string setup

in a c-module I found the definition of a string:

char acString[20] = "";

For me the initilisation looks a little bit unusual.

My questions at this point are:
- is this allowed in ansi-c?
- what happens? Is only acString[0] set to 0x00
or is the whole string set to zero similar
to memset(acString,0,20)?
- can I be shure that in futur versions of C166
the compiler always treats this in the same manner?

Thank you
...Leo