getuid 为 root 返回不同的零

Question

I am with the following problem to run

void main(void){
uid_t getuid(void);
gid_t getgid(void);

uid_t user_id;
gid_t group_id;

printf("user_id: %d\n",user_id);
printf("group_id: %d\n",group_id);
exit(0);
}

I am having the result : user_id: 134513819 and group_id: -1216946176 as the file belongs to the root and is running as root should not return:

user_id: 0
group_id: 0

also if anyone can clarify why it is returning a negative value appreciate .

Answer 1

Wrong types in printf. %d is a signed decimal. You probably want %u.

The number is negative because of the different binary representations of numbers. For a signed decimal the first bit in the sequence of zeros and ones in your PC represents the sign: plus or minus.

For unsigned decimals the first bit represents an exponential of 2.

So if you try to interpret a sequence of bits that represents an unsigned decimal (%u) as a signed decimal (%d), that first bit, which should translate to a numerical value like 2^7, gets interpreted as a plus or minus.

Unsigned 8 bit long decimals range from 0 to 255, signed 8 bit decimals range from -127 to 128, which also is a total of 256 different digits.

See https://en.wikipedia.org/wiki/Signed_number_representations