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Amazon.com: C#TM Network Programming (764): Richard Blum: Books.
Arrays allow to define type of variables that can hold several data items of the same kind. Similarly structure is another user defined data type available in C that allows to combine data items of different kinds.
Structures are used to represent a record. Suppose you want to keep track of your books in a library. You might want to track the following attributes about each book −
- Title
- Author
- Subject
- Book ID
Defining a Structure
To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than one member. The format of the struct statement is as follows −
The structure tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the structure's definition, before the final semicolon, you can specify one or more structure variables but it is optional. Here is the way you would declare the Book structure −
Accessing Structure Members
To access any member of a structure, we use the member access operator (.). The member access operator is coded as a period between the structure variable name and the structure member that we wish to access. You would use the keyword struct to define variables of structure type. The following example shows how to use a structure in a program −
When the above code is compiled and executed, it produces the following result −
Structures as Function Arguments
You can pass a structure as a function argument in the same way as you pass any other variable or pointer.
When the above code is compiled and executed, it produces the following result −
Pointers to Structures
You can define pointers to structures in the same way as you define pointer to any other variable −
Now, you can store the address of a structure variable in the above defined pointer variable. To find the address of a structure variable, place the '&'; operator before the structure's name as follows −
To access the members of a structure using a pointer to that structure, you must use the → operator as follows −
Let us re-write the above example using structure pointer.
When the above code is compiled and executed, it produces the following result −
Bit Fields
Bit Fields allow the packing of data in a structure. This is especially useful when memory or data storage is at a premium. Typical examples include −
- Packing several objects into a machine word. e.g. 1 bit flags can be compacted.
- Reading external file formats -- non-standard file formats could be read in, e.g., 9-bit integers.
C allows us to do this in a structure definition by putting :bit length after the variable. For example −
Here, the packed_struct contains 6 members: Four 1 bit flags f1..f3, a 4-bit type and a 9-bit my_int.
C automatically packs the above bit fields as compactly as possible, provided that the maximum length of the field is less than or equal to the integer word length of the computer. If this is not the case, then some compilers may allow memory overlap for the fields while others would store the next field in the next word.
- C Programming Tutorial
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Pointers in C are easy and fun to learn. Some C programming tasks are performed more easily with pointers, and other tasks, such as dynamic memory allocation, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect C programmer. Let's start learning them in simple and easy steps.
As you know, every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator, which denotes an address in memory. Consider the following example, which prints the address of the variables defined −
When the above code is compiled and executed, it produces the following result −
What are Pointers?
A pointer is a variable whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before using it to store any variable address. The general form of a pointer variable declaration is −
Here, type is the pointer's base type; it must be a valid C data type and var-name is the name of the pointer variable. The asterisk * used to declare a pointer is the same asterisk used for multiplication. However, in this statement the asterisk is being used to designate a variable as a pointer. Take a look at some of the valid pointer declarations −
The actual data type of the value of all pointers, whether integer, float, character, or otherwise, is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to.
How to Use Pointers?
There are a few important operations, which we will do with the help of pointers very frequently. (a) We define a pointer variable, (b) assign the address of a variable to a pointer and (c) finally access the value at the address available in the pointer variable. This is done by using unary operator * that returns the value of the variable located at the address specified by its operand. The following example makes use of these operations −
When the above code is compiled and executed, it produces the following result −
NULL Pointers
It is always a good practice to assign a NULL value to a pointer variable in case you do not have an exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned NULL is called a null pointer.
The NULL pointer is a constant with a value of zero defined in several standard libraries. Consider the following program −
When the above code is compiled and executed, it produces the following result −
In most of the operating systems, programs are not permitted to access memory at address 0 because that memory is reserved by the operating system. However, the memory address 0 has special significance; it signals that the pointer is not intended to point to an accessible memory location. But by convention, if a pointer contains the null (zero) value, it is assumed to point to nothing.
To check for a null pointer, you can use an 'if' statement as follows −
Pointers in Detail
Pointers have many but easy concepts and they are very important to C programming. The following important pointer concepts should be clear to any C programmer −
Sr.No. | Concept & Description |
---|---|
1 | Pointer arithmetic There are four arithmetic operators that can be used in pointers: ++, --, +, - |
2 | Array of pointers You can define arrays to hold a number of pointers. |
3 | Pointer to pointer C allows you to have pointer on a pointer and so on. |
4 | Passing pointers to functions in C Passing an argument by reference or by address enable the passed argument to be changed in the calling function by the called function. |
5 | Return pointer from functions in C C allows a function to return a pointer to the local variable, static variable, and dynamically allocated memory as well. |