Basic printf implementation supporting strings and ints.

my_printf.c

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

void fmt_int(char* buf, int n) {
    int i = 0;
    if (n < 0) {
        *buf++ = '-';
        n = -n;
    }

    while (n > 0) {
        buf[i++] = (n % 10) + '0';
        n /= 10;
    }

    for (int j = 0; j < i / 2; j++) {
        char tmp = buf[j];
        buf[j] = buf[i - j - 1];
        buf[i - j - 1] = tmp;
    }

    buf[i] = '\0';
}

int my_printf(const char* format, ...) {
    va_list args;
    va_start(args, format);

    int n = 0;

    while (*format != '\0') {
        switch (*format) {
        case '%':
            format++;
            switch (*format) {
            case '%':
                putchar('%');
                n++;
                break;
            case 's':
                char* s = va_arg(args, char*);
                while (*s != '\0') {
                    putchar(*s);
                    n++;
                    s++;
                }
                break;
            case 'd':
                int d = va_arg(args, int);
                char buf[11];
                fmt_int(buf, d);
                for (char* pbuf = buf; *pbuf != '\0'; pbuf++) {
                    putchar(*pbuf);
                    n++;
                }
                break;
            default:
                putchar('?');
                n++;
            }
            break;
        default:
            putchar(*format);
            n++;
        }

        format++;
    }

    va_end(args);

    return n;
}

int main(int argc, char** argv) {
    my_printf("Hello %s %d\n", "world", 567);

    return 0;
}