ASCII Chart - 4 Column Char, Dec, Oct, Hex - POSIX printf

/*
 * ISO C does not support POSIX %n$ operand number formats, but a nice twist
 * with POSIX printf conversion (will generate expected -Wformat warning).
 *
 * (for pure standard C, see "ASCII Chart - 4 Column Char, Dec, Oct, Hex")
 */

#include <stdio.h>

enum { TWO = 1u << 1, R1 = 1u << 5, R2 = 1u << 6, R3 = R1 + R2 };

#define HDG "Char  Dec  Oct  Hex"
#define SEP "-------------------------------------------"
#define FMT "%-5s %3d %04o 0x%02x"

int main (void) {

    /* non-printing characters */
    char *special[] = { "(nul)", "(soh)", "(stx)", "(etx)",
                        "(eot)", "(enq)", "(ack)", "(bel)",
                        "(bs)",  "(ht)",  "(nl)",  "(vt)",
                        "(np)",  "(cr)",  "(so)",  "(si)",
                        "(dle)", "(dc1)", "(dc2)", "(dc3)",
                        "(dc4)", "(nak)", "(syn)", "(etb)",
                        "(can)", "(em)",  "(sub)", "(esc)",
                        "(fs)",  "(gs)",  "(rs)",  "(us)",
                        "(sp)",  "(del)" };

    /* print heading (with non-ISO C %n$ conversion specifiers) */
    printf ("%1$s | %1$s | %1$s | %1$s\n%2$s%3$s\n", HDG, SEP, 1+SEP);

    /* print chars */
    for (unsigned i = 0; i < R1; i++) {
        char s1[TWO] = { (char)i+R1, 0 },
             s2[TWO] = { (char)i+R2, 0 },
             s3[TWO] = { (char)i+R3, 0 };

        printf (FMT " | " FMT " | " FMT " | " FMT "\n",
                special[i], i, i, i,
                i ? s1 : special[R1], i+R1, i+R1, i+R1,
                s2, i+R2, i+R2, i+R2,
                i == R1 - 1 ? special[R1+1] : s3, i+R3, i+R3, i+R3);
    }

    return 0;
}