sfseek.c

su 的源代码库

/* Copyright (c) Colorado School of Mines, 2006.*/
/* All rights reserved.                       */

#include	"sfhdr.h"

/*	Set the IO pointer to a specific location in the stream
**
**	Written by Kiem-Phong Vo.
*/

#if __STD_C
static void newpos(Sfio_t* f, Sfoff_t p)
#else
static void newpos(f, p)
Sfio_t*	f;
Sfoff_t p;
#endif
{
#ifdef MAP_TYPE
	if((f->bits&SF_MMAP) && f->data)
	{	SFMUNMAP(f, f->data, f->endb-f->data);
		f->data = NIL(uchar*);
	}
#endif
	f->next = f->endr = f->endw = f->data;
	f->endb = (f->mode&SF_WRITE) ? f->data+f->size : f->data;
	if((f->here = p) < 0)
	{	f->extent = -1;
		f->here = 0;
	}
}

#if __STD_C
Sfoff_t sfseek(Sfio_t* f, Sfoff_t p, int type)
#else
Sfoff_t sfseek(f,p,type)
Sfio_t*	f;	/* seek to a new location in this stream */
Sfoff_t	p;	/* place to seek to */
int	type;	/* 0: from org, 1: from here, 2: from end */
#endif
{
	Sfoff_t	r, s;
	int	mode, local, hardseek, mustsync;

	SFMTXSTART(f, (Sfoff_t)(-1));

	GETLOCAL(f,local);

	hardseek = (type|f->flags)&(SF_SHARE|SF_PUBLIC);

	if(hardseek && f->mode == (SF_READ|SF_SYNCED) )
	{	newpos(f,f->here);
		f->mode = SF_READ;
	}

	/* set and initialize the stream to a definite mode */
	if((int)SFMODE(f,local) != (mode = f->mode&SF_RDWR))
	{	int	flags = f->flags;

		if(hardseek&SF_PUBLIC) /* seek ptr must follow file descriptor */
			f->flags |= SF_SHARE|SF_PUBLIC;
		mode = _sfmode(f,mode,local);
		if(hardseek&SF_PUBLIC)
			f->flags = flags;

		if(mode < 0)
			SFMTXRETURN(f, (Sfoff_t)(-1));
	}

	mustsync = (type&SF_SHARE) && !(type&SF_PUBLIC) &&
		   (f->mode&SF_READ) && !(f->flags&SF_STRING);

	/* Xopen-compliant */
	if((type &= (SEEK_SET|SEEK_CUR|SEEK_END)) != SEEK_SET &&
	   type != SEEK_CUR && type != SEEK_END )
	{	errno = EINVAL;
		SFMTXRETURN(f, (Sfoff_t)(-1));
	}

	if(f->extent < 0)
	{	/* let system call set errno */
		(void)SFSK(f,(Sfoff_t)0,SEEK_CUR,f->disc);
		SFMTXRETURN(f, (Sfoff_t)(-1));
	}

	/* throw away ungetc data */
	if(f->disc == _Sfudisc)
		(void)sfclose((*_Sfstack)(f,NIL(Sfio_t*)));

	/* lock the stream for internal manipulations */
	SFLOCK(f,local);

	/* clear error and eof bits */
	f->flags &= ~(SF_EOF|SF_ERROR);

	while(f->flags&SF_STRING)
	{	SFSTRSIZE(f);

		if(type == SEEK_CUR)
			r = p + (f->next - f->data);
		else if(type == SEEK_END)
			r = p + f->extent;
		else	r = p;

		if(r >= 0 && r <= f->size)
		{	p = r;
			f->next = f->data+p;
			f->here = p;
			if(p > f->extent)
				memclear((char*)(f->data+f->extent),(int)(p-f->extent));
			goto done;
		}

		/* check exception handler, note that this may pop stream */
		if(SFSK(f,r,SEEK_SET,f->disc) != r)
		{	p = -1;
			goto done;
		}
		else if(!(f->flags&SF_STRING))
		{	p = r;
			goto done;
		}
	}

	if(f->mode&SF_WRITE)
	{	/* see if we can avoid flushing buffer */
		if(!hardseek && type < SEEK_END && !(f->flags&SF_APPENDWR) )
		{	s = f->here + (f->next - f->data);
			r = p + (type == SEEK_SET ? 0 : s);
			if(r == s)
			{	p = r;
				goto done;
			}
		}

		if(f->next > f->data && SFSYNC(f) < 0)
		{	p = -1;
			goto done;
		}
	}

	if(type == SEEK_END || (f->mode&SF_WRITE) )
	{	if((hardseek&SF_PUBLIC) || type == SEEK_END)
			p = SFSK(f, p, type, f->disc);
		else
		{	r = p + (type == SEEK_CUR ? f->here : 0);
			p = (hardseek || r != f->here) ? SFSK(f,r,SEEK_SET,f->disc) : r;
		}
		if(p >= 0)
			newpos(f,p);

		goto done;
	}

	/* if get here, must be a read stream */
	s = f->here - (f->endb - f->next);
	r = p + (type == SEEK_CUR ? s : 0);
	if(r <= f->here && r >= (f->here - (f->endb-f->data)) )
	{	if((hardseek || (type == SEEK_CUR && p == 0)) )
		{	if((s = SFSK(f, (Sfoff_t)0, SEEK_CUR, f->disc)) == f->here ||
			   (s >= 0 && !(hardseek&SF_PUBLIC) &&
			    (s = SFSK(f, f->here, SEEK_SET, f->disc)) == f->here) )
				goto near_done;
			else if(s < 0)
			{	p = -1;
				goto done;
			}
			else
			{	newpos(f,s);
				hardseek = 0;
			}
		}
		else
		{ near_done:
			f->next = f->endb - (f->here - r);
			p = r;
			goto done;
		}
	}

	/* desired position */
	if((p += type == SEEK_CUR ? s : 0) < 0)
		goto done;

#ifdef MAP_TYPE
	if(f->bits&SF_MMAP)
	{	/* if mmap is not great, stop mmaping if moving around too much */
#if _mmap_worthy < 2
		if((f->next - f->data) < ((f->endb - f->data)/4) )
		{	SFSETBUF(f,(Void_t*)f->tiny,(size_t)SF_UNBOUND);
			hardseek = 1; /* this forces a hard seek below */
		}
		else
#endif
		{	/* for mmap, f->here can be virtual except for hardseek */
			newpos(f,p);
			if(!hardseek)
				goto done;
		}
	}
#endif

	if(f->endb > f->next)
	{	/* reduce wastage in future buffer fillings */
		f->iosz = (f->next - f->data) + (f->endb - f->next)/2;
		f->iosz = ((f->iosz + f->blksz-1)/f->blksz)*f->blksz;
	}
	if(f->iosz >= f->size)
		f->iosz = 0;

	/* buffer is now considered empty */
	f->next = f->endr = f->endb = f->data;

	/* small backseeks often come in bunches, so seek back as far as possible */
	if(p < f->lpos && f->size > f->blksz && (p + f->blksz) > s)
	{	if((r = s - f->size) < 0)
			r = 0;
	}
	/* try to align buffer to block boundary to enhance I/O speed */
	else if(f->blksz > 0 && f->size >= 2*f->blksz)
		r = p - (p%f->blksz);
	else
	{	r = p;

		/* seeking around and wasting data, be conservative */
		if(f->iosz > 0 && (p > f->lpos || p < f->lpos-f->size) )
			f->bits |= SF_JUSTSEEK;
	}

	if((hardseek || r != f->here) && (f->here = SFSK(f,r,SEEK_SET,f->disc)) != r)
	{	if(r < p) /* now try to just get to p */
			f->here = SFSK(f,p,SEEK_SET,f->disc);
		if(f->here != p)
			p = -1;
		goto done;
	}

	if(r < p) /* read to cover p */
	{	(void)SFRD(f, f->data, f->size, f->disc);
		if(p <= f->here && p >= (f->here - (f->endb - f->data)) )
			f->next = f->endb - (size_t)(f->here-p);
		else /* recover from read failure by just seeking to p */
		{	f->next = f->endb = f->data;
			if((f->here = SFSK(f,p,SEEK_SET,f->disc)) != p)
				p = -1;
		}
	}

done :
	if(f->here < 0) /* hasn't been the best of time */
	{	f->extent = -1;
		f->here = 0;
	}

	f->lpos = p;

	SFOPEN(f,local);

	if(mustsync)
		sfsync(f);
	SFMTXRETURN(f, p);
}