Variable length arguments for Macros in C

In C, we can use variable length arguments for macros, similar to how we use them with functions. This is achieved using ellipsis (…) and the special __VA_ARGS__ identifier. The concatenation operator ## is used to handle cases where no variable arguments are provided.

Syntax

#define MACRO_NAME(fixed_params, ...) macro_body __VA_ARGS__

Where __VA_ARGS__ represents all the variable arguments passed to the macro, and ## can be used for token pasting.

Example: Logging Macro with Variable Arguments

This example demonstrates a logging macro that accepts variable arguments like printf(). The macro prints the filename, line number, and custom error messages −

#include <stdio.h>

#define INFO 1
#define ERR 2
#define STD_OUT stdout
#define STD_ERR stderr

#define LOG_MESSAGE(pr, strm, msg, ...) do {\
    char *str;\
    if (pr == INFO)\
        str = "INFORMATION";\
    else if (pr == ERR)\
        str = "ERROR";\
    fprintf(strm, "[%s] : %s : %d : "msg" <br>", \
        str, __FILE__, __LINE__, ##__VA_ARGS__);\
} while (0)

int main() {
    char *s = "Test String";
    
    /* Simple message without additional arguments */
    LOG_MESSAGE(ERR, STD_ERR, "Unable to open the file");
    
    /* Message with string argument */
    LOG_MESSAGE(INFO, STD_OUT, "%s is passed as argument", s);
    
    /* Message with multiple integer arguments */
    LOG_MESSAGE(INFO, STD_OUT, "%d + %d = %d", 14, 16, (14 + 16));
    
    return 0;
}

[ERROR] : /tmp/code.c : 18 : Unable to open the file 
[INFORMATION] : /tmp/code.c : 21 : Test String is passed as argument 
[INFORMATION] : /tmp/code.c : 24 : 14 + 16 = 30 

Key Points

• __VA_ARGS__ expands to all the variable arguments passed to the macro.
• The ## operator before __VA_ARGS__ removes the preceding comma if no arguments are provided.
• The do-while(0) construct ensures the macro behaves like a single statement.
• Variable argument macros are available in C99 and later standards.

Conclusion

Variable length argument macros in C provide a powerful way to create flexible, printf-like macros. Using __VA_ARGS__ and the ## operator, we can create robust logging and debugging utilities that accept any number of arguments.