C Interview Questions Part 9

C interview question: What is a stack ?

Answer: The stack is a region of memory within which our programs temporarily store data as they execute. For example, when a program passes parameters to functions, C places the parameters on the stack. When the function completes, C removes the items from the stack. Similarly, when a function declares local variables, C stores the variable's values on the stack during the function's execution. Depending on the program's use of functions and parameters, the amount of stack space that a program requires will differ.

C interview question:Allocating memory for a 3-D array

Answer: #include "alloc.h"

#define MAXX 3

#define MAXY 4

#define MAXZ 5

main( )

{

int ***p, i, j, k ;

p = ( int *** ) malloc ( MAXX * sizeof ( int ** ) ) ;

for ( i = 0 ; i < MAXX ; i++ )

{

p[i] = ( int ** ) malloc ( MAXY * sizeof ( int * ) ) ;

for ( j = 0 ; j < MAXY ; j++ )

p[i][j] = ( int * ) malloc ( MAXZ * sizeof ( int ) ) ;

}

for ( k = 0 ; k < MAXZ ; k++ )

{

for ( i = 0 ; i < MAXX ; i++ )

{

for ( j = 0 ; j < MAXY ; j++ )

{

p[i][j][k] = i + j + k ;

printf ( "%d ", p[i][j][k] ) ;

}

printf ( "\n" ) ;

}

printf ( "\n\n" ) ;

}

}

C interview question:How to distinguish between a binary tree and a tree?

Answer: A node in a tree can have any number of branches. While a binary tree is a tree structure in which any node can have at most two branches. For binary trees we distinguish between the subtree on the left and subtree on the right, whereas for trees the order of the subtrees is irrelevant.

Consider two binary trees, but these binary trees are different. The first has an empty right subtree while the second has an empty left subtree. If the above are regarded as trees (not the binary trees), then they are same despite the fact that they are drawn differently. Also, an empty binary tree can exist, but there is no tree having zero nodes.

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C Interview Questions Part 8

C interview question:If we have declared an array as global in one file and we are using it in another file then why doesn't the sizeof operator works on an extern array?

Answer: An extern array is of incomplete type as it does not contain the size. Hence we cannot use sizeof operator, as it cannot get the size of the array declared in another file. To resolve this use any of one the following two solutions:

1. In the same file declare one more variable that holds the size of array. For example,

array.c

int arr[5] ;

int arrsz = sizeof ( arr ) ;

myprog.c

extern int arr[] ;

extern int arrsz ;

2. Define a macro which can be used in an array

declaration. For example,

myheader.h

#define SZ 5

array.c

#include "myheader.h"

int arr[SZ] ;

myprog.c

#include "myheader.h"

extern int arr[SZ] ;

C interview question:How do I write printf( ) so that the width of a field can be specified at runtime?

Answer: This is shown in following code snippet.

main( )

{

int w, no ;

printf ( "Enter number and the width for the

number field:" ) ;

scanf ( "%d%d", &no, &w ) ;

printf ( "%*d", w, no ) ;

}

Here, an '*' in the format specifier in printf( ) indicates that an int value from the argument list should be used for the field width.

C interview question:How to find the row and column dimension of a given 2-D array?

Answer: Whenever we initialize a 2-D array at the same place where it has been declared, it is not necessary to mention the row dimension of an array. The row and column dimensions of such an array can be determined programmatically as shown in following program.

void main( )

{

int a[][3] = { 0, 1, 2,9,-6, 8,7, 5, 44,23, 11,15 } ;

int c = sizeof ( a[0] ) / sizeof ( int ) ;

int r = ( sizeof ( a ) / sizeof ( int ) ) / c ;

int i, j ;

printf ( "\nRow: %d\nCol: %d\n", r, c ) ;

for ( i = 0 ; i < r ; i++ )

{

for ( j = 0 ; j < c ; j++ )

printf ( "%d ", a[i][j] ) ;

printf ( "\n" ) ;

}

}

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C Interview Questions Part 7

C interview question:What is atexit() ?

Answer:Function atexit( ) recevies parameter as the address of function of the type void fun ( void ). The function whose address is passed to atexit( ) gets called before the termination of program. If atexit( ) is called for more than one function then the functions are called in "first in last out" order. You can verify that from the output.

.

#include

void fun1( )

{

printf("Inside fun1\n");

}

void fun2( )

{

printf("Inside fun2\n");

}

main( )

{

atexit ( fun1 ) ;

/* some code */

atexit ( fun2 ) ;

printf ( "This is the last statement of

program?\n" );

}

C interview question:How do I write a user-defined function, which deletes each character in a string str1, which matches any character in string str2?

Answer: The function is as shown below:

Compress ( char str1[], char str2[] )

{

int i, j, k ;

for ( i = k = 0 ; str1[i] != ‘\0’ ; i++ )

{

for ( j = 0 ; str2[j] != ‘\0’ && str2[j] !=

str1[i] ; j++ );

if ( str2[j] == ‘\0’ )

str1[k++] = str1[I] ;

}

str1[k] = ‘\0’

}

C interview question:How does free( ) know how many bytes to free?

Answer: The malloc( ) / free( ) implementation remembers the size of each block allocated and returned, so it is not necessary to remind it of the size when freeing.

C interview question:What is the use of randomize( ) and srand( ) function?

Answer: While generating random numbers in a program, sometimes we require to control the series of numbers that random number generator creates. The process of assigning the random number generators starting number is called seeding the generator. The randomize( ) and srand( ) functions are used to seed the random number generators. The randomize( ) function uses PC's clock to produce a random seed, whereas the srand( ) function allows us to specify the random number generator's starting value.

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C Interview Questions Part 6

C interview question:The sizeof( ) function doesn’t return the size of the block of memory pointed to by a pointer. Why?

Answer:The sizeof( ) operator does not know that malloc( ) has been used to allocate a pointer. sizeof( ) gives us the size of pointer itself. There is no handy way to find out the size of a block allocated by malloc( ).

C interview qustion: What is FP_SEG And FP_OFF…

Answer:Sometimes while working with far pointers we need to break a far address into its segment and offset. In such situations we can use FP_SEG and FP_OFF macros. Following program illustrates the use of these two macros.

#include

main( )

{

unsigned s, o ;

char far *ptr = "Hello!" ;

s = FP_SEG ( ptr ) ;

o = FP_OFF ( ptr ) ;

printf ( "\n%u %u", s, o ) ;

}

C interview question:How do I write a program to convert a string containing number in a hexadecimal form to its equivalent decimal?

Answer: The following program demonstrates this:

main( )

{

char str[] = "0AB" ;

int h, hex, i, n ;

n = 0 ; h = 1 ;

for ( i = 0 ; h == 1 ; i++ )

{

if ( str[i] >= '0' && str[i] <= '9' )

hex = str[i] - '0' ;

else

{

if ( str[i] >= 'a' && str[i] <= 'f' )

hex = str[i] - 'a' + 10 ;

else

if ( str[i] >= 'A' && str[i] <= 'F' )

hex = str[i] - 'A' + 10 ;

else

h = 0 ;

}

if ( h == 1 )

n = 16 * n + hex ;

}

printf ( "\nThe decimal equivalent of %s is %d",

str, n ) ;

}

The output of this program would be the decimal equivalent of 0AB is 171.

C interview question:How do I write code that reads the segment register settings?

Answer: We can use segread( ) function to read segment register settings. There are four segment registers—code segment, data segment, stack segment and extra segment. Sometimes when we use DOS and BIOS services in a program we need to know the segment register's value. In such a situation we can use segread( ) function. The following program illustrates the use of this function.

#include

main( )

{

struct SREGS s ;

segread ( &s ) ;

printf ( "\nCS: %X DS: %X SS: %X ES: %X",s.cs,

s.ds, s.ss, s.es ) ;

}

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C Interview Questions Part 5

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 ) ;

}

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C Interview Questions Part 4

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 ) ;

}

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C Interview Questions Part 3

C interview question:The Spawnl( ) function...

Answer:DOS is a single tasking operating system, thus only one program runs at a time. The Spawnl( ) function provides us with the capability of starting the execution of one program from within another program. The first program is called the parent process and the second program that gets called from within the first program is called a child process. Once the second program starts execution, the first is put on hold until the second program completes execution. The first program is then restarted. The following program demonstrates use of spawnl( ) function.

/* Mult.c */

int main ( int argc, char* argv[ ] )

{

int a[3], i, ret ;

if ( argc < 3 || argc > 3 )

{

printf ( "Too many or Too few arguments..." ) ;

exit ( 0 ) ;

}

for ( i = 1 ; i < argc ; i++ )

a[i] = atoi ( argv[i] ) ;

ret = a[1] * a[2] ;

return ret ;

}

/* Spawn.c */

#include

main( )

{

int val ;

val = spawnl ( P_WAIT, "C:\\Mult.exe", "3", "10",

"20", NULL ) ;

printf ( "\nReturned value is: %d", val ) ;

}

Here, there are two programs. The program 'Mult.exe' works as a child process whereas 'Spawn.exe' works as a parent process. On execution of 'Spawn.exe' it invokes 'Mult.exe' and passes the command-line arguments to it.'Mult.exe' in turn on execution, calculates the product of 10 and 20 and returns the value to val in 'Spawn.exe'. In our call to spawnl( ) function, we have passed 6 parameters, P_WAIT as the mode of execution, path of '.exe' file to run as child process, total number of arguments to be passed to the child process, list of command line arguments and NULL. P_WAIT will cause our application to freeze execution until the child process has completed its execution.This parameter needs to be passed as the default parameter if you are working under DOS. under other operating systems that support multitasking, this parameter can be P_NOWAIT or P_OVERLAY. P_NOWAIT will cause the parent process to execute along with the child process, P_OVERLAY will load the child process on top of the parent process in the memory.

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C Interview Questions Part 2

C interview question:What will be the output of the following code?

void main ()

{ int i = 0 , a[3] ;

a[i] = i++;

printf (“%d",a[i]) ;

}

Answer: The output for the above code would be a garbage value. In the statement a[i] = i++; the value of the variable i would get assigned first to a[i] i.e. a[0] and then the value of i would get incremented by 1. Since a[i] i.e. a[1] has not been initialized, a[i] will have a garbage value.

C interview question:How do I know how many elements an array can hold?

Answer: The amount of memory an array can consume depends on the data type of an array. In DOS environment, the amount of memory an array can consume depends on the current memory model (i.e. Tiny, Small, Large, Huge, etc.). In general an array cannot consume more than 64 kb. Consider following program, which shows the maximum number of elements an array of type int, float and char can have in case of Small memory model.

main( )

{

int i[32767] ;

float f[16383] ;

char s[65535] ;

}

C interview question:How do I write code that reads data at memory location specified by segment and offset?

Answer: Use peekb( ) function. This function returns byte(s) read from specific segment and offset locations in memory. The following program illustrates use of this function. In this program from VDU memory we have read characters and its attributes of the first row. The information stored in file is then further read and displayed using peek( ) function.

#include

main( )

{

char far *scr = 0xB8000000 ;

FILE *fp ;

int offset ;

char ch ;

if ( ( fp = fopen ( "scr.dat", "wb" ) ) == NULL )

{

printf ( "\nUnable to open file" ) ;

exit( ) ;

}

// reads and writes to file

for ( offset = 0 ; offset < 160 ; offset++ )

fprintf ( fp, "%c", peekb ( scr, offset ) ) ;

fclose ( fp ) ;

if ( ( fp = fopen ( "scr.dat", "rb" ) ) == NULL )

{

printf ( "\nUnable to open file" ) ;

exit( ) ;

}

// reads and writes to file

for ( offset = 0 ; offset < 160 ; offset++ )

{

fscanf ( fp, "%c", &ch ) ;

printf ( "%c", ch ) ;

}

fclose ( fp ) ;

}

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C Interview Questions Part 1

C interview question: In printf() Function- What is the difference between "printf(...)" and "sprintf(...)"?

Answer: sprintf(...) writes data to the character array whereas printf(...) writes data to the standard output device.

C interview question:Compilation How to reduce a final size of executable?

Answers:Size of the final executable can be reduced using dynamic linking for libraries.

C interview question:Linked Lists -- Can you tell me how to check whether a linked list is circular?

Answers:Create two pointers, and set both to the start of the list. Update each as follows:

while (pointer1) {

pointer1 = pointer1->next;

pointer2 = pointer2->next;

if (pointer2) pointer2=pointer2->next;

if (pointer1 == pointer2) {

print ("circular");

}

}

If a list is circular, at some point pointer2 will wrap around and be either at the item just before pointer1, or the item before that. Either way, its either 1 or 2 jumps until they meet.

C interview question:string Processing --- Write out a function that prints out all the permutations of a string. For example, abc would give you abc, acb, bac, bca, cab, cba.

Answers:void PrintPermu (char *sBegin, char* sRest) {

int iLoop;

char cTmp;

char cFLetter[1];

char *sNewBegin;

char *sCur;

int iLen;

static int iCount;

iLen = strlen(sRest);

if (iLen == 2) {

iCount++;

printf("%d: %s%s\n",iCount,sBegin,sRest);

iCount++;

printf("%d: %s%c%c\n",iCount,sBegin,sRest[1],sRest[0]);

return;

} else if (iLen == 1) {

iCount++;

printf("%d: %s%s\n", iCount, sBegin, sRest);

return;

} else {

// swap the first character of sRest with each of

// the remaining chars recursively call debug print

sCur = (char*)malloc(iLen);

sNewBegin = (char*)malloc(iLen);

for (iLoop = 0; iLoop <>

strcpy(sCur, sRest);

strcpy(sNewBegin, sBegin);

cTmp = sCur[iLoop];

sCur[iLoop] = sCur[0];

sCur[0] = cTmp;

sprintf(cFLetter, "%c", sCur[0]);

strcat(sNewBegin, cFLetter);

debugprint(sNewBegin, sCur+1);

}

}

}

void main() {

char s[255];

char sIn[255];

printf("\nEnter a string:");

scanf("%s%*c",sIn);

memset(s,0,255);

PrintPermu(s, sIn);

}

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