CODEWITHSUNDEEP
cfile-iobinarywav
speedyTeh
speedyTeh

Reputation: 257

C: editing binary file

Here is the problem: I have to change header of WAVE file, to be exact I have to change ChunkSize and SubChunk2Size. The problem is that those values use 4bytes but it seemt that using fwrite i overwrite 8 bytes:

the original:

RIFFđ WAVEfmt

edited:

RIFF(}   }  fmt

code:

FILE *nova;
nova=fopen ( "nova.wav", "wb" );

fseek ( nova, 4, SEEK_SET );
fwrite ( &brojacC,4,1,nova );
fseek ( zvuk, 44, SEEK_SET );
fwrite ( &brojacCS2,4,1,nova );

In edited file WAVE is overwritten. Something went wrong because I started at 4th byte and wrote 4 bytes and WAVE starts at 8th byte.

I hope I was at least a bit clear. Can this be done in some other way?

Upvotes: 1

Views: 4144

Answers (2)

zwol
zwol

Reputation: 140796

This code has at least one bug on every line shown.

FILE *nova;

It is easier to get the error handling right if you do this sort of thing with open, write, and lseek rather than fopen, fwrite, and fseek.

nova=fopen ( "nova.wav", "wb" );

The second string should be "r+b" instead of "wb" so you don't truncate the file. You need to check for errors.

fseek ( nova, 4, SEEK_SET );

You need to check for errors.

fwrite ( &brojacC,4,1,nova );

fwrite should always be called with second argument 1 and third argument equal to the size of the data to be written; otherwise it is impossible to recover from short writes. You need to check for short writes and write errors.

You don't show the code that initializes brojacC so I can't assess whether you have any endianness or structure-padding problems, but I bet you do.

fseek ( zvuk, 44, SEEK_SET );

This operates on the unrelated file handle zvuk rather than nova. And you need to check for errors.

fwrite ( &brojacCS2,4,1,nova );

Since the fseek call on the previous line was applied to zvuk, this writes at offset 4+4=8, not offset 44 as was intended. All the comments on the previous fwrite line also apply to this line. (Psst: You need to check for errors.)

Inconsistent spacing around commas, by the way, invites the gods to strike you with lightning. So does putting spaces on the inside of your parentheses.

Upvotes: 2

Misguided
Misguided

Reputation: 1312

Well, according to my man fopen output:

   r      Open  text  file  for  reading.  The stream is positioned at the
          beginning of the file.

   r+     Open for reading and writing.  The stream is positioned  at  the
          beginning of the file.

   w      Truncate  file  to  zero length or create text file for writing.
          The stream is positioned at the beginning of the file.

   w+     Open for reading and writing.  The file is created  if  it  does
          not  exist, otherwise it is truncated.  The stream is positioned
          at the beginning of the file.

   a      Open for appending (writing at end of file).  The file  is  cre‐
          ated  if it does not exist.  The stream is positioned at the end
          of the file.

   a+     Open for reading and appending (writing at end  of  file).   The
          file is created if it does not exist.  The initial file position
          for reading is at the beginning  of  the  file,  but  output  is
          always appended to the end of the file.

That being said, I would definitely go forfopen("nova.wav", "r+b"), as w seems to truncate the file, and you're reading before writing, while a appends to the end of the file, and you want to rewrite part of the file.

Upvotes: 3

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