seekf
static off_t seekf(void *, off_t, int);
if (!(cshin = funopen((void *) &SHIN, readf, writef, seekf, closef)))
if (!(cshout = funopen((void *) &SHOUT, readf, writef, seekf, closef)))
if (!(csherr = funopen((void *) &SHERR, readf, writef, seekf, closef)))