safe_strtoi
static int safe_strtoi(char const *, char *);
interval = safe_strtoi(argv[optind], "invalid interval");
count = safe_strtoi(argv[optind], "invalid count");
t = safe_strtoi(argv[optind], "invalid sampling interval");
t = safe_strtoi(argv[optind], "invalid sampling interval");
n = 1 + safe_strtoi(argv[optind+1], "invalid sample count");
static int safe_strtoi(char const *, char *);
df.if_max_iodevs = safe_strtoi(optarg, "invalid limit");
interval = safe_strtoi(argv[optind], "invalid interval");
iter = safe_strtoi(argv[optind], "invalid count");
static int safe_strtoi(char const *val, char *errmsg);