Question

Good afternoon! I have some doubts about following code.

[1] What does the middle condition in the for loop mean? ; *p; and ; *sval; – reaching an end of input string? If yes, how is it determined?

[2] I do not understand that complicated for loop.

Let’s suppose that if condition is not satisfied *p == % so we go into switch right away. Now let’s consider opposite case, we enter every other char despite %, if is satisfied so we use continue and also go to switch after that. What’s the difference between those 2 cases then ?
I must be terribly wrong with sth, but I can’t find my mistake for over 2 hours now…

#include <stdarg.h>
/* minprintf: minimal printf with variable argument list */
void minprintf(char *fmt, ...)
{
    va_list ap; /* points to each unnamed arg in turn */
    char *p, *sval;
    int ival;
    double dval;

    va_start(ap, fmt); /* make ap point to 1st unnamed arg */

    for (p = fmt; *p; p++) {         /* [1] */
        if (*p != '%') {
            putchar(*p);
            continue;                /* [2] */
        }
        switch (*++p) {
        case 'd':
            ival = va_arg(ap, int);
            printf("%d", ival);
            break;
        case 'f':
            dval = va_arg(ap, double);
            printf("%f", dval);
            break;
        case 's':
            for (sval = va_arg(ap, char *); *sval; sval++)
                putchar(*sval);
            break;
        default:
            putchar(*p);
            break;
        }
    }
    va_end(ap); /* clean up when done */
}

Help is greatly appreciated!

Answer 1

When we continue, we just print the character, and skip the switch altogether. The loop goes through the format line looking for (1) a terminating zero, or (2) a percent sign.

When the terminating zero is reached, the loop ends:

for (p = fmt; *p; p++)
//            ^^^ Here is the condition: *p != 0 is implied

When a percent sign is reached, the following symbol is taken as a format character, and the next vararg is interpreted according to it.

---

There is a bug in this program – it produces a read past the end of the format string (an undefined behavior) when the percent symbol is the last character of the format string. Try this:

minprintf("%\0bug!bug!bug!");

The simulated end-of-line marker \0 is skipped, and the bug!bug!bug! output is produced. The reason for this is an unconditional pre_increment of p in the header of the switch statement. To fix this problem, add the following case to the switch:

 case '\0':
    p--; // reverse the ++p from the header of the switch
    break;