#include <string.h>char *strncat(char *str1, const char *str2, size_t count);
The strncat( ) function concatenates not more than count characters of the string pointed to by str2 to the string pointed to by str1 and terminates str1 with a null. The null terminator originally ending str1 is overwritten by the first character of str2. The string str2 is untouched by the operation. If the strings overlap, the behavior is undefined.
In C99, str1 and str2 are qualified by restrict.
The strncat( ) function returns str1.
Remember that no bounds checking takes place, so it is the programmer’s responsibility to ensure that str1 is large enough to hold both its original contents and also those of str2.
Related functions are strcat( ), strnchr( ), strncmp( ), and strncpy( ).