strncat - Man Page

concatenate a string with part of another


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>

char *strncat(char *restrict s1, const char *restrict 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 strncat() function shall append not more than n bytes (a NUL character and bytes that follow it are not appended) from the array pointed to by s2 to the end of the string pointed to by s1. The initial byte of s2 overwrites the NUL character at the end of s1. A terminating NUL character is always appended to the result. If copying takes place between objects that overlap, the behavior is undefined.

Return Value

The strncat() function shall return s1; no return value shall be reserved to indicate an error.


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

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

2017 IEEE/The Open Group POSIX Programmer's Manual