#define string with numeric define - c

Question

For example, here is a C common #define:
#define USERNAME_LEN 100
#define SCAN_FMT "%100s"
// str is input from somewhere
char username[USERNAME_LEN + 1];
ret = sscanf(str, SCAN_FMT, username);
// check ret == 1 ?
can we have something like:
#define SCAN_FMT "%" USERNAME_LEN "s"
of course, this syntax is not what we want, but the ultimate goal
is to mix numeric #define into string #define
Note: I know we can do something like:
sprintf(SCAN_FMT, "%%ds", USERNAME_LEN); // char SCAN_FMT[10];
but this is not what I am looking for, because it requires run-time generation,
the best is to base on ANSI-C or std99.

Answer 1

You might like to do it like this:
#define SCAN_FMT_STRINGIFY(max) "%"#max"s"
#define SCAN_FMT(max) SCAN_FMT_STRINGIFY(max)
#define USERNAME_MAXLEN (100)
...
char username[USERNAME_MAXLEN + 1] = ""; /* Add one for the `0`-terminator. */
int ret = sscanf(str, SCAN_FMT(USERNAME_MAXLEN), username);

Answer 2

You can use the preprocessor directives for these kind of tasks.
1.The first directive is # allows you to do such things:
#define str(x) #x
cout << str(test);
This will be translated into:
cout << "test";
2.The second directive is ##:
#define glue(a,b) a ## b
glue(c,out) << "test";
will be translated into:
cout << "test";
Look here for more info preprocessor