C/C++ Check to see if bit 31 is set in an unsigned int -

i'm trying check , see if bit set in unsigned int. i'm not sure how can this, assume this. i'm trying make cdq instruction in c++ (but function)

here have

unsigned int cdq(unsigned int eax) {      unsigned int edx = 0;       if( (eax >> 31) & 1 ) { edx = 0xffffffff; }      return edx } 

when use function following values:

cdq(0x12345678) bit 31 set (1) should return (unsigned int)-1 cdq(0x01) bit 31 not set (0) should return 0

the problem returns 0, , i'm not sure why

cdq(0x12345678) bit 31 set (1)

no, it's not ... highest bit set bit 28:

    1    2    3    4    5    6    7    8  0001 0010 0011 0100 0101 0110 0111 1000  ^  ^   |  | 31  28 

you code should work, use

if( eax & (1u << 31) ) edx = 0xffffffff; 

since it's bit more direct , shifts constant rather variable less work @ run time (although optimizing compiler should produce same code both).

actually write like

int cdq(int eax) {     return eax < 0? -1 : 0; } 

by way, code doesn't implement cdq because eax , edx variables not hardware eax , edx registers. , it's not idea replicate asm instructions c functions anyway ... c has own features doing these sorts of things, e.g.,

int32_t foo = -0x12345678; int64_t bar = (int64_t)foo;