274 lines
9.5 KiB
C
274 lines
9.5 KiB
C
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/***
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*fseek.c - reposition file pointer on a stream
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*
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* Copyright (c) 1985-2001, Microsoft Corporation. All rights reserved.
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*
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*Purpose:
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* defines fseek() - move the file pointer to new place in file
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*
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*Revision History:
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* 10-13-83 RN initial version
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* 06-26-85 TC added code to allow variable buffer lengths
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* 02-10-87 BCM fixed '%' mistakenly used for '/'
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* 03-04-87 JCR added errno settings
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* 04-16-87 JCR added _IOUNGETC support for bug fix and changes whence
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* from unsigned int to int (ANSI conformance)
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* 04-17-87 JCR fseek() now clears end-of-file indicator flag _IOEOF
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* (for ANSI conformance)
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* 04-21-87 JCR be smart about lseek'ing to the end of the file and
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* back
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* 09-17-87 SKS handle case of '\n' at beginning of buffer (FCRLF flag)
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* 09-24-87 JCR fixed an incorrect access to flag _IOEOF
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* 09-28-87 JCR Corrected _iob2 indexing (now uses _iob_index() macro).
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* 09-30-87 JCR Fixed buffer allocation bug, now use _getbuf()
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* 11-04-87 JCR Multi-thread support
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* 12-11-87 JCR Added "_LOAD_DS" to declaration
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* 01-13-88 JCR Removed unnecessary calls to mthread fileno/feof/ferror
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* 03-04-88 JCR Return value from read() must be treated as unsigned
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* value
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* 05-27-88 PHG Merged DLL and normal versions
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* 06-06-88 JCR Optimized _iob2[] references
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* 06-15-88 JCR Near reference to _iob[] entries; improve REG variables
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* 08-25-88 GJF Don't use FP_OFF() macro for the 386
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* 12-02-88 JCR Added _IOCTRLZ support (fixes bug pertaining to ^Z at
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* eof)
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* 04-12-89 JCR Ripped out all of the special read-only code. See the
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* comments in the routine header for more information.
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* 08-17-89 GJF Clean up, now specific to OS/2 2.0 (i.e., 386 flat
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* model). Also fixed copyright and indents.
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* 02-15-90 GJF Fixed copyright
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* 03-19-90 GJF Made calling type _CALLTYPE1, added #include
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* <cruntime.h> and removed #include <register.h>.
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* 05-29-90 SBM Use _flush, not [_]fflush[_lk]
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* 07-23-90 SBM Replaced <assertm.h> by <assert.h>
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* 10-02-90 GJF New-style function declarators.
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* 01-21-91 GJF ANSI naming.
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* 03-27-92 DJM POSIX support.
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* 08-08-92 GJF Use seek method constants!
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* 08-26-92 GJF Include unistd.h for POSIX build.
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* 04-06-93 SKS Replace _CRTAPI* with __cdecl
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* 05-24-93 GJF If the stream was opened for read-access-only, reduce
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* _bufsiz after flushing the stream. This should reduce
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* the expense of the next _filbuf call, and the overall
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* burden of seek-and-do-small-reads patterns of file
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* input.
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* 06-22-93 GJF Check _flag for _IOSETVBUF (new) before changing
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* buffer size.
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* 11-05-93 GJF Merged with NT SDK version. Also, replaced MTHREAD
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* with _MT.
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* 02-06-94 CFW assert -> _ASSERTE.
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* 02-20-95 GJF Merged in Mac version.
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* 03-07-95 GJF _[un]lock_str macros now take FILE * arg.
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* 03-02-98 GJF Exception-safe locking.
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* 05-17-99 PML Remove all Macintosh support.
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*
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*******************************************************************************/
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#include <cruntime.h>
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#include <stdio.h>
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#include <file2.h>
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#include <dbgint.h>
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#ifdef _POSIX_
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#include <unistd.h>
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#else
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#include <msdos.h>
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#endif
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#include <errno.h>
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#include <malloc.h>
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#include <io.h>
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#include <stddef.h>
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#include <internal.h>
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#ifndef _POSIX_
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#include <mtdll.h>
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#endif
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/***
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*int fseek(stream, offset, whence) - reposition file pointer
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*
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*Purpose:
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*
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* Reposition file pointer to the desired location. The new location
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* is calculated as follows:
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* { whence=0, beginning of file }
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* <offset> bytes + { whence=1, current position }
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* { whence=2, end of file }
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*
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* Be careful to coordinate with buffering.
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*
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* - - - - - - - - - - - - -
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*
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* [NOTE: We used to bend over backwards to try and preserve the current
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* buffer and maintain disk block alignment. This ended up making our
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* code big and slow and complicated, and slowed us down quite a bit.
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* Some of the things pertinent to the old implimentation:
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*
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* (1) Read-only: We only did the special code path if the file was
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* opened read-only (_IOREAD). If the file was writable, we didn't
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* try to optimize.
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*
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* (2) Buffering: We'd assign a buffer, if necessary, since the
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* later code might need it (i.e., call _getbuf).
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*
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* (3) Ungetc: Fseek had to be careful NOT to save the buffer if
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* an ungetc had ever been done on the buffer (flag _IOUNGETC).
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*
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* (4) Control ^Z: Fseek had to deal with ^Z after reading a
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* new buffer's worth of data (flag _IOCTRLZ).
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*
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* (5) Seek-to-end-and-back: To determine if the new seek was within
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* the current buffer, we had to 'normalize' the desired location.
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* This means that we sometimes had to seek to the end of the file
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* and back to determine what the 0-relative offset was. Two extra
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* lseek() calls hurt performance.
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*
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* (6) CR/LF accounting - When trying to seek within a buffer that
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* is in text mode, we had to go account for CR/LF expansion. This
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* required us to look at every character up to the new offset and
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* see if it was '\n' or not. In addition, we had to check the
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* FCRLF flag to see if the new buffer started with '\n'.
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*
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* Again, all of these notes are for the OLD implimentation just to
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* remind folks of some of the issues involving seeking within a buffer
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* and maintaining buffer alignment. As an aside, I think this may have
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* been a big win in the 'old days' on floppy-based systems but on newer
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* fast hard disks, the extra code/complexity overwhelmed any gain.
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*
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* - - - - - - - - - - - - -
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*
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*Entry:
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* FILE *stream - file to reposition file pointer on
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* long offset - offset to seek to
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* int whence - origin offset is measured from (0=beg, 1=current pos,
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* 2=end)
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*
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*Exit:
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* returns 0 if succeeds
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* returns -1 and sets errno if fails
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* fields of FILE struct will be changed
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*
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*Exceptions:
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*
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*******************************************************************************/
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#ifdef _MT /* multi-thread; define both fseek() and _lk_fseek() */
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int __cdecl fseek (
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FILE *stream,
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long offset,
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int whence
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)
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{
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int retval;
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_ASSERTE(stream != NULL);
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_lock_str(stream);
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__try {
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retval = _fseek_lk (stream, offset, whence);
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}
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__finally {
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_unlock_str(stream);
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}
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return(retval);
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}
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/***
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*_fseek_lk() - Core fseek() routine (stream is locked)
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*
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*Purpose:
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* Core fseek() routine; assumes that caller has the stream locked.
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*
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* [See fseek() for more info.]
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*
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*Entry: [See fseek()]
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*
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*Exit: [See fseek()]
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*
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*Exceptions:
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*
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*******************************************************************************/
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int __cdecl _fseek_lk (
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#else /* non multi-thread; just define fseek() */
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int __cdecl fseek (
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#endif /* rejoin common code */
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FILE *str,
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long offset,
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int whence
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)
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{
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REG1 FILE *stream;
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_ASSERTE(str != NULL);
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/* Init stream pointer */
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stream = str;
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if ( !inuse(stream) || ((whence != SEEK_SET) && (whence != SEEK_CUR) &&
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(whence != SEEK_END)) ) {
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errno=EINVAL;
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return(-1);
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}
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/* Clear EOF flag */
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stream->_flag &= ~_IOEOF;
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/* If seeking relative to current location, then convert to
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a seek relative to beginning of file. This accounts for
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buffering, etc. by letting fseek() tell us where we are. */
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if (whence == SEEK_CUR) {
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offset += _ftell_lk(stream);
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whence = SEEK_SET;
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}
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/* Flush buffer as necessary */
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#ifdef _POSIX_
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/*
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* If the stream was last read, we throw away the buffer so
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* that a possible subsequent write will encounter a clean
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* buffer. (The Win32 version of fflush() throws away the
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* buffer if it's read.) Write buffers must be flushed.
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*/
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if ((stream->_flag & (_IOREAD | _IOWRT)) == _IOREAD) {
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stream->_ptr = stream->_base;
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stream->_cnt = 0;
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} else {
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_flush(stream);
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}
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#else
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_flush(stream);
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#endif
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/* If file opened for read/write, clear flags since we don't know
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what the user is going to do next. If the file was opened for
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read access only, decrease _bufsiz so that the next _filbuf
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won't cost quite so much */
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if (stream->_flag & _IORW)
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stream->_flag &= ~(_IOWRT|_IOREAD);
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else if ( (stream->_flag & _IOREAD) && (stream->_flag & _IOMYBUF) &&
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!(stream->_flag & _IOSETVBUF) )
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stream->_bufsiz = _SMALL_BUFSIZ;
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/* Seek to the desired locale and return. */
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#ifdef _POSIX_
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return(lseek(fileno(stream), offset, whence) == -1L ? -1 : 0);
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#else
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return(_lseek(_fileno(stream), offset, whence) == -1L ? -1 : 0);
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#endif
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}
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