Good Questions In
C
!!Enjoy!!!
1.
What will be the output of the following code?
void main ()
{ int i = 0 , a[3] ;
a[i] = i++;
printf (“%d",a[i]) ;
}
Ans: 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.
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2.
Why doesn't the following code give the desired result?
int x = 3000, y = 2000 ;
long int z = x * y ;
Ans: Here the multiplication is carried out between two
ints x and y, and the result that would overflow would be
truncated before being assigned to the variable z of type
long int. However, to get the correct output, we should use an
explicit cast to force long arithmetic as shown below:
long int z = ( long int ) x * y ;
Note that ( long int )( x * y ) would not give the
desired effect.
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3.
Why doesn't the following statement work?
char str[ ] = "Hello" ;
strcat ( str, '!' ) ;
Ans: The string function strcat( ) concatenates strings and
not a character. The basic difference between a string and a
character is that a string is a collection of characters,
represented by an array of characters whereas a character is a
single character. To make the above statement work writes the
statement as shown below:
strcat ( str, "!" ) ;
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4.
How do I know how many elements an array can hold?
Ans: 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] ;
}
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5.
How do I write code that reads data at memory location
specified by segment and offset?
Ans: 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 <stdio.h>
#include <dos.h>
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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6.
How do I compare character data stored at two different
memory locations?
Ans: Sometimes in a program we require to compare memory
ranges containing strings. In such a situation we can use
functions like memcmp( ) or memicmp( ). The basic difference
between two functions is that memcmp( ) does a case-sensitive
comparison whereas memicmp( ) ignores case of characters.
Following program illustrates the use of both the functions.
#include <mem.h>
main( )
{
char *arr1 = "Kicit" ;
char *arr2 = "kicitNagpur" ;
int c ;
c = memcmp ( arr1, arr2, sizeof ( arr1 ) ) ;
if ( c == 0 )
printf ( "\nStrings arr1 and arr2 compared using memcmp
are identical" ) ;
else
printf ( "\nStrings arr1 and arr2 compared using memcmp
are not identical"
) ;
c = memicmp ( arr1, arr2, sizeof ( arr1 ) ) ;
if ( c == 0 )
printf ( "\nStrings arr1 and arr2 compared using memicmp
are identical" )
;
else
printf ( "\nStrings arr1 and arr2 compared using memicmp
are not
identical" ) ;
}
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7.
Fixed-size objects are more appropriate as compared to
variable size data objects. Using variable-size data objects
saves very little space. Variable size data objects usually
have some overhead. Manipulation of fixed-size
data objects is usually faster
and easier. Use fixed size when maximum size is clearly
bounded and close to average. And use variable-size data
objects when a few of the data items are bigger than the
average size. For example,
char *num[10] = { "One", "Two",
"Three", "Four",
"Five", "Six", "Seven", "Eight",
"Nine", "Ten" } ;
Instead of using the above, use
char num[10][6] = { "One",
"Two", "Three", "Four",
"Five", "Six", "Seven", "Eight",
"Nine", "Ten" } ;
The first form uses variable-size data objects. It allocates
10 pointers, which are pointing to 10 string constants of
variable size. Assuming each pointer is of 4 bytes, it
requires 90 bytes. On the other hand, the second form uses
fixed size data objects. It allocates 10 arrays of 6
characters each. It requires only 60 bytes of space. So, the
variable-size in this case does not offer any advantage over
fixed size.
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8.
The Spawnl( ) function...
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 <process.h>
#include <stdio.h>
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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9.
Are the following two statements identical?
char str[6] = "Kicit" ;
char *str = "Kicit" ;
Ans: 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.
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10.
Is the following code fragment correct?
const int x = 10 ;
int arr[x] ;
Ans: 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.
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11.
How do I write code to retrieve current date and time from
the system and display it as a string?
Ans: 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 <sys\types.h>
void main( )
{
time_t curtime ;
char ctm[50] ;
time ( &curtime ) ; //retrieves
current time &
stores in curtime
printf ( "\nCurrent Date & Time:
%s", ctime (
&curtime ) ) ;
}
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12.
How do I change the type of cursor and hide a cursor?
Ans: 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 <conio.h>
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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13.
How do I write code that would get error number and display
error message if any standard error occurs?
Ans: Following code demonstrates this.
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
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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14.
How do I write code to get the current drive as well as set
the current drive?
Ans: 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 <dir.h>
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 ) ;
}
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15.
The functions memcmp( ) and memicmp( )
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 <stdio.h>
#include <mem.h>
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" ) ;
}
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16.
How do I write code to find an amount of free disk space
available on current drive?
Ans: Use getdfree( ) function as shown in follow code.
#include <stdio.h>
#include <stdlib.h>
#include <dir.h>
#include <dos.h>
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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17.
Use of array indices...
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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18.
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 <stdio.h>
#include <stdlib.h>
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" );
}
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19.
How do I write a user-defined function, which deletes each
character in a string str1, which matches any character in
string str2?
Ans: 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’
}
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20.
How does free( ) know how many bytes to free?
Ans: 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.
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21.
What is the use of randomize( ) and srand( ) function?
Ans: 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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22.
How do I determine amount of memory currently available for
allocating?
Ans: 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.
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23.
How does a C program come to know about command line
arguments?
Ans: 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 table, 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.
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24.
When we open a file, how does functions like fread( )/fwrite(
), etc. get to know from where to read or to write the data?
Ans: 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.
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25.
The sizeof( ) function doesn’t return the size of the block
of memory pointed to by a pointer. Why?
Ans: 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( ).
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26.
FP_SEG And FP_OFF…
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 <dos.h>
main( )
{
unsigned s, o ;
char far *ptr = "Hello!" ;
s = FP_SEG ( ptr ) ;
o = FP_OFF ( ptr ) ;
printf ( "\n%u %u", s, o ) ;
}
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27.
How do I write a program to convert a string containing
number in a hexadecimal form to
its equivalent decimal?
Ans: 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.
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28.
How do I write code that reads the segment register settings?
Ans: 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 <dos.h>
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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29.
What is environment and how do I get environment for a
specific entry?
Ans: 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 <stdio.h>
#include <stdlib.h>
main( )
{
char *path = NULL ;
path = getenv ( "PATH" ) ;
if ( *path != NULL )
printf ( "\nPath: %s", path ) ;
else
printf ( "\nPath is not set" ) ;
}
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30.
How do I display current date in the format given below?
Saturday October 12, 2002
Ans: Following program illustrates how we can display date in
above given format.
#include <stdio.h>
#include <time.h>
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.
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