strncmp - Man Page

compare part of two strings


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <string.h>

int strncmp(const char *s1, const char *s2, size_t n);


The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard.

The strncmp() function shall compare not more than n bytes (bytes that follow a NUL character are not compared) from the array pointed to by s1 to the array pointed to by s2.

The sign of a non-zero return value is determined by the sign of the difference between the values of the first pair of bytes (both interpreted as type unsigned char) that differ in the strings being compared.

Return Value

Upon successful completion, strncmp() shall return an integer greater than, equal to, or less than 0, if the possibly null-terminated array pointed to by s1 is greater than, equal to, or less than the possibly null-terminated array pointed to by s2 respectively.


No errors are defined.

The following sections are informative.



Application Usage




Future Directions


See Also


The Base Definitions volume of POSIX.1-2017, <string.h>

Referenced By

strcmp(3p), string.h(0p).

2017 IEEE/The Open Group POSIX Programmer's Manual