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Date: Fri, 1 Dec 2017 17:02:14 +0000 (UTC)
From: "Xiaofeng Liu via cygwin" <cygwin AT cygwin DOT com>
Reply-To: Xiaofeng Liu <liuxf09 AT yahoo DOT com>
Reply-To: Xiaofeng Liu <liuxf09 AT yahoo DOT com>
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Message-ID: <1470375482.5430561.1512147734234@mail.yahoo.com>
Subject: fseek(...,SEEK_CUR) calls _fflush_r(ptr, fp) in read only mode
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In ftello.c, it intentionally skipped fflush for read mode:
https://github.com/openunix/cygwin/blob/master/newlib/libc/stdio/ftello.c#L121

/* Find offset of underlying I/O object, then adjust for buffered     bytes.  Flush a write stream, since the offset may be altered if     the stream is appending.  Do not flush a read stream, since we     must not lose the ungetc buffer.  */  if (fp->_flags & __SWR)    _fflush_r (ptr, fp);
But in fseeko.c, when fseek(..., SEEK_CUR) requires to get the current position first. But it calls _fflush_r(ptr, fp) in read mode, which screwed up the current position after fflush. 
https://github.com/openunix/cygwin/blob/master/newlib/libc/stdio/fseeko.c#L174


For my job, if I skip _fflush_r for read only mode in fseek(..., SEEK_CUR), the job runs OK. 
Considering fseek(..., SEEK_CUR) is a basic call, could my understanding be wrong? 
Thanks.
Xiaofeng Liu
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