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

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...
}

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().

The return value of scanf indicates the number of items successfully read and assigned to your variables, or EOF:

These functions return the number of input items successfully matched and assigned, which can be fewer than provided for, or even zero in the event of an early matching failure.

The value EOF is returned if the end of input is reached before either the first successful conversion or a matching failure occurs. EOF is also returned if a read error occurs, in which case the error indicator for the stream (see ferror(3)) is set, and errno is set indicate the error.

If the int returned by scanf has a value less than the amount of input specifiers (or at least the specifiers which require an argument to be passed) in the format string, then you can assume something went wrong.

However, if you want to perform a more thorough validation of your floating point number, then you should consider using strtod(). ie. for the input "1.23abc", scanf would store 1.23 into your variable and tell you that everything went well, but with strtod you can check if the last character converted is actually the end of the string being parsed.