Appends a number of characters from one string to another
#include <string.h> char *strncat ( char * restrict s1 , const char * restrict s2 , size_t n );
The strncat( ) function copies up to n characters of the string addressed by the second pointer argument, s2, to the location following the string addressed by the first pointer, s1. The first character of s2 is copied over the null character that terminates the string addressed by s1. The function returns the value of its first argument, which points to the resulting string. The locations that strncat( ) reads from and writes to must not overlap.
Unlike strcat( ), strncat( ) has a third parameter, n, to limit the length of the string written. The strncat( ) function stops copying when it has copied n characters, or when it reaches a terminating null character in the source string, whichever occurs first. In either case, strncat( ) appends a null character to the concatenated string addressed by s1. The string's length thus increases by at most n characters.
See the example for strcspn( ) in this chapter.