Overview

• Dealing with script files.
• A C program ecosystem.
• Steps in compilation.
• Including files.
• Detour: Why we have included files.
• Macros.
• CPP cnditionals.
• Detour: Standard header file format.
• Command-line flags.
• Hack: Generic data types.

Preliminaries

Admin

• Sam gone next week. Mr. Stone discusses cron and other exciting ways to make programs run unattended.
• HW forthcoming: Read about macros in C and find/develop an interesting example. We'll talk about them in two weeks.

Questions

Moving .jpg files

Code by NK.

#!/bin/sh
# first argument is where from, second argument is where too.
for file in $(find $1 | grep .jpg) ; do
  echo "copying $file to $2"
  cp $file $2
done

Observation 1: $1 and $2 refer to command-line arguments.

Question: How do we figure out what find $1 | grep .jpg does?

• Google: Nope.
• man grep - search for lines that match a pattern
• man find - when used as find dir, lists all the files in a directory.

Conclusion: The command find $1 | grep .jpg finds all the jpg files within a directory.

Question: Might it find thigns other than jpg files. We might have a filename with .jpg in the middle. (or just any character and jpg). use \.jpg$.

Other improvements: find can actually have some parameters that specify what files to find.

find . -name '*.jpg'

Note time cmd tells you how long a command takes

Detour: How do pipes work?

• Traditionally: We start both processes. The client process runs until it requests input from the server. Then the client blocks on the server. The server runs until it generates a buffer full of output (or flushes the buffer). At that point, it goes to sleep and the client is resumed.

A C program ecosystem

On the design and implementation of larger programs written in C.

• Some motivation for this comes from the time period.
• Some comes from good program design.

Simple model of C programs: Put everything in one file. (Alternate: Put things in separate files.)

• If we change one line in one function, we have to recompile everything.
  • Partially a time period issue.
  • Recompiling Gimp completely on a MathLAN workstation: about 30 minutes
• If we separate the large file into smaller files, we can resuse code. (Libraries!)
• Easier for the programmer to read, as long as there's a good naming system, a table of contents, or grep.
• Provides more protection of your code in cases of accidental deletion. (But you use a version control system ...)
• Easier for collaboration.

Traditionally, a C project includes

• The .c file for each part of the project.
• Unit tests for each part of the project (I wish).
• Question: I'm compiling partb.c which depends on parta.c. How does the compiler know about the functions in parta?
  • We could recompile everything together!
  • We could #include parta.c, but that has the slow compilation issue.

Detour: Steps in compilation

The 161 model: .c -> executable

The actual steps:

• Preprocessor: Handles #include and #define and other directives (converts C code to C code).
• Translator: Converts C code to assembly code
• Assembler: Converts assembly code to relocatable object code (.o suffix)
• Linker: Joins togethers multiple object files into an executable

For our silly program

• parta.c -> parta.o
• partb.c -> partb.o
• parta.o + partb.o + stdio.la + ... -> prog

Back to the Question

When compiling partb, how does the compiler know about parta?

We've just seen that it doesn't have to. Maybe it's smart.

Why do we not get the warning that 'foo' is not declared?

The C compiler trusts the user.

What should we do?

Quick hack:

    extern double foo(double);

Normal strategy: For every .c file, you create a .h file that gives such declarations.

Here's parta.h

    #ifndef __PARTA_H__
    #define __PARTA_H__

    /**
     * parta.h
     *   Amazing mathemtical utils by NK and cmapsdfay.
     *
     * Released under GPL 3.0.  You should have received a
     * license with this software.  If not, got to
     * Richard Stallman's abode and ask for a copy.
     */
    extern double foo(double);

    #endif __PARTA_H__

Including files

Detour: Why we have included files

Macros

CPP cnditionals

Detour: Standard header file format

Command-line flags

Hack: Generic data types