View source for Linux System Programming

== Reference ==
* [http://perl.plover.com/yak/commands-perl/ System Programming in Perl: The Unix Process Model (slides)]

== Process ==
A process contains:
* A unique '''process ID number''' ('pid')
* A '''current working directory''' ('cwd')
* A '''user ID''' and '''group ID''' number (actually more than one of each)
* An '''open file table'''
* An '''environment' (just a bunch of string data)
* A '''signal''' table
* An '''alarm''' clock
* Lots of other equipment

==Why fork & exec ==
* 2 main primitives of kernel to manage processes:
**'''fork''': create a new process
**'''exec''': replace a process's object code with the contents of a file 
* Launching a new process is done in 2 steps:
*# ''Fork'' the current process
*# In the child process, ''exec'' the new file that will then replace the code of the current child but keep environment.
* Why '''fork and exec''' with 2 separate commands?<br/>Because the child process can alter its environment before it does the exec. For example:<br/><source lang="bash">
ps > /tmp/procs                # The child shell will first change the output FD, without ps knowing
</source>

=== Fork ===
* The ''child'' will get a '''new''' copy of the '''file descriptor (FD) table'''.<br/>So closing a file in the child will not interfere with the parents.
* However the '''system open file table''' is '''not''' copied; the same table is shared between child and parent. Why? Because parent must keep the same '''Seek pointer''' as the child, even after child is dead:
<source lang="bash">
cat ./pie
# #!/bin/sh
# echo "I like pie.";                      # Will move seek pointer, also in parent
# echo "Especially blackberry.";           # Will write starting from new position of seek pointer!
./pie > piefile
</source>

== <tt>/proc</tt> filesystem ==
Some links:
* [http://blog.ksplice.com/2011/01/solving-problems-with-proc/ Solving problems with proc]
** ''phantom progress bar'' &mdash; Showing progress of a process in a file, after process was launched

* For a shell, <tt>/dev/fd/###</tt>, <tt>/proc/self/fd/###</tt> and <tt>/proc/$$/fd/###</tt> refer to the same file

* If a program only takes a filename as argument and you want them to process file from standard input, use the fake file <tt>/proc/self/fd/O</tt> ([http://unix.derkeiler.com/Newsgroups/comp.unix.questions/2003-06/0077.html]):
<source lang="bash">
antiword /proc/self/fd/0 < test.doc > test.txt
somecommand | antiword /proc/self/fd/0 > test.txt   # Not guarantee to work, because pipe does not support random access
antiword <(somecommand) >test.txt                   # Using bash process substitution
</source>
* Read the ''manpages'' for more information:
<source lang="bash">man proc</source>

== pthreads ==

* [http://www.domaigne.com/blog/computing/condvars-signal-with-mutex-locked-or-not/ condvars: signal with mutex locked or not?]
* [http://stackoverflow.com/questions/4544234/calling-pthread-cond-signal-without-locking-mutex Calling pthread_cond_signal without locking mutex]
* [https://groups.google.com/forum/?hl=ky&fromgroups#!msg/comp.programming.threads/wEUgPq541v8/ZByyyS8acqMJ basic question about concurrency]

== Input / output (IO) ==

Some traps:

* Make sure to call <code>write</code> in a loop because write may do ''partial'' writes [https://www.gnu.org/software/libc/manual/html_node/I_002fO-Primitives.html#I_002fO-Primitives].
* When <code>fsync</code> fails, it will delete the cached pages, meaning that the data will be '''lost'''. Note that next fsync would then succeed. See more on fsync [https://lwn.net/Articles/752063/ here], [https://stackoverflow.com/questions/42434872/writing-programs-to-cope-with-i-o-errors-causing-lost-writes-on-linux here], and [https://stackoverflow.com/questions/37288453/calling-fsync2-after-close2/50158433 here].

References:
* [https://danluu.com/file-consistency/ Files are hard] (how to write to disk in a robust way).
: Very detailed way on how to write to files and dealing with I/O errors.

== Memory protection ==
See [[C]] page how we used <code>mprotect</code> to change the protection (read / write / execute) on a memory zone.

== Examples ==

=== Kill on ALARM signal ===
Small script to kill a process on ALARM signal, implementing a kindof timeout kill (see [[Perl#Kill on ALARM signal|Perl page]]).