NAME¶

std::memcmp - std::memcmp

Synopsis¶

Defined in header <cstring>
int memcmp( const void* lhs, const void* rhs, std::size_t count );

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.

Parameters¶

lhs, rhs - pointers to the memory buffers to compare
count - number of bytes to examine

Return value¶

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.

Notes¶

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.

Example¶

// Run this code

#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

See also¶

strcmp compares two strings
(function)
strncmp compares a certain number of characters from two strings
(function)
C documentation for
memcmp