How scanf/fscanf/sscanf work is it scans a string and stops whenever the input doesn't match what you've specified or it finds whitespace. It's kind of like a regex -- you're creating a string which matches the data you want, with various kinds of wildcards. But it's very different from a regex in that it doesn't stop at the end of the line, it stops wherever it pleases. You can force it with %[charstoaccept] / %[^charstonotaccept], a %s-equivalent I first saw in a config file loader which suddenly had to cope with carriage returns when ported to Windows. (And people wonder why I'm paranoid about those.)

Suppose you're scanning

Code:

abcde 3.14159


c d e

with %s %lf %s. It starts with %s, and accepts 'a', 'b', 'c', 'd', 'e' into the string. Then it sees whitespace and decides the string is over.

The space betweeh %s and %lf tells it "whitespace is acceptable", so it skips past the whitespace.

After that, it gets to %lf, scans '3', '.', '1', '4', '1', '5', '9', and processes that into a float, then hits \n which tells it the number's now over.

The space between %lf and %s tells it whitespace is acceptable. So it skips past all three newlines and starts reading 'c' into a string, and stops immediately because it's hit some whitespace. Since it's completely finished the command string "%s %lf %s", it feels no need to do anything with that whitespace and leaves it there for next time.

...Which means, the next time you scan "%s %lf %s", it will quit immediately because %s sees a white space and stops. The point is, fscanf leaves the file things in an unpredictable place unless things go exactly as planned.

This is why I always read entire lines. sscanf() can't choke and leave stdin in a weird place, because the entire line has already been read. If you want sscanf to read several entire lines, it can do that too.

Code:

#include <stdio.h>
#include <string.h>

typedef struct abc {
  char a[64], b[64], c[64];
} abc;

int main(void)
{
        char buf[4096];
        abc a;

        while(!feof(stdin)) {
                int bpos=0;

                // Read three lines into the same buffer.
                if(!fgets(buf+bpos, 4096-bpos, stdin)) break;
                bpos = strlen(buf); // could be more efficient but you get the idea
                if(!fgets(buf+bpos, 4096-bpos, stdin)) break;
                bpos = strlen(buf);
                if(!fgets(buf+bpos, 4096-bpos, stdin)) break;

                sscanf(buf, "%[^\r\n] %[^\r\n] %[^\r\n]", a.a, a.b, a.c);

                printf("a: %s\nb: %s\nc: %s\n", a.a, a.b, a.c);
        }
}

Code:

$ printf "a b c\nd e f\ng h i\n" | ./a.out

a: a b c
b: d e f
c: g h i

$