use eargf.
-- 
/sys/src/cmd/disk/format.c
format.c.orig:181,187 - /n/sources/patch/applied/formateargf/format.c:181,187
  	memmove(label, "CYLINDRICAL", sizeof(label));
  	ARGBEGIN {
  	case 'c':
- 		clustersize = atoi(ARGF());
+ 		clustersize = atoi(EARGF(usage()));
  		break;
  	case 'd':
  		dos = 1;
format.c.orig:191,197 - /n/sources/patch/applied/formateargf/format.c:191,197
  		fflag = 1;
  		break;
  	case 'l':
- 		a = ARGF();
+ 		a = EARGF(usage());
  		n = strlen(a);
  		if(n > sizeof(label))
  			n = sizeof(label);
format.c.orig:200,213 - /n/sources/patch/applied/formateargf/format.c:200,213
  			label[n++] = ' ';
  		break;
  	case 'b':
- 		pbs = ARGF();
+ 		pbs = EARGF(usage());
  		writepbs = 1;
  		break;
  	case 'r':
- 		nresrv = atoi(ARGF());
+ 		nresrv = atoi(EARGF(usage()));
  		break;
  	case 't':
- 		type = ARGF();
+ 		type = EARGF(usage());
  		break;
  	case 'v':
  		chatty++;