memcmp

 

C

Language

headers

Type support

Dynamic memory management

Error handling

Program utilities

Variadic function support

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Thread support (C11)

Atomic operations (C11)

Technical Specifications

 

Strings library

Byte strings

Multibyte strings

Wide strings

 

Null-terminated byte strings

Functions
Character manipulation
isalnum isalpha islower isupper isdigit isxdigit isblank (C99)	iscntrl isgraph isspace isprint ispunct tolower toupper
Conversions to numeric formats
atof atoiatolatoll strtolstrtoll	strtoulstrtoull strtofstrtodstrtold strtoimaxstrtoumax (C99)(C99)
String manipulation
strcpystrcpy_s (C11)
strncpystrncpy_s (C11)
strcatstrcat_s (C11)
strncatstrncat_s (C11)
strxfrm
String examination
strlenstrnlen_s (C11) strcmp strncmp strcoll strchr strrchr	strspn strcspn strpbrk strstr strtokstrtok_s (C11)
Memory manipulation
memchr
memcmp
memsetmemset_s (C11)
memcpymemcpy_s (C11)
memmovememmove_s (C11)
Miscellaneous
strerrorstrerror_sstrerrorlen_s (C11)(C11)

 

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

Compares the first count characters 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.

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);
    if(rc == 0)
        printf(" compares equal to ");
    else if(rc < 0)
        printf(" precedes ");
    else if(rc > 0)
        printf(" follows ");
    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

• 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

See also

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