Q:

Is there any inbuilt function in C to check for special characters (like space and tab) just like isdigit() function

Wanted to know if there is any inbuilt function in C to check for special characters like space, tab just like the isdigit() function?

A:

Look up and learn the functions defined by the C11 standard in the §7.4 Character handling <ctype.h> functions.
The isspace() function looks for a fairly general set of white space characters:
' ', '\t', '\r', '\f', '\n', '\v'

The isblank() function looks for the set you want:
' ', '\t'

To some extent, the POSIX variants are locale dependent — the POSIX specifications for isspace() and isblank() are less emphatic about which characters are matched, but disclaim conflict with the C standard, so in the C locale, the behaviour is as required by the C standard.

