Re-write by John Gilmore, from his P.D. tar program.

diff --git a/src/is_tar.c b/src/is_tar.c
index 7f840624ac1bd69fde3d34e115665e21fcfba742..31b949994e1ba849fe2505acd7f3f35723e4a407 100644 (file)
--- a/src/is_tar.c
+++ b/src/is_tar.c
@@ -1,44 +1,81 @@
 /*
- * istar() - see if it is a tar file. Found in code written by
- *     Fred Blonder (301) 454-7690
- *     harpo!seismo!umcp-cs!fred
- *     Fred@Maryland.ARPA
- *     Net.sources, <9@gyre.uucp>, October 1983
- * I changed the calling sequence, added a cast or two, and ran through cb -sj.
+ * 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 by John Gilmore (ihnp4!hoptoad!gnu).
+ *
+ * @(#)list.c 1.18 9/23/86 Public Domain - gnu
  */
+#include <ctype.h>
+#include <sys/types.h>
+#include "tar.h"
 
-is_tar(buf)    /* Is it a tar file? */
-char *buf;
+#define        isodigit(c)     ( ((c) >= '0') && ((c) <= '7') )
+
+long from_oct();                       /* Decode octal number */
+
+/*
+ * Return 1 for old tar file, 0 if the checksum is bad, 2 for Unix Std tar file.
+ */
+int
+is_tar(header)
+       register union record *header;
 {
-       /* This stuff is copied from tar.c. */
-#define TBLOCK 512
-#define NAMSIZ 100
-       register struct header {
-               char    name[NAMSIZ];
-               char    mode[8];
-               char    uid[8];
-               char    gid[8];
-               char    size[12];
-               char    mtime[12];
-               char    chksum[8];
-               char    linkflag;
-               char    linkname[NAMSIZ];
-       };
-       register comp_chksum;
-       register char   *cp;
-       int     header_chksum;
-
-       /* Compute checksum */
-       comp_chksum = 0;
-       for (cp = buf; cp < &buf[TBLOCK]; cp++)
-               comp_chksum += (cp < ((struct header *)buf)->chksum
-                    || cp >= &((struct header *)buf)->chksum[8]) ? 
-                   *cp : ' ';
-
-       /* Convert checksum field to integer */
-       (void) sscanf(((struct header *)buf)->chksum, "%o", &header_chksum);
-
-       return (comp_chksum == header_chksum); /* Is checksum correct? */
+       register int    i;
+       register long   sum, recsum;
+       register char   *p;
+
+       recsum = from_oct(8,  header->header.chksum);
+
+       sum = 0;
+       p = header->charptr;
+       for (i = sizeof(*header); --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).
+ */
+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;
+}