Writing files using O_DIRECT in C

anchit87 · 02-28-2010, 04:02 PM

Hi,

I am trying to write .pgm images using the O_DIRECT flag in open().
I have a char* buffer which has the image data.

I know that I have to align the buffers and have done that using posix_memalign() yet only a part of the image gets written.

Has someone used O_DIRECT for writing files successfully? Can you help me on how to proceed with this?

Thanks

neonsignal · 02-28-2010, 08:51 PM

Quote:

Has someone used O_DIRECT for writing files successfully?

I haven't, because there isn't a good reason to.

However, the following code illustrates its use. Note that both the buffer start and end should be block aligned.

Code:

#define _GNU_SOURCE
#include <string.h>
#include <stdlib.h>
#include <fcntl.h>
#define BLOCKSIZE 512
char image[] =
{
	'P', '5', ' ', '2', '4', ' ', '7', ' ', '1', '5', '\n',
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	0, 3, 3, 3, 3, 0, 0, 7, 7, 7, 7, 0, 0,11,11,11,11, 0, 0,15,15,15,15, 0,
	0, 3, 0, 0, 0, 0, 0, 7, 0, 0, 0, 0, 0,11, 0, 0, 0, 0, 0,15, 0, 0,15, 0,
	0, 3, 3, 3, 0, 0, 0, 7, 7, 7, 0, 0, 0,11,11,11, 0, 0, 0,15,15,15,15, 0,
	0, 3, 0, 0, 0, 0, 0, 7, 0, 0, 0, 0, 0,11, 0, 0, 0, 0, 0,15, 0, 0, 0, 0,
	0, 3, 0, 0, 0, 0, 0, 7, 7, 7, 7, 0, 0,11,11,11,11, 0, 0,15, 0, 0, 0, 0,
	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	0,
};
int main()
{
	void *buffer;
	posix_memalign(&buffer, BLOCKSIZE, BLOCKSIZE);
	memcpy(buffer, image, sizeof(image));
	int f = open("feep.pgm", O_CREAT|O_TRUNC|O_WRONLY|O_DIRECT, S_IRWXU);
	write(f, buffer, BLOCKSIZE);
	close(f);
	free(buffer);
	return 0;
}