Random Access files

[Peter Holzer]

gordon at osiris.cso.uiuc.edu (John Gordon) writes:

>Tony R. Marasco <TRM900 at psuvm.psu.edu> writes:

>>Hello.  I am trying to use random access files and have run into some
>>problems...

>>I am using _fopen_ with a "w+b" parameter to create the file (if it doesn't
>>exist).  If it exists, I use "r+b".  Otherwise it seems to erase the file
>>like PASCAL's _rewrite_.  How am I doing so far??

>	Opening a file in "w" mode is supposed to destroy existing files.  If
>you don't want to do this, open in "a" mode.

But you would have to close it and reopen it with "r+" if you want to
modify existing records (see below). I doubt that 

	if ((fp = fopen (filename, "a+b")) == NULL) {
		/* error */
	} else {
		fclose (fp);
		if ((fp = fopen (filename, "r+b")) == NULL) {
			/* error */
		} else {
			/* everything ok */
		}
	}

is more elegant than

	if ((fp = fopen (filename, "r+b")) == NULL) {
		if ((fp = fopen (filename, "w+b")) == NULL) {
			/* error */
		} 
	}
	/* everything ok */

>>I am also using _fseek_ to position the file pointer (recnum*size of struct)
>>with SEEK_SET.  I do this before each _fread_ & _fwrite_.  The program
>>seems to "lose" a few bytes at the beginning of each _fread_.

>	The first record number should be at recnum 0.  Are you starting at
>recnum 1?

That would make no difference (except that he is wasting one record).

>>What is more astounding is I can write a record that doesn't exist, but
>>I can't update a record that already exists.

>	I think that opening in "a" might work.

No. Opening in "a" mode means that you cannot update existing records
but only append new ones (an lseek to the end of the file is done before
any write). In fact, the symptoms Tony describes sound exactly as if he
had opened the file in "a+" mode.
--
|    _  | Peter J. Holzer                       | Think of it   |
| |_|_) | Technical University Vienna           | as evolution  |
| | |   | Dept. for Real-Time Systems           | in action!    |
| __/   | hp at vmars.tuwien.ac.at                 |     Tony Rand |