读取二进制文件的问题

Question

I'm trying to read the first 6 bytes of a file, but it's giving me weird results, and I can't seem to figure out what I'm doing wrong.

My code:

struct Block {
    char fileSize[3];
    char initialDataBlockId[3];
};

int main(int c, char **a) {

    ifstream file("C\\main_file_cache.idx0", ios::binary);

    Block block;

    file.get((char*)&block, sizeof(block));

    printf("File Size: %i\n", block.fileSize);
    printf("Initial Data Block ID: %i\n", block.initialDataBlockId);

    file.close();

    system("pause");
    return 0;
}

Before I ran the code, I opened the file in a binary editor, and it showed me this hex code:

    00 00 00 00-00 00 05 af-4b 00 00 01-26 df cd 00
    00 6f 03 3f-ed 00 03 61-05 08 35 00-04 8b 01 61
    59 00 08 39-03 23 0a 00-05 6c 00 35-d0 00 06 fe
    03 69 d8 00-07 19

There are a total of 54 bytes. The first 6 bytes are just zero.

So, I expected my program to produce the following output:

File Size: 0
Initial Data Block ID: 0

Instead, the outputs is as follows:

File Size: 10419128
Initial Data Block ID: 10419131

This result makes no sense. Maybe there is something wrong with my code?

Answer 1

You should use type unsigned char in your Block structure.

You should use file.read() to read binary data instead of file.get() .

You are printing the addresses of the arrays in the Block structure, not their contents, furthermore the specifier %i expects an int , not a char * , so the behavior in undefined and you get some weird integer value but anything culd have happened, including program termination. Increasing the warning level is advisable so the compiler warns about such silly mistakes.

If the file format is little endian, you could convert these 3 byte arrays to numbers this way:

int block_fileSize =  (unsigned char)block.fileSize[0] + 
                     ((unsigned char)block.fileSize[1] << 8) + 
                     ((unsigned char)block.fileSize[2] << 16);
int block_initialDataBlockId =  (unsigned char)block.initialDataBlockId[0] +
                               ((unsigned char)block.initialDataBlockId[1] << 8) +
                               ((unsigned char)block.initialDataBlockId[2] << 16);
printf("File Size: %i\n", block_fileSize);
printf("Initial Data Block ID: %i\n", block_initialDataBlockId);

Answer 2

If you want to read a binary data you can use a read method from ifstream and also write method from ofstream .

istream & ifstream::read (char * s, streamsize n);
ostream & ofstream::write (const char * s, streamsize n);

You have to know that binary mode is useless for UNIX systems and text mode is only useful.