segmentation fault and array issue - c++

Question

I've been going back trying to find a segmentation error in my program. Very often when the program crashes it is at this point.
unsigned long data = octets[g];
So I have tracked this buffer as being created in the main loop with a fixed defined size. However since it's defined in a if statement in main does it need to be allocated with "new"? Basically after receiving from a TCP socket the char buffer is copied to an unsigned char buffer to check for certain binary data types. So only if data arrives is this called into existance.
INT8U byteArray[BUFFERSIZE];
This buffer is then passed for message ID and crc checking. Is not doing a "new" type allocation the issue because it is in the main loop? I thought it would go out of scope at the end of the "if new data is received" statement.
long calc_crc24q(byte* octets, int start, int last) //start is first byte, last is MSbyte of CRC
{
long crc = CRC24SEED;
for(int g = start; g < last; g++) //should xor from preamble to the end of data
{
unsigned long data = octets[g]; //fault occurs here often
crc = crc ^ data << 16;
for (int i = 0; i < 8; i++)
{
crc <<= 1;
if (crc & 0x1000000)
crc = crc ^ CRC24POLY;
}
}
return crc & 0x00ffffff; //returns an int value with high byte 00 then data in lower 3 bytes
}
//---------------------------------------------
Here is the message id
unsigned int id_message(INT8U* buffer, unsigned int posStart, unsigned int numberbytes, unsigned int& messageLength)
{
unsigned int messID = 0;
unsigned int posEnd;
unsigned int noBytes = 0;
if(buffer[posStart] == Preamble)
{
unsigned int dataLength = (((0x0000 | buffer[posStart+1]) << 8) | buffer[posStart+2]); //0x byte1 byte2
messID = ((0x0000 | (buffer[posStart+3] << 4)) | ((buffer[posStart+4] >> 4) & 0x0F)); //byte1 shift 4 bits add upper 4 bits of byte 2
noBytes = dataLength + 6;
//numberbytes = noBytes;
posEnd = posStart + noBytes - 1;
if(calc_crc24q( buffer, posStart, posEnd-2) != (((0x00000000 | buffer[posEnd-2]) << 16) | ((0x00000000 | buffer[posEnd-1]) << 8) | (0x00000000 | buffer[posEnd])) )
{
cout << "CRC error" << endl;
return 0;
}
//return message type extracted from data segment
messageLength = posStart + noBytes;
return messID;
}
return 255; //unknown type
}