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Read a file in binary

YamSeng asked
Medium Priority
Last Modified: 2010-08-05
I would like to know how to open/read a file in binary.  That means I should have a pointer pointing to the first bit.
And also, can I overwrite the bit while I'm reading it?
I'm programming in Win95 using Microsoft Visual C++ 4.0. I just want to prgram in C.  

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Open a file as binary mode and write/read:
   FILE *fp = fopen(filename, "r+b");

Operation for each bit:
   It seems difficult to operate each bit. As I know, we usually
   operate for each byte by stream fp. I think you can define a
   union to access each bit, then read/write to the file as one

Hope this is helpful.


can show example to operate on each byte and then retreiving every bit from every single byte?


Actually, I need to do an XOR operation on the byte.  If I can do XOR operation on a byte (given another byte), then I do not need to use the bit.

If you just need to XOR a byte, do this:

unsigned char readValue;  // The value read from the file.
unsigned char result;     // The resultant value from the XOR.
unsigned char xorValue;   // The value to XOR readValue with.

readValue = ...;
xorValue = ...;

result = readValue ^ xorValue;

and result will hold the desired value.

By the way, If you still want to access an individual bit, you can do this:

unsigned char initialValue;   // The byte containing the data.
int bitNumber;                // The bit you want to check.
                              // This will be 1 - 8 where 1 is
                              // the rightmost bit.

valueOfBit = initialValue & pow(2, bitNumber-1);

valueOfBit will contain either a 1 or a 0 depending on the value of the byte being checked.  You may also need to include math.h for pow().
YamSeng - Try out the above comments I have already added and let me know if that helps.  If not, add another comment to this and I may be able to provide come more info.

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So, if I want to check the nth value should I use

valueOfBit = initialValue & pow(2, bitNumber-n);



OK - I've just tried it myself and I was slightly wrong before.  This WILL work:

  unsigned char initialValue; // The byte containing the data.
  int bitNumber;              // The bit you want to check.
                              // This will be 1 - 8 where 1 is
                              // the rightmost bit.

  initialValue = ...;
  bitNumber = ...;
  int valueOfBit = initialValue & (int)pow(2, bitNumber-1);

if valueOfBit is 0 then the bit being checked was 0, if it is non-zero, then the bit was set to 1.

So, back to your comment, if you want to check the nth value, you would set bitNumber to be n.

Eg: If you have a byte as follows:
00001101 (13)

A value of 1, 3 and 4 for bitNumber in the above statement would return a non-zero value (bit is set) and values of 2, 5, 6, 7 and 8 would return 0 (bit is not set.

In terms of the nth value:
valueOfBit = initialValue & (int)pow(2, n-1);

Sorry for any confusion from my previous comment.  Again, if you need more info, just add a comment.

Thanks for being helpful.
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