Set - 1

Question 1 :

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 2 :

What does static variable mean?

Answer :

There are 3 main uses for the static.

1. If you declare within a function: It retains the value between function calls
2.If it is declared for a function name: By default function is extern..so it will be visible from other files if the function declaration is as static..it is invisible for the outer files 
3. Static for global variables: By default we can use the global variables from outside files If it is static global..that variable is limited to with in the file.

#include <stdio.h>
int t = 10;  
main(){
    int x = 0; 
    void funct1();
    funct1();            
    printf("After first call \n");
    funct1();            
    printf("After second call \n");
    funct1();            
    printf("After third call \n");
}
void funct1() {
    static int y = 0;  
    int z = 10;             
    printf("value of y %d z %d",y,z);
    y=y+10;
}

Output :

value of y 0 z 10 After first call
value of y 10 z 10 After second call
value of y 20 z 10 After third call

 


Question 3 :

What are the different storage classes in C ?

Answer :

C has three types of storage: automatic, static and allocated. 
Variable having block scope and without static specifier have automatic storage duration. 
Variables with block scope, and with static specifier have static scope. Global variables (i.e, file scope) with or without the the static specifier also have static scope. 
Memory obtained from calls to malloc(), alloc() or realloc() belongs to allocated storage class.


Question 4 :

What is hashing?

Answer :

To hash means to grind up, and that's essentially what hashing is all about. The heart of a hashing algorithm is a hash function that takes your nice, neat data and grinds it into some random-looking integer. 

The idea behind hashing is that some data either has no inherent ordering (such as images) or is expensive to compare (such as images). If the data has no inherent ordering, you can't perform comparison searches. 

If the data is expensive to compare, the number of comparisons used even by a binary search might be too many. So instead of looking at the data themselves, you'll condense (hash) the data to an integer (its hash value) and keep all the data with the same hash value in the same place. This task is carried out by using the hash value as an index into an array. 

To search for an item, you simply hash it and look at all the data whose hash values match that of the data you're looking for. This technique greatly lessens the number of items you have to look at. If the parameters are set up with care and enough storage is available for the hash table, the number of comparisons needed to find an item can be made arbitrarily close to one. 

One aspect that affects the efficiency of a hashing implementation is the hash function itself. It should ideally distribute data randomly throughout the entire hash table, to reduce the likelihood of collisions. Collisions occur when two different keys have the same hash value. 

There are two ways to resolve this problem. In open addressing, the collision is resolved by the choosing of another position in the hash table for the element inserted later. When the hash table is searched, if the entry is not found at its hashed position in the table, the search continues checking until either the element is found or an empty position in the table is found.

The second method of resolving a hash collision is called chaining. In this method, a bucket or linked list holds all the elements whose keys hash to the same value. When the hash table is searched, the list must be searched linearly.


Question 5 :

Can static variables be declared in a header file ?

Answer :

You can't declare a static variable without defining it as well (this is because the storage class modifiers static and extern are mutually exclusive). A static variable can be defined in a header file, but this would cause each source file that included the header file to have its own private copy of the variable, which is probably not what was intended.


Question 6 :

Can a variable be both constant and volatile ?

Answer :

Yes. The const modifier means that this code cannot change the value of the variable, but that does not mean that the value cannot be changed by means outside this code. For instance, in the example in FAQ 8, the timer structure was accessed through a volatile const pointer.

The function itself did not change the value of the timer, so it was declared const. However, the value was changed by hardware on the computer, so it was declared volatile. If a variable is both const and volatile, the two modifiers can appear in either order.


Question 7 :

Can include files be nested?

Answer :

Yes. Include files can be nested any number of times. As long as you use precautionary measures , you can avoid including the same file twice. In the past, nesting header files was seen as bad programming practice, because it complicates the dependency tracking function of the MAKE program and thus slows down compilation. Many of today's popular compilers make up for this difficulty by implementing a concept called precompiled headers, in which all headers and associated dependencies are stored in a precompiled state.

Many programmers like to create a custom header file that has #include statements for every header needed for each module. This is perfectly acceptable and can help avoid potential problems relating to #include files, such as accidentally omitting an #include file in a module.


Question 8 :

What is a null pointer ?

Answer :

There are times when it's necessary to have a pointer that doesn't point to anything. The macro NULL, defined in , has a value that's guaranteed to be different from any valid pointer. NULL is a literal zero, possibly cast to void* or char*.

Some people, notably C++ programmers, prefer to use 0 rather than NULL.

The null pointer is used in three ways:
1) To stop indirection in a recursive data structure.
2) As an error value.
3) As a sentinel value.


Question 9 :

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(...). 
malloc()


Question 10 :

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 11 :

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 12 :

How to reduce a final size of executable ?

Answer :

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


Question 13 :

Can you tell me how to check whether a linked list is circular ?

Answer :

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 14 :

Advantages of a macro over a function ?

Answer :

Macro gets to see the Compilation environment, so it can expand __ __TIME__ __FILE__ #defines. It is expanded by the preprocessor. 

For example, you can't do this without macros

#define PRINT(EXPR) printf( #EXPR "=%d\n", EXPR) 

PRINT( 5+6*7 ) // expands into printf("5+6*7=%d", 5+6*7 ); 

You can define your mini language with macros:

#define strequal(A,B) (!strcmp(A,B)) 

Macros are a necessary evils of life. The purists don't like them, but without it no real work gets done.


Question 15 :

What is the difference between strings and character arrays ?

Answer :

A major difference is: string will have static storage duration, whereas as a character array will not, unless it is explicity specified by using the static keyword. 

Actually, a string is a character array with following properties: 
* the multibyte character sequence, to which we generally call string, is used to initialize an array of static storage duration. The size of this array is just sufficient to contain these characters plus the terminating NUL character. 
* it not specified what happens if this array, i.e., string, is modified. 
* Two strings of same value[1] may share same memory area. For example, in the following declarations:

char *s1 = "Calvin and Hobbes";
char *s2 = "Calvin and Hobbes";

the strings pointed by s1 and s2 may reside in the same memory location. But, it is not true for the following: 

char ca1[] = "Calvin and Hobbes";
char ca2[] = "Calvin and Hobbes";

[1] The value of a string is the sequence of the values of the contained characters, in order.