c - What does the n stand for in `sscanf(s, "%d %n", &i, &n)`?

Question

The man page states that the signature of sscanf is

sscanf(const char *restrict s, const char *restrict format, ...);

I have seen an answer on SO where a function in which sscanf is used like this to check if an input was an integer.

bool is_int(char const* s) {
    int n;
    int i;
    return sscanf(s, "%d %n", &i, &n) == 1 && !s[n];
}

Looking at !s[n] it seems to suggest that we check if sscanf scanned the character sequence until the termination character . So I assume n stands for the index where sscanf will be in the string s when the function ends.

But what about the variable i? What does it mean?

Edit:

To be more explicit: I see the signature of sscanf wants a pointer of type char * as first parameter. A format specifier as seconf parameter so it knows how to parse the character sequence and as much variables as conversion specifiers as next parameters. I understand now that i is for holding the parsed integer.

Since there is only one format specifier, I tried to deduce the function of n.

Is my assumption above for n correct?

Answer 1

Looks like the op has his answer already, but since I bothered to look this up for myself and run the code...

From "C The Pocket Reference" (2nd Ed by Herbert Shildt) scanf() section:

%n Receives an integer of value equal to the number of characters read so far

and for the return value:

The scanf() function returns a number equal to the number of the number of fields that were successfully assigned values

The sscanf() function works the same, it just takes it's input from the supplied buffer argument ( s in this case ). The "== 1" test makes sure that only one integer was parsed and the !s[n] makes sure the input buffer is well terminated after the parsed integer and/or that there's really only one integer in the string.

Running this code, an s value like "32" gives a "true" value ( we don't have bool defined as a type on our system ) but s as "3 2" gives a "false" value because s[n] in that case is "2" and n has the value 2 ( "3 " is parsed to create the int in that case ). If s is " 3 " this function will still return true as all that white space is ingored and n has the value of 3.

Another example input, "3m", gives a "false" value as you'd expect.