A simple c function proxying standard printf slightly modifying contents on the way.

prontf.c

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

int prontf(const char *fmt, ...) {
    va_list varargs;
    va_start(varargs, fmt);
    int bufsize = vsnprintf(NULL, 0, fmt, varargs) + 1;
    va_end(varargs);

    if (bufsize <= 0)
        return bufsize;

    char buf[bufsize];
    buf[bufsize - 1] = '\0';

    va_start(varargs, fmt);
    int cwr = vsnprintf(buf, bufsize, fmt, varargs);
    va_end(varargs);

    assert(cwr <= bufsize);
    for (char *c = buf; *c != '\0'; ++c)
        *c = *c == 'i' ? 'o' : *c;
    return printf("%s", buf);
}

int main() {
    prontf("Think different");
    return 0;
}