Comparing null-terminated string with a non null-terminated string in C

The deserialization library (messagepack) I am using does not provide null-terminated strings. Instead, I get a pointer to the beginning of the string and a length. What is the fastest way to compare this string to a normal null-terminated string?

The fastest way is strncmp() which limits the length to be compared.

 if (strncmp(sa, sb, length)==0)  
    ...

This assumes however that the length you use is the maximum length of the two strings. If the null terminated string could have a bigger length, you first have to compare the length.

 if(strncmp(sa,sb, length)==0 && strlen(sa)<=length) // sa being the null terminated one
     ...

Note that the strlen() is checked on purpose after the comparison, to avoid unnecessarily iterating through all the characters o fthe null terminated string if the first caracters don't even match.

A final variant is:

 if(strncmp(sa,sb, length)==0 && sa[length]==0) // sa being the null terminated one
     ...