What is the effect of space at different places in the scanf format string?

Question

The below is what I understood so far. Please confirm, add, correct as the case may be -

scanf (" %c %d %s", &a, &b, c);

In the above - the first space before %c makes sure the buffer is cleared before scanf starts accepting new string into it from stdin for this particular function scanf call. This clears any of the delimiters from any previous input function calls..

The remaining two spaces before %d and %s allow any number of spaces or tabs but not enter key press between the user's entry of a, b and b,c , respectively.

Even with the above, none of the inputs can contain a space in it i.e. space is the delimiter for each of the three inputs. To add space it has to be specified in the string control braces [] like "%[a-z A-Z_0-9]" can contain any upper or lower case alphabets, digits 0-9, a space and an underscore - but will treat all other characters as invalid - The invalid character will go to the next input in the format string, if any, so if %[___] above was followed by %c and an astrick is pressed, the astrick is put into the character corresponding to %c.

Please confirm, correct, add. Thanks.

Answer 1

the first space before %c makes sure the buffer is cleared before scanf starts accepting new string into it from stdin for this particular function scanf call. This clears any of the delimiters from any previous input function calls.

No. The buffer is not cleared and there are no delimiters.

The remaining two spaces before %d and %s allow any number of spaces or tabs but not enter key press between the user's entry of a, b and b,c , respectively.

No. The enter key produces a newline character, which is whitespace.

Even with the above, none of the inputs can contain a space in it i.e. space is the delimiter for each of the three inputs.

No.

The invalid character will go to the next input in the format string

Yes. This isn't limited to %[ ]: If e.g. %d sees 12foo in the input stream, it will consume 12 and leave foo to be read by the rest of the format string (however, if there are no leading digits at all, %d will fail and abort processing).

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Any whitespace character in the format string reads and consumes all available whitespace characters at this point in the input stream, including spaces, tabs, and newlines. It doesn't matter whether the space appears before %c or %d or %s: All whitespace (including newlines) in the input is skipped.

%c accepts spaces just fine. Space is not a delimiter because %c has no delimiters; it always reads a single character. The only reason it can't read a space in your code is that it is preceded by in the format string, which will have skipped over all available whitespace.

As for %d and %s, they implicitly skip leading whitespace. That is, " %d" is equivalent to "%d" and " %s" is equivalent to "%s".

Answer 2

From cppreference.com:

Any single whitespace character in the format string consumes all available consecutive whitespace characters from the input

So all the spaces in the format string just mean to skip over any whitespace in the input.

There's no difference between the spaces before %c and the other spaces. The initial spaces don't clear the buffer, it just skips over any initial whitespace in the input. This ensures that %c will read the first non-whitespace character in the input.

Whitespace includes space, TAB, and newline characters. So you can put spaces or press enter between each input.

You don't actually need the spaces before %d or %s. These formats don't read anything that contains whitespace, and they automatically skip over any whitespace before the object they read. The spaces in the format string are redundant and do no harm, and may make it easier to read.

All conversion specifiers other than [, c, and n consume and discard all leading whitespace characters (determined as if by calling isspace) before attempting to parse the input.
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The conversion specifiers that do not consume leading whitespace, such as %c, can be made to do so by using a whitespace character in the format string