Why does memcmp return a negative value when there is a positive difference?

Question

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

int main()
{
    int test1 = 8410092;    // 0x8053EC
    int test2 = 8404974;    // 0x803FEE
    char *t1 = ( char*) &test1;
    char *t2 = (char*) &test2;
    int ret2 = memcmp(t1,t2,4);

    printf("%d",ret2);

}

Here's a very basic function that when run prints -2. Maybe I am totally misunderstanding memcmp, but I thought if it returns the difference between the first different bytes. Since test1 is a larger num than test2, shouldn't the printed value be positive?

I am using the standard gcc.7 compiler for ubuntu.

memcmp compares bytes, not int values. So endianness is relevant. — Weather Vane
Also memcmp does not specify what the magnitude of the return value means. Only < 0 or 0 or > 0. — Weather Vane
Effectively you are comparing the strings "\xEC\x53\x80\x00" and "\xEE\x3F\x80\x00", because of endianness. So yeah the first one is smaller than the second. — Havenard
No, not really. Why do you want do it? OK; yes, you can forcibly store the value in a big-endian format and then do the memcmp() on that, and you will get the answer you seem to seek, but why not just use test1 < test2 (or whatever comparison operator you're interested in). — Jonathan Leffler
@knowads How about just not using memcmp ? If you want to compare numerical scalars, then do so; memcmp doesn't so why try to hammer in a nail with a screwdriver ? — WhozCraig

HappyKeyboard HappyKeyboard · Accepted Answer · 2018-11-27T01:09:45

As pointed out in the comments, memcmp() runs byte comparison. Here is a man quote

int memcmp(const void *s1, const void *s2, size_t n);

RETURN VALUE: The memcmp() function returns an integer less than, equal to, or greater than zero if the first n bytes of s1 is found, respectively, to be less than, to match, or be greater than the first n bytes of s2 For a nonzero return value, the sign is determined by the sign of the difference between the first pair of bytes (interpreted as unsigned char) that differ in s1 and s2. If n is zero, the return value is zero. http://man7.org/linux/man-pages/man3/memcmp.3.html

If the bytes are not the same, the sign of the difference depends on the target endianness.

One application of memcmp() is testing if two large arrays are the same, which could be faster than writing a loop that runs element by element comparison. Refer to this stack questions for more details. Why is memcmp so much faster than a for loop check?

Why does memcmp return a negative value when there is a positive difference?

2 Answers