Command line parameter of C

Command line parameter

You may have been wondering about the empty parentheses in the first line of the main function, i.e. main().

The parentheses may contain special arguments that allow parameters to be passed to main from the operating system.

Most versions of C permit two such arguments, which are traditionally called argc and argv, respectively.

The first of these, argc, must be an integer variable, while the second, argv, is an array of pointers of characters; i.e., an array of strings.

Each string in this array will represent a parameter that is passed to main. The value of argc will indicate the number of parameters passed.

Example: The following outline indicates how the arguments argc and argv are defined within main.

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

{

.....

}

The first line can be written without the keyword void, i.e.,

main(int argc, char *argv[])

A program is normally executed by specifying the name of the program within a menu-driven environment.

Some compilers also allow a program to be executed by specifying the name of the program (actually, the name of the file containing the compiled object program) at the operating system level.

The program name is then interpreted as an operating system command. Hence, the line in which its appears is generally referred to as a command line.

In order to pass one or more parameters to the program when it is executed from the operating system, the parameters must follow the program name on the command line.

E.g.: Program-name parameter 1 parameter 2 . . . parameter n

The individual items must be separated from one another either by blank spaces or by tabs.

Some operating systems permits blank spaces to be included within a parameter provided the entire parameter is enclosed in quotation marks.

The program name will be stored as the first item in argv, followed by each of the parameters. Hence, if the program name is followed by n parameters.

There will be (n+1) entries in argv, ranging from argv [0] to argv [n]. Moreover, argc will automatically be assigned the value (n+1).

Note that the value for argc is not supplied explicitly from the command line.

An example program which will be executed from a command line:

Out put of the program

This program allows an unspecified number of parameters to be entered from the command line.

When the program is executed, the Count value for argc and the elements of argv will be displayed as separate lines of output.

Sample red white blue

then the program will be executed, resulting in the following output.

argc =4

argv [0]=sample.exe

argv [1]=red

argv [2]=white

argv [3]=blue

The output tells us that four separate items have been entered form the command line.

The first is the program name, sample. exe, followed by the three parameters., red. White and blue.

Each item is an element in the array argv. (Name that sample.exe is the name of the object file resulting from the compilation of the source code sample. C.)

Similarly, if the command line is

Sample red "white blue"

The resulting output will be

argc=3

argv [0]=sample.exe

argv [1]=red

argv [2]=white blue

In, this case the string "white blue" will be interpreted as a single parameter, because of the quotation marks.

Once the parameters have been entered, they can be utilized within the program in any desired manner.

One particularly common application is to specify the names of data files as command line parameter.