How do we test the return values from the scanf() function?

Question

I want to confirm if a value returned from the scanf() function is a floating number or not. How can I do that? My code is not running as it should if the wrong data types are supplied to the scanf() function. Similarly, how would I confirm if a value returned is a character string or not?

Answer 1

scanf() et al return the number of successful conversions.

If you have:

scanf("%f", &f);

you should test:

if (scanf("%f", &f) == 1)
    ...all OK...
else
    ...EOF or conversion failure...

If you have several conversions, check that they all completed. If you're using %n 'conversions', they aren't counted.

Although scanf() does return EOF on EOF, you should not test for that — you should always check primarily that you got the number of conversions you expected. For example, consider the buggy code:

while (scanf("%f %d %s", &f, &i, s) != EOF)  // Here be BUGS!
    ...loop body...

If you type 3.14 x23 yes, then you will have an infinite loop because scanf() will return 1 on the first iteration (it successfully converted 3.14), and 0 thereafter (not EOF).

You might be OK with:

while ((rc = scanf("%f %d %s", &f, &i, s)) != EOF)
{
    if (rc != 3)
        ...oops data problems...
    else
        ...all OK...
}

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Judging from previous questions, you should be looking at using fgets() (or possibly POSIX getline()) to read lines of data, and then using sscanf() or even functions like strtol() and strtod() to read particular values from the line. If you use sscanf(), the comments made above about checking the number of successful conversions still apply.

I don't use scanf() in production code; it is just too damn hard to control properly. I regard it as almost suitable for beginning programs — except that it causes lots of confusion. On the whole, the best advice is 'stay clear of scanf() and fscanf()'. Note that that does not mean you have to stay clear of sscanf(), though some caution is needed even with sscanf().