From: martin AT loplop DOT com (Martin Peach)
Newsgroups: comp.os.msdos.djgpp
Subject: fwrite from linear vga buffer
Date: Mon, 27 Jul 1998 20:01:39 GMT
Organization: Communications Accessibles Montreal, Quebec Canada
Lines: 60
Message-ID: <35bcd9c0.147173@nntp.hip.cam.org>
NNTP-Posting-Host: dialup-831.hip.cam.org
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Precedence: bulk

Hi all,
I want to dump the screen image to a file for debugging purposes. I
have set up a linear buffer in the way Shawn Hargreaves describes on
the delorie website. I have no problem accessing the data via a line
buffer I allocate, using movedata(linBufSelector, i, _my_ds,
(int)lineBuf, lineBytes). But I want it to go faster...it seems as
though I could just do a fwrite((void *)linBufMapping.address + i,
lineBytes, 1L, fp) to write the data directly to a file rather than
going through an intermediate line buffer. But the thing crashes when
I use linBufMapping.address (which is 0xE0000000, a reasonable looking
address, no?). Is there some other call I need to make to point the
thing to the right place or does fwrite not work on this kind of
address?
Martin.

The function:

size_t DumpScreen(char *Nom)
{// write screen image to file Nom, return number of bytes written
	FILE *fp;
	unsigned long bytes, i, x, y, numwritten;
	unsigned long lineBytes;
	char *lineBuf;
	
	 fprintf(stdout, "Doing screen dump to %s\n", Nom);
	// allocate a buffer for one line of pixels
	lineBytes = vgaModeInfo.XResolution * bytesPerPixel;
	if ((lineBuf = malloc(lineBytes)) == NULL)
	{
		fprintf(stdout, "DumpScreen: Unable to allocate %u
bytes for lineBuf\n", lineBytes);
		return 0L;
	}
	if ((fp = fopen(Nom, "wb")) == NULL)
	{
		fprintf(stdout, "DumpScreen: Unable to open %s for
writing\n", Nom);
	return 0L;
	}
	for (y = bytes = 0; y < vgaModeInfo.YResolution; y++)
	{
		i = y * vgaModeInfo.BytesPerScanLine;
		movedata(linBufSelector, i, _my_ds(), (int)lineBuf,
lineBytes);
// this crashes:
//     	if ((numwritten = fwrite((void *)linBufMapping.address + i,
lineBytes, 1L, fp))!=1L)
//			fprintf(stdout, "DumpScreen: Write Error at
offset %u\n", i);
//this works:
		if ((numwritten = fwrite((void *)lineBuf, lineBytes,
1L, fp))!=1L)
		fprintf(stdout, "DumpScreen: Write Error at offset
%u\n", i);	bytes += numwritten;
	}
	fclose(fp);
	free(lineBuf);
    	return bytes;
}