Alternate to copy + remove Post: 302957538

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Top Forums Shell Programming and Scripting Alternate to copy + remove Post 302957538 by Don Cragun on Monday 12th of October 2015 02:23:49 PM

10-12-2015

Registered User

Note, however, that you have a built-in race condition...
If someone opens file1 for appending during the cp and then writes to that file descriptor:

the data may be copied into fie2 by the cp,
may be cleared by the > /dev/null never to be seen again,
or may appear in file1 after it has been cleared by the > /dev/null.

To get rid of this race condition, we need to know more about how processes writing to file1 open the file and how the permissions of processes writing to that file are related to the owner and group of the directory in which the target file resides.

The writers obviously have write permission to the target file. If they also have write permission in the directory in which it resides, the safe thing would be to use:

Code:

mv file1 file2

and have the writers open file1 with the flags:

Code:

fd = open("file1", O_CREAT | O_APPEND, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH);

guaranteeing (assuming no write errors) that data written to that file descriptor will appear either in file1 or file2.

If you can't do that; you need to block all writers to file1 while it is being rotated.

Don Cragun

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LEARN ABOUT MINIX

ppmtosixel

ppmtosixel(1)                                                 General Commands Manual                                                ppmtosixel(1)

NAME

       ppmtosixel - convert a portable pixmap into DEC sixel format

SYNOPSIS

       ppmtosixel [-raw] [-margin] [ppmfile]

DESCRIPTION

       Reads  a  portable  pixmap  as input.  Produces sixel commands (SIX) as output.  The output is formatted for color printing, e.g. for a DEC
       LJ250 color inkjet printer.

       If RGB values from the PPM file do not have maxval=100, the RGB values are rescaled.  A printer control header and a color assignment table
       begin the SIX file.  Image data is written in a compressed format by default.  A printer control footer ends the image file.

OPTIONS

       -raw   If  specified,  each  pixel  will  be explicitly described in the image file.  If -raw is not specified, output will default to com-
              pressed format in which identical adjacent pixels are replaced by "repeat pixel" commands.  A raw file is often an order  of  magni-
              tude larger than a compressed file and prints much slower.

       -margin
              If  -margin  is not specified, the image will be start at the left margin (of the window, paper, or whatever).  If -margin is speci-
              fied, a 1.5 inch left margin will offset the image.

PRINTING

       Generally, sixel files must reach the printer unfiltered.  Use the lpr -x option or cat filename > /dev/tty0?.

BUGS

       Upon rescaling, truncation of the least significant bits of RGB values may result in poor color conversion.  If the original PPM maxval was
       greater  than  100,  rescaling  also reduces the image depth.  While the actual RGB values from the ppm file are more or less retained, the
       color palette of the LJ250 may not match the colors on your screen.  This seems to be a printer limitation.

SEE ALSO

       ppm(5)

AUTHOR

       Copyright (C) 1991 by Rick Vinci.

                                                                   26 April 1991                                                     ppmtosixel(1)