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

Question

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?

Answer 1

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
     ...

Answer 2

int compare(char *one, size_t onelen, char *two, size_t twolen)
{
int dif;

  dif = memcmp(one, two, onelen < twolen ? onelen : twolen);
  if (dif) return dif;

  if (onelen == twolen) return 0;
  return onelen > twolen? 1 : -1;
}

usage:

...
int result;
char einz[4] = "einz"; // not terminated
char *zwei = "einz";   // terminated

result = compare(einz, sizeof einz, zwei, strlen(zwei));

...

Answer 3

Here is one way:

bool is_same_string(char const *s1, char const *s2, size_t s2_len)
{
    char const *s2_end = s2 + s2_len;
    for (;;)
    {
        if ( s1[0] == 0 || s2 == s2_end )
            return s1[0] == 0 && s2 == s2_end;

        if ( *s1++ != *s2++ )
            return false;
    }
}