Defined in header <cstring>
int strncmp( const char* lhs, const char* rhs, std::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.


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.


This function is not locale-sensitive, unlike std::strcoll and std::strxfrm.


#include <cstring>
#include <iostream>
void demo(const char* lhs, const char* rhs, int sz)
    const int rc = std::strncmp(lhs, rhs, sz);
    if (rc < 0)
        std::cout << "First " << sz << " chars of ["
                  << lhs << "] precede [" << rhs << "]\n";
    else if (rc > 0)
        std::cout << "First " << sz << " chars of ["
                  << lhs << "] follow [" << rhs << "]\n";
        std::cout << "First " << sz << " chars of ["
                  << lhs << "] equal [" << rhs << "]\n";
int main()
    demo("Hello, world!", "Hello, everybody!", 13);
    demo("Hello, everybody!", "Hello, world!", 13);
    demo("Hello, everybody!", "Hello, world!", 7);
    demo("Hello, everybody!" + 12, "Hello, somebody!" + 11, 5);


First 13 chars of [Hello, world!] follow [Hello, everybody!]
First 13 chars of [Hello, everybody!] precede [Hello, world!]
First 7 chars of [Hello, everybody!] equal [Hello, world!]
First 5 chars of [body!] equal [body!]

See also

compares two strings
compares a certain amount of characters from two wide strings
compares two buffers
compares two strings in accordance to the current locale
C documentation for strncmp

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