Week 9 lab topics:

1. command line arguments in C and atoi function
2. dynamically allocated 2-D arrays in C
3. file I/O in C
4. make and makefiles
5. a reminder about using gdb and valgrind for debugging C programs

---

Create a week09 subdirectory in your weeklylab subdirectory and copy over some files:

    cd cs31/weeklylab		
    pwd
    mkdir week09
    ls
    cd week09
    pwd
    cp ~newhall/public/cs31/week09/* .
    ls

Let's look at an example C program that takes command line arguments. Open commandlineargs.c in an editor:

vim commandlineargs.c

The first thing to note is the change in main's definition:

int main(int argc, char *argv[]) { ...

The first parameter to main, argc, is the number of command line arguments. For example, if the user enters:

./a.out 10 11 200

argc will be 4 (a.out counts as one of the command line arguments, and 10, 11, and 200 as three more).

Each command line argument is passed to main in the second parameter, argv, as a string (argv is an array of strings):

        -----
argv[0]:| *-|-----> "./a.out"
        -----
argv[1]:| *-|-----> "10"
        -----
argv[2]:| *-|-----> "11"
        -----
argv[3]:| *-|-----> "200"
        -----
argv[4]:| *-|-----|     (NULL pointer: no more command line strings)
        -----

C has functions that can convert strings of numeric characters to their int, float, and other basic types, values.

int x = atoi(argv[1]);  // x gets the int value 10

See the man page for atoi for more information (man atoi).

Let's try compiling and running this program with different command line arguments and see what happens.

We are going to compare variable declaration, memory layout, and access syntax for a statically declared 2D array and one way to dynamically allocating a 2D array ( Method 1: the memory efficient way).

More information about arrays in C, including dynamically allocated 2D arrays: Arrays in C (we will use Method 1 for dynamically allocated 2D arrays).

We are going to look at an example of file I/O in C.

Here is some documentation about file I/O in C: File I/O in C

For the next lab, you will write a makefile from scratch to compile your program.

Here is some documentation about writing makefiles: Makefiles I suggest following the simple example here, and/or using a makefile from a previous lab assignment as a starting point.

We are back to C programming this week, with pointers and functions. You are all experts at using gdb to debug at the assembly level, but remember that you also know how to use gdb (and valgrind) to debug programs at the C code level.

In lab on week 5 we looked at some examples of using both gdb and valgrind. Take a look at the the week 5 weekly lab page and the examples we talked about as a reminder of how to use these debugging tools.

Also, here are links to my guides for gdb, ddd, and valgrind:

• My GDB Guide for more information about using gdb and ddd.
• Valgrind Guide