CODEWITHSUNDEEP
c
user180574
user180574

Reputation: 6084

How to set variable arguments default to empty in C?

I have a macro like this.

#define TRACE(fmt, ...) trace(__FILE__, __FUNCTION__, fmt, __VA_ARGS__);

But sometimes I don't want to supply format string. Something like TRACE() should do. Right now, I use 

TRACE("%s", "")

Is there any way to simplify?

Upvotes: 1

Views: 132

Answers (2)

Jens Gustedt
Jens Gustedt

Reputation: 78903

Adjacent string literals are merged into one single string. So the following should work

#define TRACE(...) trace(__FILE__, __func__, "" __VA_ARGS__)


TRACE();
TRACE("hello");
TRACE("integer %d", 42);

will result in something equivalent to 

trace(__FILE__, __func__, "");
trace(__FILE__, __func__, "hello");
trace(__FILE__, __func__, "integer %d", 42);

The only restriction that this has is that the first argument, if any, must really be a string literal. Another char* object wouldn't work.

(Observe the use of __func__ this one is the predefined identifier for the function name as of the C standard. And the lack of the ; in the macro to better fit into the usual function call convention in the flow of normal C code.)

Upvotes: 1

ouah
ouah

Reputation: 145829

What about?

#define TRACE(...) trace(__FILE__, __FUNCTION__, __VA_ARGS__)


TRACE("");

The ... ellipsis has to replace at least one argument, so if you use the form TRACE(fmt, ...) you cannot use a single argument when calling TRACE.

For this reason the form TRACE(...) is usually preferred over the form TRACE(fmt, ...).

Also note that I removed the ; in your macro definition. It's a bad practice to have it in the definition, the ; has to be put after the macro call instead. See this CERT document if you want more information: PRE11-C. Do not conclude macro definitions with a semicolon

Upvotes: 3

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