c faqs with answers

C interview questions:Are the following two statements identical?

char str[6] = "Kicit" ;

char *str = "Kicit" ;

Answer: No! Arrays are not pointers. An array is a single, pre-allocated chunk of contiguous elements (all of the same type), fixed in size and location. A pointer on the other hand, is a reference to any data element (of a particular type) located anywhere. A pointer must be assigned to point to space allocated elsewhere, but it can be reassigned any time. The array declaration char str[6] ; requests that space for 6 characters be set aside, to be known by name str. In other words there is a location named str at which six characters are stored. The pointer declaration char *str ; on the other hand, requests a place that holds a pointer, to be known by the name str. This pointer can point almost anywhere to any char, to any contiguous array of chars, or nowhere.

C interview question:Is the following code fragment correct?

const int x = 10 ;

int arr[x] ;

Answer: No! Here, the variable x is first declared as an int so memory is reserved for it. Then it is qualified by a const qualifier. Hence, const qualified object is not a constant fully. It is an object with read only attribute, and in C, an object associated with memory cannot be used in array dimensions.

C iInterview question:How do I write code to retrieve current date and time from the system and display it as a string?

Answer:Use time( ) function to get current date and time and then ctime( ) function to display it as a string. This is shown in following code snippet.

#include

void main( )

{

time_t curtime ;

char ctm[50] ;

time ( &curtime ) ; //retrieves current time &

stores in curtime

printf ( "\nCurrent Date & Time: %s", ctime (

&curtime ) ) ;

}

C interview question:How do I change the type of cursor and hide a cursor?

Answer: We can change the cursor type by using function _setcursortype( ). This function can change the cursor type to solid cursor and can even hide a cursor. Following code shows how to change the cursor type and hide cursor.

#include

main( )

{

/* Hide cursor */

_setcursortype ( _NOCURSOR ) ;

/* Change cursor to a solid cursor */

_setcursortype ( _SOLIDCURSOR ) ;

/* Change back to the normal cursor */

_setcursortype ( _NORMALCURSOR ) ;

}

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

Answer: 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.

C interview question:How do I write code to get the current drive as well as set the current drive?

Answer: The function getdisk( ) returns the drive number of current drive. The drive number 0 indicates 'A' as the current drive, 1 as 'B' and so on. The Setdisk( ) function sets the current drive. This function takes one argument which is an integer indicating the drive to be set. Following program demonstrates use of both the functions.

#include

main( )

{

int dno, maxdr ;

dno = getdisk( ) ;

printf ( "\nThe current drive is: %c\n", 65 + dno

) ;

maxdr = setdisk ( 3 ) ;

dno = getdisk( ) ;

printf ( "\nNow the current drive is: %c\n", 65 +

dno ) ;

}

C interview question:Explain the functions memcmp( ) and memicmp( )

Answer:The functions memcmp( ) and memicmp( ) compares first n bytes of given two blocks of memory or strings.However, memcmp( ) performs comparison as unsigned chars whereas memicmp( ) performs comparison as chars but ignores case (i.e. upper or lower case). Both the functions return an integer value where 0 indicates that two memory buffers compared are identical. If the value returned is greater than 0 then it indicates that the first buffer is bigger than the second one. The value less than 0 indicate that the first buffer is less than the second buffer. The following code snippet demonstrates use of both

#include

main( )

{

char str1[] = "This string contains some

characters" ;

char str2[] = "this string contains" ;

int result ;

result = memcmp ( str1, str2, strlen ( str2 ) ) ;

printf ( "\nResult after comapring buffer using

memcmp( )" ) ;

show ( result ) ;

result = memicmp ( str1, str2, strlen ( str2 ) ) ;

printf ( "\nResult after comapring buffer using

memicmp( )" ) ;

show ( result ) ;

}

show ( int r )

{

if ( r == 0 )

printf ( "\nThe buffer str1 and str2 hold

identical data" ) ;

if ( r > 0 )

printf ( "\nThe buffer str1 is bigger than buffer

str2" ) ;

if ( r < 0 )

printf ( "\nThe buffer str1 is less than buffer

str2" ) ;

C interview question:How do I write code to find an amount of free disk space available on current drive?

Anwer: Use getdfree( ) function as shown in follow code.

#include

main( )

{

int dr ; struct dfree disk ;

long freesp ;

dr = getdisk( ) ;

getdfree ( dr + 1 , &disk ) ;

if ( disk.df_sclus == 0xFFFF )

{

printf ( "\ngetdfree( ) function failed\n");

exit ( 1 ) ;

}

freesp = ( long ) disk.df_avail

* ( long ) disk.df_bsec

* ( long ) disk.df_sclus ;

printf ( "\nThe current drive %c: has %ld bytes

available as free space\n", 'A' + dr, freesp ) ;

}

C interview question: Explain the use of array indices ?

Answer: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 ] ;

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