fseek.c

不错的东西 请查看 WINCE OS

//
// Copyright (c) Microsoft Corporation.  All rights reserved.
//
//
// This source code is licensed under Microsoft Shared Source License
// Version 1.0 for Windows CE.
// For a copy of the license visit http://go.microsoft.com/fwlink/?LinkId=3223.
//
/***
*fseek.c - reposition file pointer on a stream
*
*
*Purpose:
*   defines fseek() - move the file pointer to new place in file
*
*******************************************************************************/

#include <cruntime.h>
#include <stdio.h>
#include <file2.h>
#include <dbgint.h>
#include <msdos.h>
#include <malloc.h>
#include <internal.h>
#include <mtdll.h>

/***
*int fseek(stream, offset, whence) - reposition file pointer
*
*Purpose:
*
*   Reposition file pointer to the desired location.  The new location
*   is calculated as follows:
*                { whence=0, beginning of file }
*       <offset> bytes + { whence=1, current position  }
*                { whence=2, end of file       }
*
*   Be careful to coordinate with buffering.
*
*           - - - - - - - - - - - - -
*
*   [NOTE: We used to bend over backwards to try and preserve the current
*   buffer and maintain disk block alignment.  This ended up making our
*   code big and slow and complicated, and slowed us down quite a bit.
*   Some of the things pertinent to the old implimentation:
*
*   (1) Read-only: We only did the special code path if the file was
*   opened read-only (_IOREAD).  If the file was writable, we didn't
*   try to optimize.
*
*   (2) Buffering:  We'd assign a buffer, if necessary, since the
*   later code might need it (i.e., call _getbuf).
*
*   (3) Ungetc: Fseek had to be careful NOT to save the buffer if
*   an ungetc had ever been done on the buffer (flag _IOUNGETC).
*
*   (4) Control ^Z: Fseek had to deal with ^Z after reading a
*   new buffer's worth of data (flag _IOCTRLZ).
*
*   (5) Seek-to-end-and-back: To determine if the new seek was within
*   the current buffer, we had to 'normalize' the desired location.
*   This means that we sometimes had to seek to the end of the file
*   and back to determine what the 0-relative offset was.  Two extra
*   lseek() calls hurt performance.
*
*   (6) CR/LF accounting - When trying to seek within a buffer that
*   is in text mode, we had to go account for CR/LF expansion.  This
*   required us to look at every character up to the new offset and
*   see if it was '\n' or not.  In addition, we had to check the
*   FCRLF flag to see if the new buffer started with '\n'.
*
*   Again, all of these notes are for the OLD implimentation just to
*   remind folks of some of the issues involving seeking within a buffer
*   and maintaining buffer alignment.  As an aside, I think this may have
*   been a big win in the 'old days' on floppy-based systems but on newer
*   fast hard disks, the extra code/complexity overwhelmed any gain.
*
*           - - - - - - - - - - - - -
*
*Entry:
*   FILEX *stream - file to reposition file pointer on
*   long offset - offset to seek to
*   int whence - origin offset is measured from (0=beg, 1=current pos,
*            2=end)
*
*Exit:
*   returns 0 if succeeds
*   returns -1 and sets errno if fails
*   fields of FILEX struct will be changed
*
*Exceptions:
*
*******************************************************************************/

int __cdecl fseek (
    FILEX *stream,
    long offset,
    int whence
    )
{
    int retval;

    if(!CheckStdioInit())
        return -1L;


    _ASSERTE(stream != NULL);

    if (! _lock_validate_str(stream))
        return -1L;

    retval = _fseek_lk (stream, offset, whence);

    _unlock_str(stream);

    return(retval);
}


/***
*_fseek_lk() - Core fseek() routine (stream is locked)
*
*Purpose:
*   Core fseek() routine; assumes that caller has the stream locked.
*
*   [See fseek() for more info.]
*
*Entry: [See fseek()]
*
*Exit:  [See fseek()]
*
*Exceptions:
*
*******************************************************************************/

int __cdecl _fseek_lk (
    FILEX *str,
    long offset,
    int whence
    )
{


    REG1 FILEX *stream;

    _ASSERTE(str != NULL);

    /* Init stream pointer */
    stream = str;

    if ( !inuse(stream) || ((whence != SEEK_SET) && (whence != SEEK_CUR) && (whence != SEEK_END)) ) 
    {
        SetLastError(ERROR_INVALID_PARAMETER);
        return(-1);
    }

    /* Clear EOF flag */

    stream->_flag &= ~_IOEOF;

    /* If seeking relative to current location, then convert to
       a seek relative to beginning of file.  This accounts for
       buffering, etc. by letting fseek() tell us where we are. */

    if (whence == SEEK_CUR) {
        offset += _ftell_lk(stream);
        whence = SEEK_SET;
    }

    /* Flush buffer as necessary */
    _flush(stream);

    /* If file opened for read/write, clear flags since we don't know
       what the user is going to do next. If the file was opened for
       read access only, decrease _bufsiz so that the next _filbuf
       won't cost quite so much */

    if (stream->_flag & _IORW)
        stream->_flag &= ~(_IOWRT|_IOREAD);
    else if ( (stream->_flag & _IOREAD) && (stream->_flag & _IOMYBUF) &&
          !(stream->_flag & _IOSETVBUF) )
        stream->_bufsiz = _SMALL_BUFSIZ;

    /* Seek to the desired locale and return. */
    return(_lseek(stream, offset, whence) == -1L ? -1 : 0);
}