memcmp

Defined in header `<string.h>`
int memcmp( const void* lhs, const void* rhs, size_t count );

Compares the first count bytes of the objects pointed to by lhs and rhs. 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.

The behavior is undefined if access occurs beyond the end of either object pointed to by lhs and rhs. The behavior is undefined if either lhs or rhs is a null pointer.

Parameters

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

Return value

Negative value if lhs appears before rhs in lexicographical order.

Zero if lhs and rhs compare equal, or if count is zero.

Positive value if lhs appears after rhs in lexicographical order.

Notes

This function reads object representations, not the object values, and is typically meaningful for byte arrays only: structs may have padding bytes whose values are indeterminate, the values of any bytes beyond the last stored member in a union are indeterminate, and a type may have two or more representations for the same value (different encodings for +0 and -0 or for +0.0 and –0.0, indeterminate padding bits within the type).

Example

Run this code

#include <stdio.h>
#include <string.h>
 
void demo(const char* lhs, const char* rhs, size_t sz)
{
    for(size_t n = 0; n < sz; ++n)
        putchar(lhs[n]);
 
    int rc = memcmp(lhs, rhs, sz);
    const char *rel = rc < 0 ? " precedes " : rc > 0 ? " follows " : " compares equal ";
    fputs(rel, stdout);
 
    for(size_t n = 0; n < sz; ++n)
        putchar(rhs[n]);
    puts(" in lexicographical order");
}
 
int main(void)
{
    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

References

C23 standard (ISO/IEC 9899:2024):

7.24.4.1 The memcmp function (p: TBD)

C17 standard (ISO/IEC 9899:2018):

7.24.4.1 The memcmp function (p: 266)

C11 standard (ISO/IEC 9899:2011):

7.24.4.1 The memcmp function (p: 365)

C99 standard (ISO/IEC 9899:1999):

7.21.4.1 The memcmp function (p: 328)

C89/C90 standard (ISO/IEC 9899:1990):

4.11.4.1 The memcmp function

Compiler support
Language
Headers
Type support
Program utilities
Variadic function support
Error handling
Dynamic memory management
Strings library
Algorithms
Numerics
Date and time utilities
Input/output support
Localization support
Concurrency support (C11)
Technical Specifications
Symbol index

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