integer.c

/* integer.c creates and reads special integers from tar headers so that 
 * they may be compatible with GNU's tar */

#include <arpa/inet.h>
#include <string.h>
#include <stdint.h>

uint32_t extract_special_int(char *where, int len) {
  /* For interoperability with GNU tar. GNU seems to
  * set the high–order bit of the first byte, then
  * treat the rest of the field as a binary integer
  * in network byte order.
  * I don’t know for sure if it’s a 32 or 64–bit int, but for
  * this version, we’ll only support 32. (well, 31)
  * returns the integer on success, –1 on failure.
  * In spite of the name of htonl(), it converts int32 t
  */
  int32_t val= -1;
  if ( (len >= sizeof(val)) && (where[0] & 0x80)) {
    /* the top bit is set and we have space
    * extract the last four bytes */
    val = *(int32_t *)(where+len-sizeof(val));
    val = ntohl(val); /* convert to host byte order */
  }
  return val;
}
int insert_special_int(char *where, size_t size, int32_t val) {
  /* For interoperability with GNU tar. GNU seems to
  * set the high–order bit of the first byte, then
  * treat the rest of the field as a binary integer
  * in network byte order.
  * Insert the given integer into the given field
  * using this technique. Returns 0 on success, nonzero
  * otherwise
  */
  int err=0;
  if ( val < 0 || ( size < sizeof(val)) ) {
    /* if it’s negative, bit 31 is set and we can’t use the flag
    * if len is too small, we can’t write it. Either way, we’re
    * done.
    */
    err++;
  } else {
    /* game on....*/
    memset(where, 0, size); /* Clear out the buffer */
    *(int32_t *)(where+size-sizeof(val)) = htonl(val); /* place the int */
    *where |= 0x80; /* set that high–order bit */
  }
  return err;
}