DAY36
Thursday, 02 August 2012

Today is NOT a LAB day, but we will meet in the lab as usual.

Today, I will talk about a special feature of the C programming language that allows you to make your C program look like a command, called the command line interface.

Things we will discuss:

• Assuming your program is named "add", a typical use of the command line might be

              ./add 12 33 
            

  and presumably the program would print out "45". Up to now, we would have had to run the program, then ask the user to enter the two numbers, and use scanf() to read the numbers.
• To use command line arguments, you must declare your main program as

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

• argc is an integer which counts the number of arguments on the line. For the example

              ./add 12 33
            

  argc will be 3.
• argv is an array of strings, containing the individual words that appeared on the command line. For the example

              ./add 12 33
            

  argv[0] will be "./add", argv[1] will be "12" and argv[2] will be "33". Example: report_args.c
• Since ARGC tells you how many arguments were entered on the command line, a program that expects a specific number of inputs can detect that some arguments are missing, and prompt the user to enter them. Example: missing.c.
• If an argument is useful as a string, then you can use it right away; Many UNIX commands work with filenames, which are strings, so this is a common occurrence. Example: copyfile.c.
• You can also use the strcpy() function to copy the argument into another string variable if that is more convenient. The strcpy() command must be declared using the statement "#include ". Example: define.c.
• If an argument is numeric, you need to call a function to convert the string to a number. atoi() converts a string to an int, atof() converts a string to a real number (double or float). Example: power.c
• Command line arguments are a very common way of allowing a user to run a program with a small amount of input information. Example: lights_out_opengl.c

Programs we might discuss:

• copyfile.c makes a copy of a file. If the executable is named "copyfile" you might use it by typing

              ./copyfile original.txt copy.txt
            

• define.c is a dictionary program which provides definitions for words. Currently, it only knows the definitions of "aardvark", "abyss", "acumen", "addle", "aerie", "affix", "agar", "ahoy", "aigrette", and "ajar". Compile the program, rename the executable to "define", and then you can give commands like "./define ajar" and the program will give you the definition.
• lights_out_opengl.c uses OpenGL to set up a game of "Lights Out". The user is expected to enter three command line arguments, the number of rows, columns, and scrambling moves. Successfully compiling this program requires including the OpenGL library.
• missing.c is a program which expects to see 4 items on the command line, namely, the program name and three words. If any items are missing, it prompts for them to be entered by the user.
• power.c is a program which expects to see a real number x followed by an integer n, and reports the value of y = x^n.
• report_args.c reports the command line arguments.