How to convert and compare byte array to int - arrays

Question

I'm trying to compare a byte array with a hex number, having a surprisingly hard time.
#include <stdio.h>
int main()
{
int bytes[4] = { 0x7c, 0x71, 0xde, 0xbb };
int number = 0x7c71debb;
printf("%u\n", number);
printf("%u\n", (int)*bytes);
return 0;
}
I'm getting:
2087837371
124
I did some reading and I tried using memcpy as suggested in various places:
#include <stdio.h>
#include <string.h>
int main()
{
int bytes[4] = { 0x7c, 0x71, 0xde, 0xbb };
int number = 0x7c71debb;
int frombytes;
memcpy(&frombytes, bytes, 4);
printf("%u\n", number);
printf("%u\n", frombytes);
return 0;
}
Still the same result:
2087837371
124
I mean, it's been like an hour if I got to be honest frustration is starting to get a hold of me.
It all started from me trying to do this:
if ((unsigned int)bytes == 0x7c71debb)
EDIT:
After switching bytes' type to char or uint8_t, here's what I'm getting:
#include <stdio.h>
#include <string.h>
#include <stdint.h>
int main()
{
uint8_t bytes[4] = { 0x7c, 0x71, 0xde, 0xbb };
int number = 0x7c71debb;
int frombytes;
memcpy(&frombytes, bytes, 4);
printf("%u\n", number);
printf("%u\n", (int)*bytes);
printf("%u\n", frombytes);
return 0;
}
Results:
2087837371
124
3151917436

Answer 1

You are making two assumptions:
You're assuming int is exactly 32 bits in size.
This might be correct, but it could be smaller or larger. You should use int32_t or uint32_t instead.
You're assuming a big-endian machine.
If you are using an x86 or x86_64, this is incorrect. These are little-endian architectures. The bytes are ordered from least-significant to most-significant.
The following code avoids those assumptions:
int32_t frombytes =
(uint32_t)bytes[0] << ( 8 * 3 ) |
(uint32_t)bytes[1] << ( 8 * 2 ) |
(uint32_t)bytes[2] << ( 8 * 1 ) |
(uint32_t)bytes[3] << ( 8 * 0 );
(It looks a bit expensive, but your compiler should optimize this.)

Answer 2

You can build the int using bitwise operators on the array values.
The below code, which assumes big-endian architecture and ints that are at least 4 bytes long, outputs:
2087837371
2087837371
#include <stdio.h>
int main()
{
int bytes[4] = { 0x7c, 0x71, 0xde, 0xbb };
int number = 0x7c71debb;
int number2 = bytes[3] | bytes[2] << 8 | bytes[1] << 16 | bytes[0] << 24;
printf("%u\n", number);
printf("%u\n", number2);
return 0;
}