C Interview Questions and Answers
Question: What is C language?
Answer: The C programming language is a standardized programming language developed in the early 1970s by Ken Thompson and Dennis Ritchie for use on the UNIX operating system. It has since spread to many other operating systems, and is one of the most widely used programming languages. C is prized for its efficiency, and is the most popular programming language for writing system software, though it is also used for writing applications.
Question: Inprintf() Function What is the output of printf("%d")?
Answer:1. When we write printf("%d",x); this means compiler will print the value of x. But as here, there is nothing after �%d� so compiler will show in output window garbage value.
2. When we use %d the compiler internally uses it to access the argument in the stack (argument stack). Ideally compiler determines the offset of the data variable depending on the format specification string. Now when we write printf("%d",a) then compiler first accesses the top most element in the argument stack of the printf which is %d and depending on the format string it calculated to offset to the actual data variable in the memory which is to be printed. Now when only %d will be present in the printf then compiler will calculate the correct offset (which will be the offset to access the integer variable) but as the actual data object is to be printed is not present at that memory location so it will print what ever will be the contents of that memory location.
3. Some compilers check the format string and will generate an error without the proper number and type of arguments for things like printf(...) and scanf(...).
Question:malloc() Function- What is the difference between "calloc(...)" and "malloc(...)"?
Answer:1. calloc(...) allocates a block of memory for an array of elements of a certain size. By default the block is initialized to 0. The total number of memory allocated will be (number_of_elements * size).
malloc(...) takes in only a single argument which is the memory required in bytes. malloc(...) allocated bytes of memory and not blocks of memory like calloc(...).
2.malloc(...) allocates memory blocks and returns a void pointer to the allocated space, or NULL if there is insufficient memory available.
calloc(...) allocates an array in memory with elements initialized to 0 and returns a pointer to the allocated space. calloc(...) calls malloc(...) in order to use the C++ _set_new_mode function to set the new handler mode.
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.
Question:Compilation How to reduce a final size of executable?
Answers:Size of the final executable can be reduced using dynamic linking for libraries.
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.
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);
}
Question: How do I compare character data stored at two different memory locations?
Answer: 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
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" ) ;
}
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.
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 )
{
printf ( "Too many or Too few arguments..." ) ;
exit ( 0 ) ;
}
for ( i = 1 ; i < ret =" a[1]" val =" spawnl">
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.
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.
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 ) ) ;
}
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 ) ;
}
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.
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 ) ;
}
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.
Question:Why doesn't the following code give the desired result?
Answer: 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.
Question: Why doesn't the following statement work?
char str[ ] = "Hello" ;
strcat ( str, '!' ) ;
Answer: 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, "!" ) ;
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] ;
}
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 < fp =" fopen" offset =" 0">
Question: What is C language?
Answer: The C programming language is a standardized programming language developed in the early 1970s by Ken Thompson and Dennis Ritchie for use on the UNIX operating system. It has since spread to many other operating systems, and is one of the most widely used programming languages. C is prized for its efficiency, and is the most popular programming language for writing system software, though it is also used for writing applications.
Question: Inprintf() Function What is the output of printf("%d")?
Answer:1. When we write printf("%d",x); this means compiler will print the value of x. But as here, there is nothing after �%d� so compiler will show in output window garbage value.
2. When we use %d the compiler internally uses it to access the argument in the stack (argument stack). Ideally compiler determines the offset of the data variable depending on the format specification string. Now when we write printf("%d",a) then compiler first accesses the top most element in the argument stack of the printf which is %d and depending on the format string it calculated to offset to the actual data variable in the memory which is to be printed. Now when only %d will be present in the printf then compiler will calculate the correct offset (which will be the offset to access the integer variable) but as the actual data object is to be printed is not present at that memory location so it will print what ever will be the contents of that memory location.
3. Some compilers check the format string and will generate an error without the proper number and type of arguments for things like printf(...) and scanf(...).
Question:malloc() Function- What is the difference between "calloc(...)" and "malloc(...)"?
Answer:1. calloc(...) allocates a block of memory for an array of elements of a certain size. By default the block is initialized to 0. The total number of memory allocated will be (number_of_elements * size).
malloc(...) takes in only a single argument which is the memory required in bytes. malloc(...) allocated bytes of memory and not blocks of memory like calloc(...).
2.malloc(...) allocates memory blocks and returns a void pointer to the allocated space, or NULL if there is insufficient memory available.
calloc(...) allocates an array in memory with elements initialized to 0 and returns a pointer to the allocated space. calloc(...) calls malloc(...) in order to use the C++ _set_new_mode function to set the new handler mode.
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.
Question:Compilation How to reduce a final size of executable?
Answers:Size of the final executable can be reduced using dynamic linking for libraries.
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.
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);
}
Question: How do I compare character data stored at two different memory locations?
Answer: 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
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" ) ;
}
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.
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 )
{
printf ( "Too many or Too few arguments..." ) ;
exit ( 0 ) ;
}
for ( i = 1 ; i < ret =" a[1]" val =" spawnl">
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.
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.
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 ) ) ;
}
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 ) ;
}
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.
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 ) ;
}
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.
Question:Why doesn't the following code give the desired result?
Answer: 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.
Question: Why doesn't the following statement work?
char str[ ] = "Hello" ;
strcat ( str, '!' ) ;
Answer: 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, "!" ) ;
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] ;
}
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 < fp =" fopen" offset =" 0">
