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Showing content from https://en.cppreference.com/w/cpp/language/../keyword/../language/../string/byte/memcmp.html below:

std::memcmp - cppreference.com

Reinterprets the objects pointed to by lhs and rhs as arrays of unsigned char and compares the first count bytes of these arrays. The comparison is done lexicographically.

The sign of the result is the sign of the difference between the values of the first pair of bytes (both interpreted as unsigned char) that differ in the objects being compared.

Negative value if the first differing byte (reinterpreted as unsigned char) in lhs is less than the corresponding byte in rhs.

​0​ if all count bytes of lhs and rhs are equal.

Positive value if the first differing byte in lhs is greater than the corresponding byte in rhs.

This function reads object representations, not the object values, and is typically meaningful for only trivially-copyable objects that have no padding. For example, memcmp() between two objects of type std::string or std::vector will not compare their contents, memcmp() between two objects of type struct { char c; int n; } will compare the padding bytes whose values may differ when the values of c and n are the same, and even if there were no padding bytes, the int would be compared without taking into account endianness.

#include <cstring>
#include <iostream>
 
void demo(const char* lhs, const char* rhs, std::size_t sz)
{
    std::cout << std::string(lhs, sz);
    const int rc = std::memcmp(lhs, rhs, sz);
    if (rc < 0)
        std::cout << " precedes ";
    else if (rc > 0)
        std::cout << " follows ";
    else
        std::cout << " compares equal to ";
    std::cout << std::string(rhs, sz) << " in lexicographical order\n";
}
 
int main()
{
    char a1[] = {'a', 'b', 'c'};
    char a2[sizeof a1] = {'a', 'b', 'd'};
 
    demo(a1, a2, sizeof a1);
    demo(a2, a1, sizeof a1);
    demo(a1, a1, sizeof a1);
}

Output:

abc precedes abd in lexicographical order
abd follows abc in lexicographical order
abc compares equal to abc in lexicographical order

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