C Programming Tutorial 

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Making maps of data.

You have a map (a plan) of the computer's memory. You need to find that essential piece of information which is stored at some unknown location. How will you find it? You need a pointer!

A pointers is a special type of variable which holds the address or location of another variable. Pointers point to these locations by keeping a record of the spot at which they were stored. Pointers to variables are found by recording the address at which a variable is stored. It is always possible to find the address of a piece of storage in C using the special & operator. For instance: if location were a float type variable, it would be easy to find a pointer to it called location_ptr.

float location;
float *location_ptr,*address;

location_ptr = &(location);


address = &(location);

The declarations of pointers look a little strange at first. The star * symbol which stands in front of the variable name is C's way of declaring that variable to be a pointer. The four lines above make two identical pointers to a floating point variable called location, one of them is called location_ptr and the other is called address. The point is that a pointer is just a place to keep a record of the address of a variable, so they are really the same thing.

A pointer is a bundle of information that has two parts. One part is the address of the beginning of the segment of memory that holds whatever is pointed to. The other part is the type of value that the pointer points to the beginning of. This tells the computer how much of the memory after the beginning to read and how to interpret it. Thus, if the pointer is of a type int, the segment of memory returned will be four bytes long (32 bits) and be interpreted as an integer. In the case of a function, the type is the type of value that the function will return, although the address is the address of the beginning of the function executable.

If, like some modern day programmers, you believe in sanctity of high level languages, it is probably a source of wonder why anyone Would ever want to know the address of these variables. Having gone to the trouble to design a high level language, like C, in which variables can be given elegant and meaningful names: it seems like a step in the backward direction to want to be able to find out the exact number of the memory location at which it is stored! The whole point of variables, after all, is that it is not necessary to know exactly where information is really stored. This is not quite fair though. It is certainly rare indeed when we should want to know the actual number of the memory location at which something is stored. That would really make the idea of a high level language a bit pointless. The idea behind pointers is that a high level programmer can now find out the exact location of a variable without ever having to know the actual number involved. Remember:

A pointer is a variable which holds the address of the storage location for another given variable.

C provides two operators & and * which allow pointers to be used in many versatile ways.