How to check whether a file is in tar format?

I want to implement a check of a tar file. I am not interested only to check the file extension, but I need surer way to check. For example for zip format I could check some lead bytes. But what is the way for tar files?

Regards, B

Solution

Check the magic bytes at offset 257. If they match "ustar" including the null terminator, the file is probably a tar.

See: http://www.gnu.org/software/tar/manual/html_node/Standard.html

/* 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 */
                                /* 500 */
};

#define TMAGIC   "ustar"        /* ustar and a null */
#define TMAGLEN  6