Why would you check that fscanf returns value < 1 in a function?

Question

I was given a problem to solve using fscanf which states the following:

Write a function to initialize array x of size SIZE with integers read from a text file. The function receives the name of the file as an argument.

The solution states the following below. Why would fscanf return a value < 1 if you're scanning each integer in the file? Shouldn't it be > 0 since fscanf returns the number of elements scanned?

void init (char *name)
{
    FILE    *fp;
    int i;

    fp = fopen (name, "r");
    if (fp == NULL)
    {
        printf ("no file\n");
        return;
    }

    for (i = 0; i < SIZE; i++)
    {
        if (fscanf (fp, "%d", &x[i]) < 1)
        {
            printf ("got %d numbers\n", i);
            return;
        }
    }

    fclose (fp);
    return;
}

So if it returns < 1 then it didn't scan anything. Perhaps you've run out of data? perhaps there is an error? — John3136, Nov 02 '17 at 05:06
Read the man page. It says `the function returns the number of items of the argument list successfully filled. This count can match the expected number of items or be less (even zero) due to a matching failure, a reading error, or the reach of the end-of-file.`. — Gaurav Pathak, Nov 02 '17 at 05:06
If you are on Linux you can simply type `man fscanf` in terminal to open the API reference manual of `fscanf` and you can read the section under _**return value**_. This goes for any function call. — Gaurav Pathak, Nov 02 '17 at 05:10
@Gaurav yeah I did read the manual and I'm still confused which is why I'm asking on here.... am I correct in thinking that this if statement is written to break the for loop when the fscanf returns a value < 1 which is essentially the EOF? — eel314, Nov 02 '17 at 05:18
@Connor, it's not just EOF, it will also fail if it finds something like `hello`, or the `.14159` bit of `3.14159`. Basically, anything that's not a valid integer. — paxdiablo, Nov 02 '17 at 05:31

paxdiablo · Answer 1 · 2017-11-02T05:17:47.793

2

You seem to be misunderstanding the intent of the code:

for (i = 0; i < SIZE; i++)
{
    if (fscanf (fp, "%d", &x[i]) < 1)
    {
        printf ("got %d numbers\n", i);
        return;
    }
}

What this will do is return if the fscanf failed to read in an integer (it returns the number of items scanned). So, at that point, the function will return^(a).

If you get a positive integer, it means you successfully read in the value and your loop will continue.

^(a) You probably should break here rather than return since the file doesn't get closed the way it is at the moment.

edited Nov 02 '17 at 05:17

answered Nov 02 '17 at 05:13

paxdiablo

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Probably, OP should replace `printf` with `perror`, in this way it should also print what error actually made `fscanf` to return 0. – Gaurav Pathak Nov 02 '17 at 05:15
1

@Gaurav -- `errno` is not set just because a user fails to enter an integer when expected by `fscanf()`.... In other words, `perror()` is probably going to be useless here. – ad absurdum Nov 02 '17 at 05:19
@paxdiablo so basically this is just a long winded way of breaking when fscanf is no longer able to read integers in the file? – eel314 Nov 02 '17 at 05:20
@DavidBowling Ohh, I thought that `perror` should print the errno in this case as well if `fscanf` fails. – Gaurav Pathak Nov 02 '17 at 05:21
@Stargateur Thanks, I just read it in the duplicate link. Got it. (y) – Gaurav Pathak Nov 02 '17 at 05:25
@Connor: yes, basically. It will loop up to `SIZE` times but break out early if you run out of valid input values. – paxdiablo Nov 02 '17 at 05:29
1

@Stargateur -- it is a POSIX requirement that `errno` be set if an error occurs before the first conversion has completed, but I don't anywhere in the C11 Standard that `fscanf()` must set `errno` or any error indicators. – ad absurdum Nov 02 '17 at 05:35

chux - Reinstate Monica · Answer 2 · 2017-11-02T14:34:39.143

Why would fscanf return a value < 1 if you're scanning each integer in the file?

fscanf (fp, "%d", &x[i]) returns:

1 when one int scanned successfully. Code loops to read additional int

0 when only non-numeric non-white-space input detected

EOF (EOF is some negative value) on end-of-file.

EOF on input error (rare).

With if (fscanf (fp, "%d", &x[i]) < 1), the branch is taken as one of the last 3 conditions occurred - likely end-of-file.

Why would you check that fscanf returns value < 1 in a function?

2 Answers2