C - Input and Output

When we say Input, it means to feed some data into a program. An input can be given in the form of a file or from the command line. C programming provides a set of built-in functions to read the given input and feed it to the program as per requirement.

When we say Output, it means to display some data on screen, printer, or in any file. C programming provides a set of built-in functions to output the data on the computer screen as well as to save it in text or binary files.

The Standard Files

C programming treats all the devices as files. So devices such as the display are addressed in the same way as files and the following three files are automatically opened when a program executes to provide access to the keyboard and screen.

Standard File	File Pointer	Device
Standard input	stdin	Keyboard
Standard output	stdout	Screen
Standard error	stderr	Your screen

The file pointers are the means to access the file for reading and writing purpose. This section explains how to read values from the screen and how to print the result on the screen.

The getchar() and putchar() Functions

The int getchar(void) function reads the next available character from the screen and returns it as an integer. This function reads only single character at a time. You can use this method in the loop in case you want to read more than one character from the screen.

The int putchar(int c) function puts the passed character on the screen and returns the same character. This function puts only single character at a time. You can use this method in the loop in case you want to display more than one character on the screen. Check the following example −

#include <stdio.h>
int main( ) {

   int c;

   printf( "Enter a value :");
   c = getchar( );

   printf( "\nYou entered: ");
   putchar( c );

   return 0;
}

When the above code is compiled and executed, it waits for you to input some text. When you enter a text and press enter, then the program proceeds and reads only a single character and displays it as follows −

$./a.out
Enter a value : this is test
You entered: t

The gets() and puts() Functions

The char *gets(char *s) function reads a line from stdin into the buffer pointed to by s until either a terminating newline or EOF (End of File).

The int puts(const char *s) function writes the string 's' and 'a' trailing newline to stdout.

#include <stdio.h>
int main( ) {

   char str[100];

   printf( "Enter a value :");
   gets( str );

   printf( "\nYou entered: ");
   puts( str );

   return 0;
}

When the above code is compiled and executed, it waits for you to input some text. When you enter a text and press enter, then the program proceeds and reads the complete line till end, and displays it as follows −

$./a.out
Enter a value : this is test
You entered: this is test

The scanf() and printf() Functions

The int scanf(const char *format, ...) function reads the input from the standard input stream stdin and scans that input according to the format provided.

The int printf(const char *format, ...) function writes the output to the standard output stream stdout and produces the output according to the format provided.

The format can be a simple constant string, but you can specify %s, %d, %c, %f, etc., to print or read strings, integer, character or float respectively. There are many other formatting options available which can be used based on requirements. Let us now proceed with a simple example to understand the concepts better −

#include <stdio.h>
int main( ) {

   char str[100];
   int i;

   printf( "Enter a value :");
   scanf("%s %d", str, &i);

   printf( "\nYou entered: %s %d ", str, i);

   return 0;
}

When the above code is compiled and executed, it waits for you to input some text. When you enter a text and press enter, then program proceeds and reads the input and displays it as follows −

$./a.out
Enter a value : seven 7
You entered: seven 7

Here, it should be noted that scanf() expects input in the same format as you provided %s and %d, which means you have to provide valid inputs like "string integer". If you provide "string string" or "integer integer", then it will be assumed as wrong input. Secondly, while reading a string, scanf() stops reading as soon as it encounters a space, so "this is test" are three strings for scanf().

C - typedef

The C programming language provides a keyword called typedef, which you can use to give a type a new name. Following is an example to define a term BYTE for one-byte numbers −

typedef unsigned char BYTE;

After this type definition, the identifier BYTE can be used as an abbreviation for the type unsigned char, for example..

BYTE  b1, b2;

By convention, uppercase letters are used for these definitions to remind the user that the type name is really a symbolic abbreviation, but you can use lowercase, as follows −

typedef unsigned char byte;

You can use typedef to give a name to your user defined data types as well. For example, you can use typedef with structure to define a new data type and then use that data type to define structure variables directly as follows −

#include <stdio.h>
#include <string.h>
 
typedef struct Books {
   char title[50];
   char author[50];
   char subject[100];
   int book_id;
} Book;
 
int main( ) {

   Book book;
 
   strcpy( book.title, "C Programming");
   strcpy( book.author, "Nuha Ali"); 
   strcpy( book.subject, "C Programming Tutorial");
   book.book_id = 6495407;
 
   printf( "Book title : %s\n", book.title);
   printf( "Book author : %s\n", book.author);
   printf( "Book subject : %s\n", book.subject);
   printf( "Book book_id : %d\n", book.book_id);

   return 0;
}

When the above code is compiled and executed, it produces the following result −

Book  title : C Programming
Book  author : Nuha Ali
Book  subject : C Programming Tutorial
Book  book_id : 6495407

typedef vs #define

#define is a C-directive which is also used to define the aliases for various data types similar to typedef but with the following differences −

• typedef is limited to giving symbolic names to types only where as #define can be used to define alias for values as well, q., you can define 1 as ONE etc.
• typedef interpretation is performed by the compiler whereas #define statements are processed by the pre-processor.

The following example shows how to use #define in a program −

#include <stdio.h>
 
#define TRUE  1
#define FALSE 0
 
int main( ) {
   printf( "Value of TRUE : %d\n", TRUE);
   printf( "Value of FALSE : %d\n", FALSE);

   return 0;
}

When the above code is compiled and executed, it produces the following result −

Value of TRUE : 1
Value of FALSE : 0

C - Bit Fields

Suppose your C program contains a number of TRUE/FALSE variables grouped in a structure called status, as follows −

struct {
   unsigned int widthValidated;
   unsigned int heightValidated;
} status;

This structure requires 8 bytes of memory space but in actual, we are going to store either 0 or 1 in each of the variables. The C programming language offers a better way to utilize the memory space in such situations.

If you are using such variables inside a structure then you can define the width of a variable which tells the C compiler that you are going to use only those number of bytes. For example, the above structure can be re-written as follows −

struct {
   unsigned int widthValidated : 1;
   unsigned int heightValidated : 1;
} status;

The above structure requires 4 bytes of memory space for status variable, but only 2 bits will be used to store the values.

If you will use up to 32 variables each one with a width of 1 bit, then also the status structure will use 4 bytes. However as soon as you have 33 variables, it will allocate the next slot of the memory and it will start using 8 bytes. Let us check the following example to understand the concept −

#include <stdio.h>
#include <string.h>

/* define simple structure */
struct {
   unsigned int widthValidated;
   unsigned int heightValidated;
} status1;

/* define a structure with bit fields */
struct {
   unsigned int widthValidated : 1;
   unsigned int heightValidated : 1;
} status2;
 
int main( ) {

   printf( "Memory size occupied by status1 : %d\n", sizeof(status1));
   printf( "Memory size occupied by status2 : %d\n", sizeof(status2));

   return 0;
}

When the above code is compiled and executed, it produces the following result −

Memory size occupied by status1 : 8
Memory size occupied by status2 : 4

Bit Field Declaration

The declaration of a bit-field has the following form inside a structure −

struct {
   type [member_name] : width ;
};

The following table describes the variable elements of a bit field −

Sr.No.	Element & Description
1	type An integer type that determines how a bit-field's value is interpreted. The type may be int, signed int, or unsigned int.
2	member_name The name of the bit-field.
3	width The number of bits in the bit-field. The width must be less than or equal to the bit width of the specified type.

The variables defined with a predefined width are called bit fields. A bit field can hold more than a single bit; for example, if you need a variable to store a value from 0 to 7, then you can define a bit field with a width of 3 bits as follows −

struct {
   unsigned int age : 3;
} Age;

The above structure definition instructs the C compiler that the age variable is going to use only 3 bits to store the value. If you try to use more than 3 bits, then it will not allow you to do so. Let us try the following example −

#include <stdio.h>
#include <string.h>

struct {
   unsigned int age : 3;
} Age;

int main( ) {

   Age.age = 4;
   printf( "Sizeof( Age ) : %d\n", sizeof(Age) );
   printf( "Age.age : %d\n", Age.age );

   Age.age = 7;
   printf( "Age.age : %d\n", Age.age );

   Age.age = 8;
   printf( "Age.age : %d\n", Age.age );

   return 0;
}

When the above code is compiled it will compile with a warning and when executed, it produces the following result −

Sizeof( Age ) : 4
Age.age : 4
Age.age : 7
Age.age : 0

C - Unions

A union is a special data type available in C that allows to store different data types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time. Unions provide an efficient way of using the same memory location for multiple-purpose.

Defining a Union

To define a union, you must use the union statement in the same way as you did while defining a structure. The union statement defines a new data type with more than one member for your program. The format of the union statement is as follows −

union [union tag] {
   member definition;
   member definition;
   ...
   member definition;
} [one or more union variables];  

The union 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 union's definition, before the final semicolon, you can specify one or more union variables but it is optional. Here is the way you would define a union type named Data having three members i, f, and str −

union Data {
   int i;
   float f;
   char str[20];
} data;  

Now, a variable of Data type can store an integer, a floating-point number, or a string of characters. It means a single variable, i.e., same memory location, can be used to store multiple types of data. You can use any built-in or user defined data types inside a union based on your requirement.

The memory occupied by a union will be large enough to hold the largest member of the union. For example, in the above example, Data type will occupy 20 bytes of memory space because this is the maximum space which can be occupied by a character string. The following example displays the total memory size occupied by the above union −

#include <stdio.h>
#include <string.h>
 
union Data {
   int i;
   float f;
   char str[20];
};
 
int main( ) {

   union Data data;        

   printf( "Memory size occupied by data : %d\n", sizeof(data));

   return 0;
}

When the above code is compiled and executed, it produces the following result −

Memory size occupied by data : 20

Accessing Union Members

To access any member of a union, we use the member access operator (.). The member access operator is coded as a period between the union variable name and the union member that we wish to access. You would use the keyword union to define variables of union type. The following example shows how to use unions in a program −

#include <stdio.h>
#include <string.h>
 
union Data {
   int i;
   float f;
   char str[20];
};
 
int main( ) {

   union Data data;        

   data.i = 10;
   data.f = 220.5;
   strcpy( data.str, "C Programming");

   printf( "data.i : %d\n", data.i);
   printf( "data.f : %f\n", data.f);
   printf( "data.str : %s\n", data.str);

   return 0;
}

When the above code is compiled and executed, it produces the following result −

data.i : 1917853763
data.f : 4122360580327794860452759994368.000000
data.str : C Programming

Here, we can see that the values of i and f members of union got corrupted because the final value assigned to the variable has occupied the memory location and this is the reason that the value of str member is getting printed very well.

Now let's look into the same example once again where we will use one variable at a time which is the main purpose of having unions −

#include <stdio.h>
#include <string.h>
 
union Data {
   int i;
   float f;
   char str[20];
};
 
int main( ) {

   union Data data;        

   data.i = 10;
   printf( "data.i : %d\n", data.i);
   
   data.f = 220.5;
   printf( "data.f : %f\n", data.f);
   
   strcpy( data.str, "C Programming");
   printf( "data.str : %s\n", data.str);

   return 0;
}

When the above code is compiled and executed, it produces the following result −

data.i : 10
data.f : 220.500000
data.str : C Programming

Here, all the members are getting printed very well because one member is being used at a time.