C Interview Questions Part 14

Q:When we open a file, how does functions like fread( )/fwrite( ), etc. get to know from where to read or to write the data?

A: When we open a file for read/write operation using function like fopen( ), it returns a pointer to the structure of type FILE. This structure stores the file pointer called position pointer, which keeps track of current location within the file. On opening file for read/write operation, the file pointer is set to the start of the file. Each time we read/write a character, the position pointer advances one character. If we read one line of text at a step from the file, then file pointer advances to the start of the next line. If the file is opened in append mode, the file pointer is placed at the very end of the file. Using fseek( ) function we can set the file pointer to some other place within the file
 
Q: Explain the use of array indices ?

A:If we wish to store a character in a char variable ch and the character to be stored depends on the value of another variable say color (of type int), then the code would be as shown below:

switch ( color )

{

case 0 :

ch = 'R' ;

break ;

case 1 :

ch = 'G' ;

break ;

case 2 :

ch = 'B' ;

break ;

}

In place of switch-case we can make use of the value in color as an index for a character array. How to do this is shown in following code snippet.

char *str = "RGB' ;

char ch ;

int color ;

// code

ch = str[ color ] ;

Q:How do I write code that would get error number and display error message if any standard error occurs?

A: Following code demonstrates this.

#include

main( )

{

char *errmsg ;

FILE *fp ;

fp = fopen ( "C:\file.txt", "r" ) ;

if ( fp == NULL )

{

errmsg = strerror ( errno ) ;

printf ( "\n%s", errmsg ) ;

}

}

Here, we are trying to open 'file.txt' file. However, if the file does not exist, then it would cause an error. As a result, a value (in this case 2) related to the error generated would get set in errno. errno is an external int variable declared in 'stdlib.h' and also in 'errno.h'. Next, we have called sterror( ) function which takes an error number and returns a pointer to standard error message related to the given error number.