Thread: Difference between scanf and fgets

Originally Posted by jimblumberg

What happens when you have different length strings?

A different macro for each of these different sizes?

Blecccch!

Anduril's previous post noted in passing an approach to that (which I extract here, with lots of snipping before and after the relevant bit).

Originally Posted by anduril462

You can get around that with some macro string quoting tricks or by sprintf'ing your format string into another string first.

Not the bit after the words "or by".

I'd still prefer a fgets() and scan the string approach.

It would usually be done something like this.

Code:

#include <stdio.h>

#define NAME_LEN 10        /* small value for testing */

#define STR2(n) #n         /* stringize */
#define STR(n) STR2(n)     /* expand n  */

int main(void) {
    char name[NAME_LEN];

    scanf("%" STR(NAME_LEN-1) "[^\n]", name);
    printf("<%s>\n", name);

    return 0;
}

What it really boils down to is: how can you reduce the work required to make a simple change like changing the size of a string. Think not just of changing the value of the macro, but if you change the macro used in the definition:

Code:

#define FOO_LEN  5
#define BAR_LEN  10
char buf[FOO_LEN];  // changing this to BAR_LEN should not require any other changes

Using the sizeof operator is the only easy way to do this. If you can't use the sizeof operator here, you are at a disadvantage when it comes to maintaining your code, you will have to make more than one change if you change the size of buf.

The macro expansion to stringify the size of the buffer and concatenate it into the format string fails in this regard. You must change the definition and the scanf call.

If you sprintf a format string, you can use the sizeof operator. But, since (as Jim pointed out) scanf does not allow for the null character, you either have to remember to do sizeof - 1. It's asking for an off-by-one error somewhere.