TAR文件格式问题

Question

我不清楚,什么是正确的.tar文件格式,因为我在三个场景中遇到了正确的功能(见下文).

根据.tar我一直在使用的规范,magic字段(ustar)是以空字符结尾的字符串,version字段是八进制数字,没有尾随空值.

但是我查看了.tar我在服务器上找到的几个文件,我发现了不同的字段和字段的实现,magic并且version它们中的三个似乎都正常工作,可能是因为系统忽略了这些字段.

在以下示例>>中查看单词ustar和root之间的不同(3)字节

场景1(20 20 00):

 000000F0      00 00 00 00 | 00 00 00 00 | 00 00 00 00      ............
 000000FC      00 00 00 00 | 00 75 73 74 | 61 72 20 20      .....ustar  
 00000108      00 72 6F 6F | 74 00 00 00 | 00 00 00 00      .root.......
 00000114      00 00 00 00 | 00 00 00 00 | 00 00 00 00      ............

场景2(00 20 20):

 000000F0      00 00 00 00 | 00 00 00 00 | 00 00 00 00      ............
 000000FC      00 00 00 00 | 00 75 73 74 | 61 72 00 20      .....ustar. 
 00000108      20 72 6F 6F | 74 00 00 00 | 00 00 00 00      root.......
 00000114      00 00 00 00 | 00 00 00 00 | 00 00 00 00      ............

场景3(00 00 00):

 000000F0      00 00 00 00 | 00 00 00 00 | 00 00 00 00      ............
 000000FC      00 00 00 00 | 00 75 73 74 | 61 72 00 00      .....ustar..
 00000108      00 72 6F 6F | 74 00 00 00 | 00 00 00 00      .root.......
 00000114      00 00 00 00 | 00 00 00 00 | 00 00 00 00      ............

哪一种格式正确？

Answer 1

在我看来,没有一个例子是正确的,至少不是POSIX格式.
你可以在这里阅读:

/* tar Header Block, from POSIX 1003.1-1990. */
/* POSIX header */

struct posix_header {   /* byte offset */
  char name[100];               /*   0 */
  char mode[8];                 /* 100 */
  char uid[8];                  /* 108 */
  char gid[8];                  /* 116 */
  char size[12];                /* 124 */
  char mtime[12];               /* 136 */
  char chksum[8];               /* 148 */
  char typeflag;                /* 156 */
  char linkname[100];           /* 157 */
  char magic[6];                /* 257 */
  char version[2];              /* 263 */
  char uname[32];               /* 265 */
  char gname[32];               /* 297 */
  char devmajor[8];             /* 329 */
  char devminor[8];             /* 337 */
  char prefix[155];             /* 345 */
};

#define TMAGIC   "ustar"        /* ustar and a null */
#define TMAGLEN  6
#define TVERSION "00"           /* 00 and no null */
#define TVERSLEN 2

您的第一个示例(Scenario 1)的格式似乎与旧的GNU标头格式匹配:

/* OLDGNU_MAGIC uses both magic and version fields, which are contiguous.
   Found in an archive, it indicates an old GNU header format, which will be
   hopefully become obsolescent.  With OLDGNU_MAGIC, uname and gname are
   valid, though the header is not truly POSIX conforming */

#define OLDGNU_MAGIC "ustar  "  /* 7 chars and a null */

在第二个和第三个示例(Scenario 2和Scenario 3)中,该version字段设置为意外值(根据上述文档,正确的值应为00ASCII或0x30 0x30十六进制),因此该字段很可能被忽略.