---
title: Eboard 11  Understanding your code
number: 11
section: eboards
held: 2017-04-20
current: true
---
CSC 282.01, Class 11:  Understanding your code
==============================================

_Overview_

* Preliminaries
    * Notes and news
    * Upcoming work
    * Questions
* The joy of segfaults
* Other memory problems
* Tracing errors with `gdb`
* Checking memory usage with `valgrind`

### News / Etc.

* Happy 20/4.  Please be moderate.

### Upcoming work

* Probably none, given how things have been going.

### Good things to do

* Be moderate.
* Go to convo.
* Go to tonight's talk on digital humanities.
* CS table next Tuesday.
* Contra dance tomorrow night
* And more.

### Questions

Can you explain the `cs282` thing you type?

* If you use `bash`, you have two important files that are usually
  run at startup.  `.bashrc` and `.bash_profile`
* Because they start with a period, you won't see them when you type
  `ls` unless you type `ls -a`.
* Your bashrc can contain all sorts of instructions
     * Settings for important variables, like `CC`, or `PATH`, or ...
     * aliases - words you type to do more complex things
     * `alias word="commands" - defines the alias
     * We can also put function definitions in our bashrc.  Then our
       aliased commands can have parameters.
     * `function alex() { pushd /home/mitchell17/$1; ls -alF }`

What is the difference between cd and pushd.

* `push` normally pushes something onto a stack.  pushd pushes your current
  directory before going to another directory
* `popd` lets you go back to where you were
* `dirs` lists all the directories on your stack

How do I set my prompt to something awesome?

* `export PS1="something awesome "`
* http://www.gnu.org/software/bash/manual/bash.html#Controlling-the-Prompt
* `function servant { export PS1="$1 says: What is your command? "; }`

An example
----------

What does this program do?

```
/**
 * example.c
 *   An example for our discussion of GDB and valgrind.
 */

// +---------+---------------------------------------------------------
// | Headers |
// +---------+

#include <assert.h>
#include <stdio.h>

// +---------+---------------------------------------------------------
// | Helpers |
// +---------+

/**
 * A function that returns an array on the stack.
 */
void
vals (int *result[])
{
  int values[10];
  *result = values;
} // vals

/**
 * A function that fills in some values in an array.
 */
void
f (int n)
{
  int i;
  int values[10];
  for (i = 0; i < 10; i++)
    {
      values[i] = n;
      assert(values[i] == n);
    } // for
} // f

// +------+------------------------------------------------------------
// | Main |
// +------+

int
main (void)
{
  int *stuff;
  vals (&stuff);
  stuff[0] = 42;
  f (5);
  assert (stuff[0] == 42);
  return 0;
} // main
```

Whoops!  We allocated space on the stack, rather than on the heap.

Detour: Why foobar?
-------------------

* Observation: At some point, folks at MIT started using `foo` in the
  programs they wrote.
    * Others soon started using `bar`
    * WW II military acronym, FUBAR, Fouled up beyond all recognition
* Observation 2: The Smokey Stover comic strip in the 1920's also used
  the term foo, as did an early Daffy Duck cartoon.


The joy of segfaults
--------------------

Why might your program segfault?

* When you reference something that doesn't exist.
     * `perpetual_motion_machine = 3`
     * Referencing a non-existent field is a compiler error
       `mac.field = 2`.
* Go too far in an array.
     * Files may behave differently
* You attempt to write your own linked lists, and you are not yet 
  competent.
* malloc and free
     * Assign to pointer that has not been allocated

What do you do when it does?

* Find someone who knows C better than I do and ask for help.
* Rely on my knowledge of likely causes of errors and think carefully
  about any places in the code.
* Curse myself for celebrating 4/20 in Sam's class too often.

Other memory problems
---------------------

What memory issues might crop up that don't cause (immediate) segfaults?

* Go too far in an array.

How do you find those issues?

Tracing errors with `gdb`
-------------------------

Checking memory usage with `valgrind`
-------------------------------------