Why doesn't fscanf convert set and then char?

Question

If the file foobar.txt contains the text foobar , then in the following code

char s[16], c;
fp = fopen("foobar.txt", "r");
fscanf(fp, "%[f]s%c", s, &c);

fscanf returns 1 and not 2 as expected.

Whereas if I split the call to fscanf :

fscanf(fp, "%[f]s", s);
fscanf(fp, "%c", &c);

then both return 1 as expected.

Why doesn't the single fscanf work?

Answer 1

Because the s shouldn't be there, instead it should be

fscanf("%[f]%c", s, &c);

When scanf() finds the s in the format but not in the input it stops and returns 1 because up until then 1 argument matched.

The thing is, the s is not part of the format, the [] is the format specifier and it only contains a single character the f , so you should read the documentation more carefully and you can confirm that the s is not required for the format specifier, and in fact fscanf() is considering it as part of the requested input, but it's not there because after the f there is a o , try fsobar .

In the second case, it also stops and does not consume any character after the f and then the second call consumes a single character thus both return 1 correctly.

Answer 2

Just remove the s and modify it as follows.

fscanf("%[f]%c", s, &c);

In the first case, when fscanf() cannot find s in the input it stops and returns 1. In the second case, as you mentioned, its obvious that fscanf() will return 1.