From: michael DOT mauch AT gmx DOT de (Michael Mauch)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Newbie question -- debug this plz
Date: Sun, 12 Jul 1998 17:47:04 +0200
Organization: Gerhard-Mercator-Universitaet -GH- Duisburg
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References: <35A8106A DOT 6B1FD58D AT bc DOT sympatico DOT ca>
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On Sat, 11 Jul 1998 18:24:59 -0700, Jim Barlow
<Jim_Barlow AT bc DOT sympatico DOT ca> wrote:

> I'm trying to write a program that does this (again, C, DJGPP):
> 1) takes the first command line parameter as a filename (obvious)
> 2) opens the file. coughs up an error if any.  File is binary and can contain
> nulls in-line. Also very large (~500 kb).

Hmm, 500 KB is not very large. With DJGPP, you can read that file into
memory without problems.

> 3) reads the file into a buffer.  Search for a specific string.  If found it
> needs to be able to write to that offset.
> 4) write changes to the file without corrupting anything.  It's overwritting
> information, not inserting anything new.
> 
> FILE *fp;
> char *my_buffer, *offset;
> if (argc) {
>   fp = fopen (argv[1], "rb+"); // or whatever open for read/write binary is
>   if (ferror (fp)) {
>     printf ("There was an error...");
>     return;
>   }

That's a bad idea: if fopen() returns a NULL pointer (what it does in
case of an error), your ferror() will crash. The normal way is:

	if (!fp) {
		perror("There was an error");
		return EXIT_FAILURE;
	}

>   // best way to read the data into my_buffer?
>   my_buffer = offset = (char *)malloc (sizeof(char) *
> func_that_gets_file_size(fp));

A portable function that is suitable for files with up to 2 GB:

long func_that_gets_file_size(FILE* fp)
{
	fseek(fp,0,SEEK_END);
	return ftell(fp);
}

You could use fsetpos()/fgetpos() for larger files.

>   readinto(fp); // should i use fread() or something else?

Yes, if your file is only 500 KB, you can fread() it completely into
memory. If you have a file that is larger than your RAM, you will get a
speed penalty because of swapping, but that's not a problem here.
 
>   strchr (offset, "my_string");

strchr() is a bad idea because the string functions (str...) all stop at
the first NUL character. memchr() is suitable.

>   if (offset) {
>     strcpy (offset, "newstring");
>   }
>   // write buffer to file

Just fwrite() it out again.

> }

And don't forget to fclose() the file.
 
> Thanks for your patience.

No problem; have a look at the libc reference for the functions I
pointed out.

Regards...
		Michael