Date: Wed, 31 Jan 2001 15:06:43 -0600 From: Lucas Bergman <lucas@slb.to> To: Theo van Klaveren <t.vanklaveren@student.utwente.nl> Cc: freebsd-questions@freebsd.org Subject: Re: Printf question Message-ID: <20010131150643.C28173@billygoat.slb.to> In-Reply-To: <FHEHJFHKDJMJCKHHPLFGEEMFCCAA.t.vanklaveren@student.utwente.nl>; from t.vanklaveren@student.utwente.nl on Wed, Jan 31, 2001 at 09:31:30PM %2B0100 References: <FHEHJFHKDJMJCKHHPLFGEEMFCCAA.t.vanklaveren@student.utwente.nl>
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On Wed, Jan 31, 2001 at 09:31:30PM +0100, Theo van Klaveren wrote:
>
> Hello,
>
> I have a small(?) question about printf. If I have a struct with two 64-bit
> integers, defined as follows:
>
> typedef struct _smbfind_result {
> uint64_t r_id;
> uint64_t c_id;
> .
> .
> } smbfind_result;
>
> Now, let's take for example res->r_id=1 and res->c_id=255676. The
> following statement:
>
> printf ("result (result %ld, cid %ld)\n",
> res->r_id, res->c_id);
>
> Outputs "result (result 1, cid 0)", whereas the following:
>
> printf ("result (result %ld, cid %ld)\n",
> (long)res->r_id, (long)res->c_id);
>
> Outputs "result (result 1, cid 255676)", which is of course the
> correct answer. Why are the typecasts required? One would say the
> %ld would require printf() to format correctly, which it doesn't. Am
> I mixing types or something?
First, uint64_t is defined as `unsigned long long', no? Since `%ld'
specifies a `long' argument, you've got an implicit conversion from
`unsigned long long' to `long' happening. How that happens is going
to depend on how your machine stores those types. Enabling compiler
warnings helps (unless you're naughty and forget to include stdio.h):
% cat foo.c
#include <stdio.h>
typedef unsigned long long uint64_t;
struct thingy {
uint64_t r_id;
uint64_t c_id;
};
int
main(void)
{
struct thingy x;
struct thingy *res = &x;
x.r_id = 1;
x.c_id = 255676;
printf("result (result %ld, cid %ld)\n", res->r_id, res->c_id);
return 0;
}
% gcc -Wall foo.c
foo.c: In function `main':
foo.c:19: warning: long int format, different type arg (arg 2)
foo.c:19: warning: long int format, different type arg (arg 3)
What you probably want is the `%llu' specifier instead of `%ld'.
Note that `long long' is a _very_ recent addition to the C standard,
so proceed with caution if you want your program to be portable.
Lucas
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