Question

Basically I have

void FileReader::parseBuffer(char * buffer, int length)
{
  //start by looking for a vrsn
  //Header seek around for a vrns followed by 32 bit size descriptor
  //read 32 bits at a time
  int cursor = 0;
  char vrsn[5] = "vrsn";
  cursor = this->searchForMarker(cursor, length, vrsn, buffer);
  int32_t size = this->getObjectSizeForMarker(cursor, length, buffer);
  cursor = cursor + 8; //advance cursor past marker and size
  wchar_t *version = this->getObjectForSizeAndCursor(size, cursor, buffer);
  cout << version << "\n";
  delete[] version;
}

wchar_t* FileReader::getObjectForSizeAndCursor(int32_t size, int cursor, char *buffer) {
  wchar_t *destination = NULL;
  destination = new wchar_t[(size/2)+1];
  memcpy(destination, buffer + cursor, size);
  return destination;
}

in my example say i have the following bytes

7672736E – marker vrsn

00000040 – size of string to follow

0032002E0030002F00530065007200610074006F002000530063007200610074006300680020004C004900560045002000440061007400610062006100730065 – string

the string uses 16 bytes per character, so i cannot use a char * for the actual string, wchar_t seems like the best bet.

However when i memcpy these bytes to a wchar_t i get 0x7fe7abc037e0 in cout which i assume is a pointer?

which seems wrong. when i use wcout i get nothing in the terminal.

Will memcpy not work for this?

also should my wchar_t size be halved since i only have half as many wchar_t’s as i would have chars?

size is a byte count.

Answer 1

Your string looks like it’s in big endian order, but you are (probably) using a little-endian machine. Therefore, your wchar_t string is full of characters like U+3200 which your terminal might not correctly print out.

You will have to properly flip every one of those characters. Your best bet is to just treat the input as a unsigned char * and copy the elements one-at-a-time into your wchar_t * with the correct endian reversal:

int wlen = size/2;
uint16_t *dest = new uint16_t[wlen + 1];
unsigned char *ptr = (unsigned char *)(buffer + cursor);
for(int i=0; i<wlen; i++) {
    dest[i] = (ptr[0] << 8) + ptr[1];
    ptr += 2;
}
dest[wlen] = 0;
return dest;

Note that you can’t assume wchar_t is 16 bits wide. If you can use C++11 features, use char16_t.