Variadic macros in C

__VA_ARGS__ forwards any number of arguments, which is how logging macros work.

#include <stdio.h>

#define LOG(fmt, ...) \
    fprintf(stderr, "[%s:%d] " fmt "\n", __FILE__, __LINE__, ##__VA_ARGS__)

#define TRACE() LOG("reached %s", __func__)

void noisy(int value) {
    LOG("starting");
    LOG("value is %d", value);
    TRACE();
}

How it works

1. ... in the parameter list captures the rest.
2. __VA_ARGS__ expands to them, commas included.
3. The leading ## swallows the comma when nothing was passed.

Keywords and builtins used here

• int
• noisy
• void

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Step 2 of 3 in Text manipulation, step 6 of 12 in Preprocessor & headers.

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