Question

As the topic says – for what characters would isblank(int c) return true ?

Haven’t found any entry about that function here on SO.

Answer 1

Unless you’ve set the current locale to something other than "C", it will only return true for ' ' and '\t‘.

N1570:

7.4.1.3 The isblank function

…
2 The isblank function tests for any character that is a standard blank character or is one
of a locale-specific set of characters for which isspace is true and that is used to
separate words within a line of text. The standard blank characters are the following:
space (' '), and horizontal tab ('\t'). In the "C" locale, isblank returns true only
for the standard blank characters.