Tony Finch (fanf) wrote,
Tony Finch
fanf

Named and optional function arguments in C99

Here's an amusing trick that I was just discussing with Mark Wooding on IRC.

Did you know you can define functions with optional and/or named arguments in C99? It's not even completely horrible!

The main limitation is there must be at least one non-optional argument, and you need to compile with -std=c99 -Wno-override-init.

(I originally wrote that this code needs C11, but Miod Vallat pointed out it works fine in C99)

The pattern works like this, using a function called repeat() as an example.

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>

    // Define the argument list as a structure. The dummy argument
    // at the start allows you to call the function with either
    // positional or named arguments.

    struct repeat_args {
        void *dummy;
        char *str;
        int n;
        char *sep;
    };

    // Define a wrapper macro that sets the default values and
    // hides away the rubric.

    #define repeat(...) \
            repeat((struct repeat_args){ \
                    .n = 1, .sep = " ", \
                    .dummy = NULL, __VA_ARGS__ })

    // Finally, define the function,
    // but remember to suppress macro expansion!

    char *(repeat)(struct repeat_args a) {
        if(a.n < 1)
            return(NULL);
        char *r = malloc((a.n - 1) * strlen(a.sep) +
                         a.n * strlen(a.str) + 1);
        if(r == NULL)
            return(NULL);
        strcpy(r, a.str);
        while(a.n-- > 1) { // accidentally quadratic
            strcat(r, a.sep);
            strcat(r, a.str);
        }
        return(r);
    }

    int main(void) {

        // Invoke it like this
        printf("%s\n", repeat(.str = "ho", .n = 3));

        // Or equivalently
        printf("%s\n", repeat("ho", 3, " "));

    }
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