read 32bit integer from binary file

问题:

My binary file looks like this.

00000000: 0000 0803 0000 ea60 0000 001c 0000 001c
00000010: 0000 0000 0000 0000 0000 0000 0000 0000

left column is address.

I just tried to read 0000 0803(=2051) as follows

ifstream if;
if.open("file");
uint32_t a;
if >> a;

As expected...It did not work :-(
a was just 0 after execution.
I tried long, int, unsigned int, unsigned long. All failed.

Why these are not working and how can I achieve the goal?

回答1:

You have two issues:

1. Insuring you read the bytes you intend (no fewer, no more) from the stream.

   I'd recommend this syntax:

   uint32_t a;

   inFILE.read(reinterpret_cast<char *>(&a), sizeof(a));

2. Insure you're interpreting those bytes with the correct byte order.

   Q: If you're on a PC, your CPU is probably little endian. Do you know if your data stream is also little-endian, or is it big endian?

   If the data is big-endian, I'd consider the standard networking functions to accomodate byte order: ntohl(), etc: http://www.retran.com/beej/htonsman.html

ALSO:

Follow Hcorg's and Daniel Jour's advice: don't forget about the "open mode" parameter, and don't forget to check for "file open" errors.

回答2:

Open file in binary mode and then use read() method, something like:

uint32_t a;
ifstream file ("file", ios::in | ios::binary);
if (file.is_open())
{
     file.read ((char*)&a, sizeof(a));
}