placemaker | Ruby library for accessing the Yahoo Placemaker service

if comma , is omitted, like hash_hash(), then does this differ from standard's explanation (undefined behavior)? And is the placemaker the same as white-space?

Yes, If a macro is defined with arguments, the arguments must be separated with a comma delimiter. Attempts to define a multiple argument macro otherwise would violate the rules of the standard, and it would not compile:

Function-like macros can take arguments, just like true functions. To define a macro that uses arguments, you insert parameters between the pair of parentheses in the macro definition that make the macro function-like. The parameters must be valid C identifiers, separated by commas and optionally whitespace.

Furthermore,

• Because this requirement is well defined, it would not be an example of undefined behavior.
• In this phrase of the reference paragraph and optionally whitespace, the and indicates that white space can be used along with a delimiter, but by itself is not sufficient as a delimiter.