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Re: Bash uses lseek while reading from serial device

From: Linda Walsh
Subject: Re: Bash uses lseek while reading from serial device
Date: Sun, 17 Aug 2014 15:21:47 -0700
User-agent: Thunderbird

Thanks, I'll pass this on...(back)?   (busybodiesRus?)

Chet Ramey wrote:

The original poster's speculation is correct.  Bash is not allowed to
read more input from stdin than it actually consumes, so commands that
it runs get the intended input.  To that end, it tries to detect whether
or not the fd it is using for standard input is seekable: if it is, bash
assumes that it can correctly position the file pointer by seeking
backwards; if it is not, bash reads a character at a time.

Bash uses lseek to the current file position to check this.  If the lseek
returns -1/EPIPE, bash assumes the fd is not seekable.  If it returns 0,
bash assumes that it can move around freely.  Since bash is trying to seek
backwards in the file, stdin is either a regular file or a tty (in which
case bash assumes that reads are newline-delimited by the device driver).

I suspect what happens is that isatty() returns 1 for serial devices, but
reads are not newline-delimited.


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