newlib-cygwin/newlib/libc/stdio/fseek.c

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2000-02-18 03:39:52 +08:00
/*
* Copyright (c) 1990 The Regents of the University of California.
* All rights reserved.
*
* Redistribution and use in source and binary forms are permitted
* provided that the above copyright notice and this paragraph are
* duplicated in all such forms and that any documentation,
* advertising materials, and other materials related to such
* distribution and use acknowledge that the software was developed
* by the University of California, Berkeley. The name of the
* University may not be used to endorse or promote products derived
* from this software without specific prior written permission.
* THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
* WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
*/
/*
FUNCTION
<<fseek>>---set file position
INDEX
fseek
ANSI_SYNOPSIS
#include <stdio.h>
int fseek(FILE *<[fp]>, long <[offset]>, int <[whence]>)
TRAD_SYNOPSIS
#include <stdio.h>
int fseek(<[fp]>, <[offset]>, <[whence]>)
FILE *<[fp]>;
long <[offset]>;
int <[whence]>;
DESCRIPTION
Objects of type <<FILE>> can have a ``position'' that records how much
of the file your program has already read. Many of the <<stdio>> functions
depend on this position, and many change it as a side effect.
You can use <<fseek>> to set the position for the file identified by
<[fp]>. The value of <[offset]> determines the new position, in one
of three ways selected by the value of <[whence]> (defined as macros
in `<<stdio.h>>'):
<<SEEK_SET>>---<[offset]> is the absolute file position (an offset
from the beginning of the file) desired. <[offset]> must be positive.
<<SEEK_CUR>>---<[offset]> is relative to the current file position.
<[offset]> can meaningfully be either positive or negative.
<<SEEK_END>>---<[offset]> is relative to the current end of file.
<[offset]> can meaningfully be either positive (to increase the size
of the file) or negative.
See <<ftell>> to determine the current file position.
RETURNS
<<fseek>> returns <<0>> when successful. If <<fseek>> fails, the
result is <<EOF>>. The reason for failure is indicated in <<errno>>:
either <<ESPIPE>> (the stream identified by <[fp]> doesn't support
repositioning) or <<EINVAL>> (invalid file position).
PORTABILITY
ANSI C requires <<fseek>>.
Supporting OS subroutines required: <<close>>, <<fstat>>, <<isatty>>,
<<lseek>>, <<read>>, <<sbrk>>, <<write>>.
*/
#include <stdio.h>
#include <time.h>
#include <fcntl.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/stat.h>
#include "local.h"
#define POS_ERR (-(fpos_t)1)
/*
* Seek the given file to the given offset.
* `Whence' must be one of the three SEEK_* macros.
*/
int
fseek (fp, offset, whence)
register FILE *fp;
long offset;
int whence;
{
struct _reent *ptr;
fpos_t _EXFUN ((*seekfn), (void *, fpos_t, int));
fpos_t target, curoff;
size_t n;
struct stat st;
int havepos;
/* Make sure stdio is set up. */
CHECK_INIT (fp);
ptr = fp->_data;
/* If we've been doing some writing, and we're in append mode
then we don't really know where the filepos is. */
if (fp->_flags & __SAPP && fp->_flags & __SWR)
{
/* So flush the buffer and seek to the end. */
fflush (fp);
}
/* Have to be able to seek. */
if ((seekfn = fp->_seek) == NULL)
{
ptr->_errno = ESPIPE; /* ??? */
return EOF;
}
/*
* Change any SEEK_CUR to SEEK_SET, and check `whence' argument.
* After this, whence is either SEEK_SET or SEEK_END.
*/
switch (whence)
{
case SEEK_CUR:
/*
* In order to seek relative to the current stream offset,
* we have to first find the current stream offset a la
* ftell (see ftell for details).
*/
fflush(fp); /* may adjust seek offset on append stream */
if (fp->_flags & __SOFF)
curoff = fp->_offset;
else
{
curoff = (*seekfn) (fp->_cookie, (fpos_t) 0, SEEK_CUR);
if (curoff == -1L)
return EOF;
}
if (fp->_flags & __SRD)
{
curoff -= fp->_r;
if (HASUB (fp))
curoff -= fp->_ur;
}
else if (fp->_flags & __SWR && fp->_p != NULL)
curoff += fp->_p - fp->_bf._base;
offset += curoff;
whence = SEEK_SET;
havepos = 1;
break;
case SEEK_SET:
case SEEK_END:
havepos = 0;
break;
default:
ptr->_errno = EINVAL;
return (EOF);
}
/*
* Can only optimise if:
* reading (and not reading-and-writing);
* not unbuffered; and
* this is a `regular' Unix file (and hence seekfn==__sseek).
* We must check __NBF first, because it is possible to have __NBF
* and __SOPT both set.
*/
if (fp->_bf._base == NULL)
__smakebuf (fp);
if (fp->_flags & (__SWR | __SRW | __SNBF | __SNPT))
goto dumb;
if ((fp->_flags & __SOPT) == 0)
{
if (seekfn != __sseek
|| fp->_file < 0
|| _fstat_r (ptr, fp->_file, &st)
|| (st.st_mode & S_IFMT) != S_IFREG)
{
fp->_flags |= __SNPT;
goto dumb;
}
#ifdef HAVE_BLKSIZE
fp->_blksize = st.st_blksize;
#else
fp->_blksize = 1024;
#endif
fp->_flags |= __SOPT;
}
/*
* We are reading; we can try to optimise.
* Figure out where we are going and where we are now.
*/
if (whence == SEEK_SET)
target = offset;
else
{
if (_fstat_r (ptr, fp->_file, &st))
goto dumb;
target = st.st_size + offset;
}
if (!havepos)
{
if (fp->_flags & __SOFF)
curoff = fp->_offset;
else
{
curoff = (*seekfn) (fp->_cookie, 0L, SEEK_CUR);
if (curoff == POS_ERR)
goto dumb;
}
curoff -= fp->_r;
if (HASUB (fp))
curoff -= fp->_ur;
}
/*
* Compute the number of bytes in the input buffer (pretending
* that any ungetc() input has been discarded). Adjust current
* offset backwards by this count so that it represents the
* file offset for the first byte in the current input buffer.
*/
if (HASUB (fp))
{
curoff += fp->_r; /* kill off ungetc */
n = fp->_up - fp->_bf._base;
curoff -= n;
n += fp->_ur;
}
else
{
n = fp->_p - fp->_bf._base;
curoff -= n;
n += fp->_r;
}
/*
* If the target offset is within the current buffer,
* simply adjust the pointers, clear EOF, undo ungetc(),
* and return. (If the buffer was modified, we have to
* skip this; see fgetline.c.)
*/
if ((fp->_flags & __SMOD) == 0 &&
target >= curoff && target < curoff + n)
{
register int o = target - curoff;
fp->_p = fp->_bf._base + o;
fp->_r = n - o;
if (HASUB (fp))
FREEUB (fp);
fp->_flags &= ~__SEOF;
return 0;
}
/*
* The place we want to get to is not within the current buffer,
* but we can still be kind to the kernel copyout mechanism.
* By aligning the file offset to a block boundary, we can let
* the kernel use the VM hardware to map pages instead of
* copying bytes laboriously. Using a block boundary also
* ensures that we only read one block, rather than two.
*/
curoff = target & ~(fp->_blksize - 1);
if ((*seekfn) (fp->_cookie, curoff, SEEK_SET) == POS_ERR)
goto dumb;
fp->_r = 0;
if (HASUB (fp))
FREEUB (fp);
fp->_flags &= ~__SEOF;
n = target - curoff;
if (n)
{
if (__srefill (fp) || fp->_r < n)
goto dumb;
fp->_p += n;
fp->_r -= n;
}
return 0;
/*
* We get here if we cannot optimise the seek ... just
* do it. Allow the seek function to change fp->_bf._base.
*/
dumb:
if (fflush (fp) || (*seekfn) (fp->_cookie, offset, whence) == POS_ERR)
return EOF;
/* success: clear EOF indicator and discard ungetc() data */
if (HASUB (fp))
FREEUB (fp);
fp->_p = fp->_bf._base;
fp->_r = 0;
/* fp->_w = 0; *//* unnecessary (I think...) */
fp->_flags &= ~__SEOF;
return 0;
}