2011-04-26



Low-level C programming  revisited.   Just  some  excercises  for
university, but they often provide good bases to go flying ...

Take this code snippet:

     printf("parent\n");
     fork();
     printf("child\n");

It will print:

     parent
     child
     child

But if you pipe the output through cat(1) it prints:

     parent
     child
     parent
     child

Why?

And this code:

     printf("parent ");
     fork();
     printf("child\n");

will print:

     parent child
     parent child

no matter if you pipe the output or not.

This shows that you really should understand the  facilities  you
use.   In  this case the buffering of stdio:  Newline flushes the
buffer if, and only if, it outputs to a terminal.




I write these tiny excercise programs on some  Sun  workstations.
They run SunOS 5.9 ... and have some strange keyboard layout. :-/
However, why not?  As the programs I write are small, this  is  a
nice  opportunity to use ed(1) for it.  I like ed but maintaining
large source files with it is a pain, even for me.   Those  small
programs, in contrast, are perfectly suited.

But ... the version of ed they have on those systems is terrible.
I always thought ed would be ed, but no, this one requires ending
delimiter for regexps -- how unneccesary!  And it doesn't  under-
stand:

     s,foo,bar

(Note the missing last delimiter, causing the line to be  printed
afterwards.)

Heirloom ed is luxury in contrast!





Finally, Michi pointed this out to me:

     mkdir new
     cd new
     ls -a >foo

What does `cat foo' print?

Hint: You need to find the core of the question.

Then think about fork(2) and exec(2) in some shell.



http://marmaro.de/lue/        markus schnalke <meillo@marmaro.de>