Add parseoffset()

This is a utility function to allow easy parsing of file or other
offsets, automatically taking in regard suffixes, proper bases and
so on, for instance used in split(1) -b or od -j, -N(1).
Of course, POSIX is very arbitrary when it comes to defining the
parsing rules for different tools.
The main focus here lies on being as flexible and consistent as
possible. One central utility-function handling the parsing makes
this stuff a lot more trivial.

libutil/parseoffset.c

@@ -0,0 +1,59 @@
+/* See LICENSE file for copyright and license details. */
+#include <ctype.h>
+#include <inttypes.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "../util.h"
+
+off_t
+parseoffset(const char *str)
+{
+	off_t res;
+	size_t scale;
+	int base = 10;
+	char *end;
+
+	/* bases */
+	if (!strncasecmp(str, "0x", strlen("0x"))) {
+		base = 16;
+	} else if (*str == '0') {
+		str++;
+		base = 8;
+	}
+
+	res = strtol(str, &end, base);
+	if (res < 0) {
+		weprintf("invalid file offset: %s\n", str);
+		return -1;
+	}
+
+	/* suffix */
+	if (*end) {
+		switch (toupper((int)*end)) {
+		case 'B':
+			scale = 512L;
+			break;
+		case 'K':
+			scale = 1024L;
+			break;
+		case 'M':
+			scale = 1024L * 1024L;
+			break;
+		case 'G':
+			scale = 1024L * 1024L * 1024L;
+			break;
+		default:
+			weprintf("invalid file offset suffix: %s\n", str);
+			return -1;
+		}
+	}
+
+	/* prevent overflow */
+	if (res > (SIZE_MAX / scale)) {
+		weprintf("file offset out of range: %s\n", str);
+		return -1;
+	}
+
+	return res;
+}