The C Programming Language, 2nd Edition, by Kernighan and Ritchie
Exercise 7.03 on page 156

Revise minprintf to handle more of the other facilities of printf .

Solution by Gregory Pietsch

/* Gregory Pietsch - K&R2 Exercise 7-3 - 2001-01-28 gkp@flash.net */

#include <stdarg.h>
#include <stdio.h>

/* minprintf:  minimal printf with variable argument list */
void minprintf(char *fmt, ...)
{
    va_list ap;
    char *p, *sval;
    int ival;
    double dval;
    unsigned uval;

    va_start(ap, fmt);    /* make ap point to the first unnamed arg */
    for (p = fmt; *p; p++) {
        if (*p != '%') {
            putchar(*p);
            continue;
        }
        switch (*++p) {
        case 'd':
        case 'i':
            ival = va_arg(ap, int);
            printf("%d", ival);
            break;
        case 'c':
            ival = va_arg(ap, int);
            putchar(ival);
            break;
        case 'u':
            uval = va_arg(ap, unsigned int);
            printf("%u", uval);
            break;
        case 'o':
            uval = va_arg(ap, unsigned int);
            printf("%o", uval);
            break;
        case 'x':
            uval = va_arg(ap, unsigned int);
            printf("%x", uval);
            break;
        case 'X':
            uval = va_arg(ap, unsigned int);
            printf("%X", uval);
            break;
        case 'e':
            dval = va_arg(ap, double);
            printf("%e", dval);
            break;
        case 'f':
            dval = va_arg(ap, double);
            printf("%f", dval);
            break;
        case 'g':
            dval = va_arg(ap, double);
            printf("%g", dval);
            break;
        case 's':
            for (sval = va_arg(ap, char *); *sval; sval++)
                putchar(*sval);
            break;
        default:
            putchar(*p);
            break;
        }
    }
    va_end(ap);
}

/* end of function */

Solution by anonymous

I implemented all of the basic format specifiers for C that I could find except for %n. Also, I didn't include support for flags, field width, precision, character length, and length modifiers.

#include <stdio.h>
#include <stdarg.h>

/*
    Exercise 7-3. Revise minprintf to handle more of the other facilities of printf.
*/

void minprintf(char *fmt, ...);

int main()
{
    char *sval = "10";
    int ival = 11;
    double dval = 12.13;
    unsigned int uival = 14;
    char cval = 'F';
    void *pval = sval;
    minprintf("%%s=%s, %%d=%d, %%i=%i, %%f=%f, %%e=%e, %%E=%E, %%g=%g, %%G=%G, %%o=%o, %%x=%x\n", sval, ival, ival, dval, dval, dval, dval, dval, uival, uival);
       printf("%%s=%s, %%d=%d, %%i=%i, %%f=%f, %%e=%e, %%E=%E, %%g=%g, %%G=%G, %%o=%o, %%x=%x\n", sval, ival, ival, dval, dval, dval, dval, dval, uival, uival);
    minprintf("%%X=%X, %%u=%u, %%c=%c, %%p=%p, %%%%=%%\n", uival, uival, cval, pval);
       printf("%%X=%X, %%u=%u, %%c=%c, %%p=%p, %%%%=%%\n", uival, uival, cval, pval);
}

// minimal printf with variable argument list
void minprintf(char *fmt, ...)
{
    va_list ap; // points to each unnamed arg in turn
    char *p, *sval, format[3] = { '%', '\0', '\0' };
    int ival;
    double dval;
    unsigned int uival;
    void *pval;

    va_start(ap, fmt); // make ap point to 1st unnamed arg
    for (p = fmt; *p; p++)
    {
        if (*p != '%')
        {
            putchar(*p);
            continue;
        }
        switch (*++p)
        {
        case 'd': case 'i': case 'c': // char is promoted to int when passed through '...'
            ival = va_arg(ap, int);
            format[1] = *p;
            printf(format, ival);
            break;
        case 'f': case 'e': case 'E': case 'g': case 'G':
            dval = va_arg(ap, double);
            format[1] = *p;
            printf(format, dval);
            break;
        case 's':
            for (sval = va_arg(ap, char *); *sval; sval++)
                putchar(*sval);
            break;
        case 'x': case 'X': case 'o':  case 'u':
            uival = va_arg(ap, unsigned int);
            format[1] = *p;
            printf(format, uival);
            break;
        case 'p':
            pval = va_arg(ap, void *);
            printf("%p", pval);
            break;
        case '%':
            putchar(*p);
            break;
        default: // something is not handled, so print out the format string
            putchar('%');
            putchar(*p);
            break;
        }
    }
    va_end(ap); // clean up when done
}