Double hash before parameter in function call

Question

I see this line in C:

#define log(format, args...) snprintf(buffer + strlen(buffer), 1023 - strlen(buffer), format, ##args);

What does the double pound / hash mean before the last param in snprintf()?

Answer 1

In standard C, the "##" is for concatenating tokens together within a macro. Here, this macro is not in standard C, but in "Gnu C", the dialect implemented by GCC. The "##" is used to remove the comma if the extra arguments (in args) turn out to be empty. See the GCC manual.

Answer 2

That's the "token-pasting" preprocessor operator, and I don't think that macro uses it correctly.