1 files changed, 100 insertions, 0 deletions
diff --git a/usr.bin/file/is_tar.c b/usr.bin/file/is_tar.c
new file mode 100644
index 0000000..5b036be
--- /dev/null
+++ b/usr.bin/file/is_tar.c
@@ -0,0 +1,100 @@
+/*
+ * is_tar() -- figure out whether file is a tar archive.
+ *
+ * Stolen (by the author!) from the public domain tar program:
+ * Pubic Domain version written 26 Aug 1985 John Gilmore (ihnp4!hoptoad!gnu).
+ *
+ * @(#)list.c 1.18 9/23/86 Public Domain - gnu
+ * $Id: is_tar.c,v 1.8 1993/09/16 21:09:35 christos Exp $
+ *
+ * Comments changed and some code/comments reformatted
+ * for file command by Ian Darwin.
+ */
+
+#include <string.h>
+#include <ctype.h>
+#include <sys/types.h>
+#include "tar.h"
+
+#define	isodigit(c)	( ((c) >= '0') && ((c) <= '7') )
+
+#if	defined(__STDC__) || defined(__cplusplus)
+static long from_oct(int, char*);	/* Decode octal number */
+#else
+static long from_oct();
+#endif
+
+/*
+ * Return 
+ *	0 if the checksum is bad (i.e., probably not a tar archive), 
+ *	1 for old UNIX tar file,
+ *	2 for Unix Std (POSIX) tar file.
+ */
+int
+is_tar(buf, nbytes)
+unsigned char *buf;
+int nbytes;
+{
+	register union record *header = (union record *)buf;
+	register int	i;
+	register long	sum, recsum;
+	register char	*p;
+
+	if (nbytes < sizeof(union record))
+		return 0;
+
+	recsum = from_oct(8,  header->header.chksum);
+
+	sum = 0;
+	p = header->charptr;
+	for (i = sizeof(union record); --i >= 0;) {
+		/*
+		 * We can't use unsigned char here because of old compilers,
+		 * e.g. V7.
+		 */
+		sum += 0xFF & *p++;
+	}
+
+	/* Adjust checksum to count the "chksum" field as blanks. */
+	for (i = sizeof(header->header.chksum); --i >= 0;)
+		sum -= 0xFF & header->header.chksum[i];
+	sum += ' '* sizeof header->header.chksum;	
+
+	if (sum != recsum)
+		return 0;	/* Not a tar archive */
+	
+	if (0==strcmp(header->header.magic, TMAGIC)) 
+		return 2;		/* Unix Standard tar archive */
+
+	return 1;			/* Old fashioned tar archive */
+}
+
+
+/*
+ * Quick and dirty octal conversion.
+ *
+ * Result is -1 if the field is invalid (all blank, or nonoctal).
+ */
+static long
+from_oct(digs, where)
+	register int	digs;
+	register char	*where;
+{
+	register long	value;
+
+	while (isspace(*where)) {		/* Skip spaces */
+		where++;
+		if (--digs <= 0)
+			return -1;		/* All blank field */
+	}
+	value = 0;
+	while (digs > 0 && isodigit(*where)) {	/* Scan til nonoctal */
+		value = (value << 3) | (*where++ - '0');
+		--digs;
+	}
+
+	if (digs > 0 && *where && !isspace(*where))
+		return -1;			/* Ended on non-space/nul */
+
+	return value;
+}