sfdcsubstream.c   [plain text]

#include	"sfdchdr.h"


/*	Discipline to treat a contiguous segment of a stream as a stream
**	in its own right. The hard part in all this is to allow multiple
**	segments of the stream to be used as substreams at the same time.
**
**	Written by David G. Korn and Kiem-Phong Vo (03/18/1998)
*/

typedef struct _subfile_s
{
	Sfdisc_t	disc;	/* sfio discipline */
	Sfio_t*		parent;	/* parent stream */
	Sfoff_t		offset;	/* starting offset */
	Sfoff_t		extent;	/* size wanted */
	Sfoff_t		here;	/* current seek location */
} Subfile_t;

#if __STD_C
static ssize_t streamio(Sfio_t* f, Void_t* buf, size_t n, Sfdisc_t* disc, int type)
#else
static ssize_t streamio(f, buf, n, disc, type)
Sfio_t*		f;
Void_t*		buf;
size_t		n;
Sfdisc_t*	disc;
int		type;
#endif
{
	reg Subfile_t	*su;
	reg Sfoff_t	here, parent;
	reg ssize_t	io;

	su = (Subfile_t*)disc;

	/* read just what we need */
	if(su->extent >= 0 && (ssize_t)n > (io = (ssize_t)(su->extent - su->here)) )
		n = io;
	if(n <= 0)
		return n;

	/* save current location in parent stream */
	parent = sfseek(su->parent,(Sfoff_t)0,1);

	/* read data */
	here = su->here + su->offset;
	if(sfseek(su->parent,here,0) != here)
		io = 0;
	else
	{	if(type == SF_WRITE) 
			io = sfwrite(su->parent,buf,n);
		else	io = sfread(su->parent,buf,n);
		if(io > 0)
			su->here += io;
	}

	/* restore parent current position */
	sfseek(su->parent,parent,0);

	return io;
}

#if __STD_C
static ssize_t streamwrite(Sfio_t* f, const Void_t* buf, size_t n, Sfdisc_t* disc)
#else
static ssize_t streamwrite(f, buf, n, disc)
Sfio_t*		f;
Void_t*		buf;
size_t		n;
Sfdisc_t*	disc;
#endif
{
	return streamio(f,(Void_t*)buf,n,disc,SF_WRITE);
}

#if __STD_C
static ssize_t streamread(Sfio_t* f, Void_t* buf, size_t n, Sfdisc_t* disc)
#else
static ssize_t streamread(f, buf, n, disc)
Sfio_t*		f;
Void_t*		buf;
size_t		n;
Sfdisc_t*	disc;
#endif
{
	return streamio(f,buf,n,disc,SF_READ);
}

#if __STD_C
static Sfoff_t streamseek(Sfio_t* f, Sfoff_t pos, int type, Sfdisc_t* disc)
#else
static Sfoff_t streamseek(f, pos, type, disc)
Sfio_t*		f;
Sfoff_t		pos;
int		type;
Sfdisc_t*	disc;
#endif
{
	reg Subfile_t*	su;
	reg Sfoff_t	here, parent;

	su = (Subfile_t*)disc;

	switch(type)
	{
	case 0:
		here = 0;
		break;
	case 1:
		here = su->here;
		break;
	case 2:
		if(su->extent >= 0)
			here = su->extent;
		else
		{	parent = sfseek(su->parent,(Sfoff_t)0,1);
			if((here = sfseek(su->parent,(Sfoff_t)0,2)) < 0)
				return -1;
			else	here -= su->offset;
			sfseek(su->parent,parent,0);
		}
		break;
	default:
		return -1;
	}

	pos += here;
	if(pos < 0 || (su->extent >= 0 && pos >= su->extent))
		return -1;

	return (su->here = pos);
}

#if __STD_C
static int streamexcept(Sfio_t* f, int type, Void_t* data, Sfdisc_t* disc)
#else
static int streamexcept(f, type, data, disc)
Sfio_t*		f;
int		type;
Void_t*		data;
Sfdisc_t*	disc;
#endif
{
	if(type == SF_FINAL || type == SF_DPOP)
		free(disc);
	return 0;
}

#if __STD_C
int sfdcsubstream(Sfio_t* f, Sfio_t* parent, Sfoff_t offset, Sfoff_t extent)
#else
int sfdcsubstream(f, parent, offset, extent)
Sfio_t*	f;	/* stream */
Sfio_t*	parent; /* parent stream */
Sfoff_t	offset;	/* offset in parent stream */
Sfoff_t	extent;	/* desired size */
#endif
{
	reg Subfile_t*	su;
	reg Sfoff_t	here;

	/* establish that we can seek to offset */
	if((here = sfseek(parent,(Sfoff_t)0,1)) < 0 || sfseek(parent,offset,0) < 0)
		return -1;
	else	sfseek(parent,here,0);

	if(!(su = (Subfile_t*)malloc(sizeof(Subfile_t))) )
		return -1;

	su->disc.readf = streamread;
	su->disc.writef = streamwrite;
	su->disc.seekf = streamseek;
	su->disc.exceptf = streamexcept;
	su->parent = parent;
	su->offset = offset;
	su->extent = extent;

	if(sfdisc(f, (Sfdisc_t*)su) != (Sfdisc_t*)su)
	{	free(su);
		return -1;
	}

	return 0;
}