YamSeng
asked on
Read a file in binary
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.
thanks!
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.
thanks!
ASKER
can show example to operate on each byte and then retreiving every bit from every single byte?
ASKER
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.
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().
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().
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So, if I want to check the nth value should I use
valueOfBit = initialValue & pow(2, bitNumber-n);
?
valueOfBit = initialValue & pow(2, bitNumber-n);
?
ASKER
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.
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.
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
byte.
Hope this is helpful.