exchild
exchild(t->left, flags|XPIPEO|XCCLOSE, NULL, pv[0]);
i = exchild(t, flags|XPCLOSE, xerrok, 0);
exchild(t->left, flags|XBGND|XFORK|XCOPROC|XCCLOSE,
rv = exchild(&texec, flags, xerrok, -1);
return exchild(t, flags & ~XTIME, xerrok, -1); /* run in sub-process */
int exchild(struct op *, int, volatile int *, int);