12-08-2012
See the 0 (zero) in the char a[] array? When printf tries to to display a string (the %s format) it stops with a byte that is zero - ascii NUL. That is the very first byte in the array. Nothing prints, because %s says stop at NUL. The strlen() function works the same way - it stops with NUL. So "length" is zero.
printf promotes a[2] from a char to an int, then prints it -- the %d version of printf.
When you deal with a single signed char (one byte) C has standards. One of those standards or rules discusses promoting values. A signed char datatype when used in a context which deals with aritmetic (number) operations gets placed into a register just like as if it were an int datatype. So the %d format specifier on a char produces a number on the tty.
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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)