Does strncat() always null terminate?

Question

Considering this code:

limit = sizeof(str1)-strlen(str1)-1;
strncat(str1,str2,limit);

If str2 length is greater than limit , does strncat Nul terminates str1 or I have to add this code, like in the case of strncpy ?

str1[sizeof(str1)-1] = '\0'

Answer 1

It always null-terminate.

Quoting C11 , chapter §7.24.3.2, ( emphasis mine )

The strncat function appends not more than n characters (a null character and characters 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 character of s2 overwrites the null character at the end of s1 . A terminating null character is always appended to the result.

and, the footnote

^{Thus, the maximum number of characters that can end up in the array pointed to by s1 is strlen(s1)+n+1 .}

Answer 2

C++ version lower than C11 doesn't append null character in Case like when your source string has not enough space inside for destination string.

char str[5];
str="Ami"

char str2[10];
str2="NotGoing"

str has 2 free space but needed is 7 to concat str2 and 1 for the null character. strncat(str,str2,);// case with no null termination.

now if str don't have space to write the whole destination (str2) inside it along with str pre-written data so in this case, it will not add a null char at the end

char str[10];
str="Ami"

char str2[3];

str2="Hello"

str got enough space for str2 inside it. so will add a null character at the end.

strncat(str,str2,);// case with null termination.

Formal I made to check on my own

length allocated to str >= strlen(str)+ strlen(str2)+1 ;

if this condition satisfies you will have a null terminated result otherwise you not.**