c - Copying a 32 bit integer to a 64 bit integer is undefined? -

well, know << 32 undefined on 32-bit integers... know pointer casts , dereferences don't mix much.

but 1 kind of none of them.

test.c:

#include <stdio.h> #include <stdlib.h> #define __stdc_format_macros #include <inttypes.h>  int main(void) {     uint64_t tmp = 1 << 31;     printf("%" prix64 "\n", tmp);     return 0; } 

then this:

$ gcc test.c -o test $ ./test ffffffff80000000 

why did corrupt first 4 bytes?

lower shifts work fine. if printf("%x\n", 1 << 31), yields 80000000 expected. if tmp 32 bits, works fine.

u64 , __u64 have quirk.

you didn't corrupt anything, didn't shift 64-bit number. in c, single 1 defaults int. if want use 1 unsigned long (or unsigned), tell compiler suffixing number (ul or u -- (ull on x86)), otherwise shifting signed number:

uint64_t tmp = 1ull << 31; 

output

80000000