0S WORLD: C interview Question Answers

C interview Question Answers

Posted by OS WORLD at Friday, February 10, 2012

What will print out? main()
{
        char *p1=“name”;
        char *p2;
        p2=(char*)malloc(20);
        memset (p2, 0, 20);
        while(*p2++ = *p1++);
        printf(“%sn”,p2);
}
Answer:empty string.
What will be printed as the result of the operation below:

main()
{
    int x=20,y=35;
    x=y++ + x++;
    y= ++y + ++x;
    printf(“%d%dn”,x,y); }
Answer : 5794
What will be printed as the result of the operation below:

main()
{
int x=5;
printf(“%d,%d,%dn”,x,x< <2,x>>2); }
Answer: 5,20,1
What will be printed as the result of the operation below:

#define swap(a,b) a=a+b;b=a-b;a=a-b; void main()
{
    int x=5, y=10;
    swap (x,y);
    printf(“%d %dn”,x,y);
    swap2(x,y);
    printf(“%d %dn”,x,y);
}
int swap2(int a, int b)
{
    int temp;
    temp=a;
    b=a;
    a=temp;
    return 0;
}
Answer: 10, 5
10, 5
What will be printed as the result of the operation below:

main()
{
    char *ptr = ” Cisco Systems”;
    *ptr++; printf(“%sn”,ptr);
    ptr++;
    printf(“%sn”,ptr); }
Answer:Cisco Systems
isco systems
What will be printed as the result of the operation below:

main()
{
    char s1[]=“Cisco”;
    char s2[]= “systems”;
    printf(“%s”,s1);
}
Answer: Cisco
What will be printed as the result of the operation below:

main()
{
    char *p1;
    char *p2;     p1=(char *)malloc(25);
    p2=(char *)malloc(25);
    strcpy(p1,”Cisco”);
    strcpy(p2,“systems”);
    strcat(p1,p2);
    printf(“%s”,p1);
}
Answer: Ciscosystems
The following variable is available in file1.c, who can access it?:
```
static int average;
```
Answer: all the functions in the file1.c can access the variable.
WHat will be the result of the following code?

#define TRUE 0 // some code while(TRUE)
{
// some code
}
Answer: This will not go into the loop as TRUE is defined as 0.
What will be printed as the result of the operation below:

int x;
int modifyvalue()
{
    return(x+=10);
} int changevalue(int x)
{
    return(x+=1);
}
void main()
{
    int x=10;
    x++;
    changevalue(x);
    x++;
    modifyvalue();
    printf("First output:%dn",x);
    x++;
    changevalue(x);
    printf("Second output:%dn",x);
    modifyvalue();
    printf("Third output:%dn",x);
}
Answer: 12 , 13 , 13
What will be printed as the result of the operation below:

main()
{
    int x=10, y=15;
    x = x++;
    y = ++y;
    printf(“%d %dn”,x,y); }
Answer: 11, 16
What will be printed as the result of the operation below:

main()
{
    int a=0;
    if(a==0)
        printf(“Cisco Systemsn”);
        printf(“Cisco Systemsn”); }
Answer: Two lines with “Cisco Systems” will be printed.

13. 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.

14.:Compilation How to reduce a final size of executable?

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

15.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.

16.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);

}

17.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.

18.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] ;

}

19.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 ) ;

}

20.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.

21.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.

22.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.

23.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 ) ) ;

}

24.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 ) ;

}

25.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" ) ;

26.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 ) ;

}

27.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( ).

28. 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 ) ;

}

29.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.

30.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 ) ;

}

31.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" );

}

32.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’

}

33.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.

34.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.

35. 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.

36.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" ) ;

}

}

37.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.

38.How do I write code that executes certain function only at program termination?

Answer: Use atexit( ) function as shown in following program.

#include

main( )

{

int ch ;

void fun ( void ) ;

atexit ( fun ) ;

// code

}

void fun( void )

{

printf ( "\nTerminate program......" ) ;

getch( ) ;

}

39.What are memory models?

Answer: The compiler uses a memory model to determine how much memory is allocated to the program. The PC divides memory into blocks called segments of size 64 KB. Usually, program uses one segment for code and a second segment for data. A memory model defines the number of segments the compiler can use for each. It is important to know which memory model can be used for a program. If we use wrong memory model, the program might not have enough memory to execute. The problem can be solved using larger memory model. However, larger the memory model, slower is your program execution. So we must choose the smallest memory model that satisfies our program needs. Most of the compilers support memory models like tiny, small, medium, compact, large and huge.

40.How does C compiler store elements in a multi-dimensional array?

Answer:The compiler maps multi-dimensional arrays in two ways—Row major order and Column order. When the compiler places elements in columns of an array first then it is called column-major order. When the compiler places elements in rows of an array first then it is called row-major order. C compilers store multidimensional arrays in row-major order. For example, if there is a multi-dimensional array a[2][3], then according row-major order, the elements would get stored in memory following order:

a[0][0], a[0][1], a[0][2], a[1][0], a[1][1], a[1][2]

41.If the result of an _expression has to be stored to one of two variables, depending on a condition, can we use conditional operators as shown below?

( ( i < 10 ) ? j : k ) = l * 2 + p ;

Answer: No! The above statement is invalid. We cannot use the conditional operators in this fashion. The conditional operators like most operators, yields a value, and we cannot assign the value of an _expression to a value. However, we can use conditional operators as shown in following code snippet.

main( )

{

int i, j, k, l ;

i = 5 ; j = 10 ; k = 12, l = 1 ;

* ( ( i < 10 ) ? &j : &k ) = l * 2 + 14 ;

printf ( "i = %d j = %d k = %d l = %d", i, j, k, l ) ;

}

The output of the above program would be as given below:

i = 5 j = 16 k = 12 l = 1

42.How can I find the day of the week of a given date?

Answer: The following code snippet shows how to get the day of week from the given date.

dayofweek ( int yy, int mm, int dd )

{

/*Monday = 1 and Sunday = 0 */

/* month number >= 1 and <= 12, yy > 1752 or so */

static int arr[ ] = { 0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4 } ;

yy = yy - mm < 3 ;

return ( yy + yy / 4 - yy / 100 + yy / 400 + arr[ mm - 1] + dd ) % 7 ;

}

void main( )

{

printf ( "\n\n\nDay of week : %d ", dayofweek ( 2002, 5, 18 ) ) ;

}

43.What's the difference between these two declarations?

struct str1 { ... } ;

typedef struct { ... } str2 ;

Answer : The first form declares a structure tag whereas the second declares a typedef. The main difference is that the second declaration is of a slightly more abstract type -- its users don't necessarily know that it is a structure, and the keyword struct is not used when declaring instances of it.

44. How do I print the contents of environment variables?

Answer: The following program shows how to achieve this:

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

{

int i = 0 ;

clrscr( ) ;

while ( env[ i ] )

printf ( "\n%s", env[ i++ ] ) ;

}

main( ) has the third command line argument env, which is an array of pointers to the strings. Each pointer points to an environment variable from the list of environment variables.

45.How do I use the function ldexp( ) in a program?

Answer: The math function ldexp( ) is used while solving the complex mathematical equations. This function takes two arguments, a double value and an int respectively. The order in which ldexp( ) function performs calculations is ( n * pow ( 2, exp ) ) where n is the double value and exp is the integer. The following program demonstrates the use of this function.

#include

void main( )

{

double ans ;

double n = 4 ;

ans = ldexp ( n, 2 ) ;

printf ( "\nThe ldexp value is : %lf\n", ans ) ;

}

Here, ldexp( ) function would get expanded as ( 4 * 2 * 2 ), and the output would be the ldexp value is : 16.000000

46.Can we get the mantissa and exponent form of a given number?

Answer:The function frexp( ) splits the given number into a mantissa and exponent form. The function takes two arguments, the number to be converted as a double value and an int to store the exponent form. The function returns the mantissa part as a double value. Following example demonstrates the use of this function.

#include

void main( )

{

double mantissa, number ;

int exponent ;

number = 8.0 ;

mantissa = frexp ( number, &exponent ) ;

printf ( "The number %lf is ", number ) ;

printf ( "%lf times two to the ", mantissa ) ;

printf ( "power of %d\n", exponent ) ;

return 0 ;

}

47.What is access( ) function...

Answer:The access( ) function checks for the existence of a file and also determines whether it can be read,written to or executed. This function takes two arguments the filename and an integer indicating the access mode. The values 6, 4, 2, and 1 checks for read/write, read, write and execute permission of a given file, whereas value 0 checks whether the file exists or not. Following program demonstrates how we can use access( ) function to check if a given file exists.

#include

main( )

{

char fname[67] ;

printf ( "\nEnter name of file to open" ) ;

gets ( fname ) ;

if ( access ( fname, 0 ) != 0 )

{

printf ( "\nFile does not exist." ) ;

return ;

}

}

48.How do I convert a floating-point number to a string?

Answer: Use function gcvt( ) to convert a floating-point number to a string. Following program demonstrates the use of this function.

#include

main( )

{

char str[25] ;

float no ;

int dg = 5 ; /* significant digits */

no = 14.3216 ;

gcvt ( no, dg, str ) ;

printf ( "String: %s\n", str ) ;

}

49.How do I determine amount of memory currently available for allocating?

Answer: We can use function coreleft( ) to get the amount of memory available for allocation. However, this function does not give an exact amount of unused memory. If, we are using a small memory model, coreleft( ) returns the amount of unused memory between the top of the heap and stack. If we are using a larger model, this function returns the amount of memory between the highest allocated memory and the end of conventional memory. The function returns amount of memory in terms of bytes.

50.How does a C program come to know about command line arguments?

Answer: When we execute our C program, operating system loads the program into memory. In case of DOS, it first loads 256 bytes into memory, called program segment prefix. This contains file tables,environment segment, and command line information. When we compile the C program the compiler inserts additional code that parses the command, assigning it to the argv array, making the arguments easily accessible within our C program.

51.What is environment and how do I get environment for a specific entry?

Answer: While working in DOS, it stores information in a memory region called environment. In this region we can place configuration settings such as command path, system prompt, etc. Sometimes in a program we need to access the information contained in environment. The function getenv( ) can be used when we want to access environment for a specific entry. Following program demonstrates the use of this function.

#include

main( )

{

char *path = NULL ;

path = getenv ( "PATH" ) ;

if ( *path != NULL )

printf ( "\nPath: %s", path ) ;

else

printf ( "\nPath is not set" ) ;

}

52.How do I display current date in the format given below?

Saturday October 12, 2002

Answer: Following program illustrates how we can display date in above given format.

#include

main( )

{

struct tm *curtime ;

time_t dtime ;

char str[30] ;

time ( &dtime ) ;

curtime = localtime ( &dtime ) ;

strftime ( str, 30, "%A %B %d, %Y", curtime ) ;

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

}

Here we have called time( ) function which returns current time. This time is returned in terms of seconds, elapsed since 00:00:00 GMT, January 1, 1970. To extract the week day, day of month, etc.from this value we need to break down the value to a tm structure. This is done by the function localtime( ). Then we have called strftime( ) function to format the time and store it in a string str.

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

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 ] ;

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.

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