strncmp

 

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Strings library

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Functions
Character manipulation
isalnum isalpha islower isupper isdigit isxdigit isblank (C99)	iscntrl isgraph isspace isprint ispunct tolower toupper
Conversions to and from numeric formats
atoiatolatoll (C99) atof strtolstrtoll (C99) strtoulstrtoull (C99)	strtoimaxstrtoumax (C99)(C99) strtofstrtodstrtold (C99)(C99) strfromfstrfromdstrfroml (C23)(C23)(C23)
String manipulation
strcpystrcpy_s (C11) strncpystrncpy_s (C11) strcatstrcat_s (C11)	strncatstrncat_s (C11) strxfrm strdup (C23) strndup (C23)
String examination
strlenstrnlen_s (C11) strcmp strncmp strcoll strchr strrchr	strspn strcspn strpbrk strstr strtokstrtok_s (C11)
Memory manipulation
memchr memcmp memsetmemset_explicitmemset_s (C23)(C11)	memcpymemcpy_s (C11) memmovememmove_s (C11) memccpy (C23)
Miscellaneous
strerrorstrerror_sstrerrorlen_s (C11)(C11)

 

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

Compares at most count characters of two possibly null-terminated arrays. The comparison is done lexicographically. Characters following the null character are not compared.

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

The behavior is undefined when access occurs past the end of either array lhs or rhs. The behavior is undefined when either lhs or rhs is the null pointer.

Parameters

lhs, rhs	-	pointers to the possibly null-terminated arrays to compare
count	-	maximum number of characters to compare

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 is not locale-sensitive, unlike strcoll and strxfrm.

Example

Run this code

#include <stdio.h>
#include <string.h>
 
void demo(const char* lhs, const char* rhs, int sz)
{
    const int rc = strncmp(lhs, rhs, sz);
    if (rc < 0)
        printf("First %d chars of [%s] precede [%s]\n", sz, lhs, rhs);
    else if (rc > 0)
        printf("First %d chars of [%s] follow [%s]\n", sz, lhs, rhs);
    else
        printf("First %d chars of [%s] equal [%s]\n", sz, lhs, rhs);
}
int main(void)
{
    const char* string = "Hello World!";
    demo(string, "Hello!", 5);
    demo(string, "Hello", 10);
    demo(string, "Hello there", 10);
    demo("Hello, everybody!" + 12, "Hello, somebody!" + 11, 5);
}

Output:

First 5 chars of [Hello World!] equal [Hello!]
First 10 chars of [Hello World!] follow [Hello]
First 10 chars of [Hello World!] precede [Hello there]
First 5 chars of [body!] equal [body!]

References

• C23 standard (ISO/IEC 9899:2024):

• 7.24.4.4 The strncmp function (p: TBD)

• C17 standard (ISO/IEC 9899:2018):

• 7.24.4.4 The strncmp function (p: TBD)

• C11 standard (ISO/IEC 9899:2011):

• 7.24.4.4 The strncmp function (p: 366)

• C99 standard (ISO/IEC 9899:1999):

• 7.21.4.4 The strncmp function (p: 329)

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

• 4.11.4.4 The strncmp function

See also

strcmp	compares two strings (function)
wcsncmp (C95)	compares a certain amount of characters from two wide strings (function)
memcmp	compares two buffers (function)
strcoll	compares two strings in accordance to the current locale (function)
C++ documentation for strncmp