Выравнивание строк

например, так:

#include <stdio.h>

int main(void)
{
        char *s0 = (char *) malloc(16), *s1 = (char *) malloc(16);
        char *t0 = "first", *t1 = "this is longest line", *t2 = "last";
        sprintf(s0, "%%%is\n", (strlen(t1) + strlen(t0))/2);
        printf(s0, t0);
        printf("%s\n", t1);
        printf("%s\n", t2);
}