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Full Discussion: what is the main difference between difference between using nonatomic lseek and O_APPEND
Top Forums Programming what is the main difference between difference between using nonatomic lseek and O_APPEND Post 302715083 by jim mcnamara on Saturday 13th of October 2012 06:36:52 PM
Old 10-13-2012
O_APPEND guarantees that every write to the file will be at the end of the file. lseek guarantees that the file pointer is positioned to the current EOF of the file. It does not guarantee that subsequent writes will be at the end of the file.

The above is something that you only worry about when multiple processes or threads are writing to one file.

Neither guarantees any kind of sequence of I/O writes when there are multiple writers. If you need that, then you have to resort to something like SYSV semaphores for multi-process apps or mutxes in the case of multi-threaded apps.
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LEARN ABOUT MINIX

pnmpsnr

pnmpsnr(1)                                                    General Commands Manual                                                   pnmpsnr(1)

NAME

       pnmpsnr - compute the difference between two portable anymaps

SYNOPSIS

       pnmpsnr [pnmfile1] [pnmfile2]

DESCRIPTION

       Reads  two  PBM,  PGM, or PPM files, or PAM equivalents, as input.  Prints the peak signal-to-noise ratio (PSNR) difference between the two
       images.  This metric is typically used in image compression papers to rate the distortion between original and decoded image.

       If the inputs are PBM or PGM, pnmpsnr prints the PSNR of the luminance only.  Otherwise, it prints the separate PSNRs of the luminance, and
       chrominance (Cb and Cr) components of the colors.

       The  PSNR  of  a  given component is the ratio of the mean square difference of the component for the two images to the maximum mean square
       difference that can exist betwee any two images.  It is expressed as a decibel value.

       The mean square difference of a component for two images is the mean square difference of the component value, comparing  each  pixel  with
       the pixel in the same position of the other image.  For the purposes of this computation, components are normalized to the scale [0..1].

       The maximum mean square difference is identically 1.

       So the higher the PSNR, the closer the images are.  A luminance PSNR of 20 means the mean square difference of the luminances of the pixels
       is 100 times less than the maximum possible difference, i.e. 0.01.

SEE ALSO

       pnm(5)

                                                                   04 March 2001                                                        pnmpsnr(1)
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