On 09/23/2017 12:35 PM, Hans van Kranenburg wrote:
> Hi,
>
> When looking around in the kernel code, I ran into this (hash.h):
>
> u32 btrfs_crc32c(u32 crc, const void *address, unsigned int length);
>
> [...]
>
> static inline u64 btrfs_extref_hash(u64 parent_objectid, const char *name,
> int len)
> {
> return (u64) btrfs_crc32c(parent_objectid, name, len);
> }
>
> [...]
>
> What is the "official" behaviour of just stuffing a 64-bit
> (parent_objectid) value into a 32-bit function argument (crc)? Does it
> get truncated? Does this compile without a warning?
>
> I would expect that the caller should do the housekeeping of deciding
> how to transform the 64 bit parent_objectid into some 32 bit value.
In the meantime I can of course answer this myself...
#include <stdlib.h>
#include <stdio.h>
typedef unsigned int __u32;
typedef unsigned long __u64;
void i_eat_32_for_breakfast(__u32 food) {
printf("%u\n", food);
}
int main(void) {
__u64 eat_this = 4294968320;
i_eat_32_for_breakfast(eat_this);
return 0;
}
-$ ./breakfast
1024
--
Hans van Kranenburg
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