it with %ld and on a 64 bit machine you need to print
> > it with %lld.
>
> On a 64-bit machine, "long" is probably 64 bits.
>
> > One could code this as follows:
> >
> >     if (sizeof(off_t) == 4)
> >        printf("%ld\n", off_t_var);
> >     else
> >        printf("%lld\n", off_t_var);
>
> It's probably better to use the preprocessor, i.e.
>
> 	#if sizeof(off_t) == 4
>
Does the sizeof operator work in the preprocessor? I tried this but it
does not seem to work for me.

> > But is this portable?
>
> No. This should be portable:
>
> 	#if sizeof(off_t) == sizeof(int)
> 		printf("%d\n", off_t_var);
> 	#elif sizeof(off_t) == sizeof(long)
> 		printf("%ld\n", off_t_var);
> 	#else
> 	#error cannot print off_t
> 	#endif
>
But is not on most 32 bit system, both long and int 4 bytes long, so %d
will be used, but off_t in that case is mostly of type long?

> You could also include tests for specific platforms which support "ll"
> or "q".
>
C99 does have a 64 bit integer type, how does one print it here?

Thanks,
Holger

Re: Howto print off_t

[Glynn Clements]

Holger Kiehl wrote:

> > > One could code this as follows:
> > >
> > >     if (sizeof(off_t) == 4)
> > >        printf("%ld\n", off_t_var);
> > >     else
> > >        printf("%lld\n", off_t_var);
> >
> > It's probably better to use the preprocessor, i.e.
> >
> > 	#if sizeof(off_t) == 4
> >
> Does the sizeof operator work in the preprocessor? I tried this but it
> does not seem to work for me.

No, sorry; I'd forgotten about the "types" exception for #if.

The argument to #if is a "constant expression", with the same
definition as in C itself (i.e. what can occur on the RHS of a static
initialiser), *EXCEPT* that it can't use "sizeof", a cast, or an enum
constant, but can use "defined()".

If you're using autoconf, you can use AC_CHECK_SIZEOF, e.g.

	AC_CHECK_SIZEOF(off_t)

will define SIZEOF_OFF_T to the appropriate value, so you can then
use e.g.

	#include <config.h>
	...
	#if SIZEOF_OFF_T == 4
	...

> > > But is this portable?
> >
> > No. This should be portable:
> >
> > 	#if sizeof(off_t) == sizeof(int)
> > 		printf("%d\n", off_t_var);
> > 	#elif sizeof(off_t) == sizeof(long)
> > 		printf("%ld\n", off_t_var);
> > 	#else
> > 	#error cannot print off_t
> > 	#endif
> >
> But is not on most 32 bit system, both long and int 4 bytes long, so %d
> will be used, but off_t in that case is mostly of type long?

If int and long are both 32 bits, then they're the same type, so it
doesn't matter whether you use "%d" or "%ld".

> > You could also include tests for specific platforms which support "ll"
> > or "q".
> 
> C99 does have a 64 bit integer type, how does one print it here?

C99 defines "%lld" for the *printf family. But C99 support is far from
widespread. [Aside: a significant number of people are still using
Unices which don't provide the wcs* functions, and they are C89.]

-- 
Glynn Clements <glynn.clements@virgin.net>

Re: Howto print off_t

[Holger Kiehl]

On Thu, 27 Jun 2002, Glynn Clements wrote:

>
> Holger Kiehl wrote:
>
> > > > One could code this as follows:
> > > >
> > > >     if (sizeof(off_t) == 4)
> > > >        printf("%ld\n", off_t_var);
> > > >     else
> > > >        printf("%lld\n", off_t_var);
> > >
> > > It's probably better to use the preprocessor, i.e.
> > >
> > > 	#if sizeof(off_t) == 4
> > >
> > Does the sizeof operator work in the preprocessor? I tried this but it
> > does not seem to work for me.
>
> No, sorry; I'd forgotten about the "types" exception for #if.
>
> The argument to #if is a "constant expression", with the same
> definition as in C itself (i.e. what can occur on the RHS of a static
> initialiser), *EXCEPT* that it can't use "sizeof", a cast, or an enum
> constant, but can use "defined()".
>
> If you're using autoconf, you can use AC_CHECK_SIZEOF, e.g.
>
> 	AC_CHECK_SIZEOF(off_t)
>
> will define SIZEOF_OFF_T to the appropriate value, so you can then
> use e.g.
>
> 	#include <config.h>
> 	...
> 	#if SIZEOF_OFF_T == 4
> 	...
>
I already had plans to use autoconf to do the configuration, so this
will be one more reason for using it.

Many thanks for the help!

Holger