c++cc-preprocessor

Double hash before parameter in function call

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()?

Solution

  • 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.