using memcpy to convert from array to int

Your code has at best implementation defined behavior and in some cases undefined behavior.

Type int may have a size different from 4: on 16-bit systems, int typically has a size of only 2 bytes. You would have undefined behavior on such systems.

On regular 32-bit systems, int has 4 bytes, but the order in which the 4 bytes are stored in memory is implementation defined, a problem referred to as endianness:

• some systems use big-endian representation, where the first byte is the most significant part of the integer. Bytes 01 02 03 04 represent the value 0x01020304 on big-endian systems, such as older Macs, some mobile phones and embedded systems.

• conversely, most personal computers today use little-endian representation, where the first byte contains the least significant part of the integer. Bytes 01 02 03 04 represent the value 0x04030201 on little-endian systems, such as yours.

• The C Standard does not exclude other representations, where bytes would be in some other order. This was the case on some ancient DEC systems: the PDP-11, where the C language was originally developped (middle-endian or mixed-endian).

Albeit surprising, the little-endian order is very logical as the byte at offset n contains the bits representing values between 2^n*8 and 2^n*8+7. Endianness is a cultural issue, both choices seem natural to long time users.

The same variations are found in other contexts, such as the ordering of date components:

• Japan uses big-endian representation: February 17 2021 is written 2021.02.17,

• Europe uses little-endian representation: February 17 2021 is written 17/02/2021,

• The USA use a middle-endian representation: February 17 2021 is written 02/17/2021.

• 21 is pronounced twenty-one in English (big-endian) whereas Germans say einundzwanzig (one and twenty, little endian and actually middle-endian for 3-digit numbers). But then 17 is seventeen (little-endian) and in French dix-sept (big-endian).

• Western languages write numbers in big-endian format (I am 42 years old) but semitic scripts use little-endian order: Hebrew (אני בת 42) and Arabic (أنا ٤٢ سنة) both use little-endian as they are read from right to left.

Here is a more portable version to test memory representation:

#include <stdio.h>
#include <string.h>

int main() {
    unsigned int aux2 = 0x01020304;
    unsigned char aux[sizeof(unsigned int)]; 
    memcpy(&aux, aux2, sizeof(aux));
    printf("%X is represented in memory as", aux2);
    for (size_t i = 0; i < sizeof(aux); i++)
        printf(" %02X", aux[i]);
    printf("\n");
    return 0;
}