octal dump / command line arguments

Lab: Command Line Arguments & od
Source: Katrin Becker, 1999

Command Line Arguments:

Up till now all we have seen is:

int main ()
{
return(0);
}

use: ./a.out

Most Unix commands are written in C so there must be a way to make our programs look like Unix commands. This is done by passing arguments to main in the command line. Main can take two arguments:

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

The names "argc" and "argv" are traditional names; not required but all C/C++ programmers will know what they are if you use these names.

argc is a count of the number of arguments on the line
argv is an array of strings (1 string for each argument)

example:

g++ -g -o myprog myprog.cc

argc = 5
argv[0] = "g++"
argv[1] = "-g"
argv[2] = "-o"
argv[3] = "myprog"
argv[4] = "myprog.cc"

You decide how many arguments should be there and in what order they should appear. Most Unix command have some optional 'switches' which always start with "-" and if file names are required, they always come last. again this is tradition rather than a requirement.

Note: all arguments are strings; those destined to become numbers must be converted by you inside your program.

How to use UNIX od (octal dump)

- if you don't know this facility make sure you read the man pages and learn it!

od [options] [file] [[+] offset[. | b]]

Octal dump; produce a dump (normally octal) of the named file. File is displayed from its beginning unless you specify an offset (normally in octal bytes)

OPTIONS:

-b : display as octal
-c : display as ASCII
-d : display words as unsigned decimal (word = 16 bits)
-D : display words = 32 bits as unsigned
-f : display as 32-bit words as floating point
-F : display as 64 bit floating point
-o : unsigned octal
-O : 32-bit unsigned octal
-s : signed decimal
-S : 32 bit signed decimal
-v : verbose
-x : hex
-X : 32 bit hex
+ : required before offset if file isn't specified

Modifiers for offset:

. : offset is decimal
b : offset is 512-byte blocks

Drawing by: Mike Polowick
CPSC 233, January 1999