Meaning of _t postfix of sigset_t, time_t [duplicate]

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What does a type followed by _t (underscore-t) represent?

I have seen many data structures in Linux environment have post-fix of '_t'.

Is there a specific meaning for it?

It's an abbreviation for "type".

The 't' stands for 'type'. POSIX and possibly also C99 reserve all names ending in '_t' for implementation-defined typedefs. Despite this, many libraries use such names for their own user-exposed types (I suppose such libraries could be considered part of the implementation, if you wish to look at it that way).