Dumping bits/bytes to a file in binary mode

Question

Hello,

I have below code.

FILE *fp;
int a;
fp=fopen("dump.bin","wb");         
a = 0xffafbcdf;
fprintf(fp,"%x",a&0x3ffff);

I am trying to dump only 18 LSBits of variable a. But the value dumped in the file is 3bcdf.

My question - Is it not possible to dump/fwrite/fprintf desired number of bits which is not a multiple of 4(nibble) to a file?

Thank you

EDIT:

1.) When i checked the output of my file dump i realized, since i am opening file in binrary mode, i should not be using fprintf, but i should use fwrite.

2.) What i see in the output is DF BC 03 , it writes in multiples of 8 bits, so it writes 24 bits(3 bytes) but i was interested in only 18 bits. But then i realised thatno file write library will be able to write non-multiple of 8 bits to a file. it will always add leading zero bits to complete the byte and then dump it. "Least count unit" for a file data is a byte.

-AD.

Answer 1

0x3bcdf are the 18 LSBits of 0xffafbcdf, so it seems to be working as expected. What did you expect?

Answer 2

Files are written in bytes (8 bits), not nibbles (4bits).

Edit after your edit: fprintf will print text, thus you'll get 6 bytes, where-as if you used fwrite you will write an int (32 bit) to a file, but masking the value recorded.

Answer 3

As others have said, the character is the minimal sized entity you can write to a file. I'd just like to observe that there is nothing wrong with using fprintf() (or any other stream output function) on binary files.