C Questions And Answers – 2018B4

There are many commonly asked questions regarding C programming language. Below are some collected such question-answer examples. The questions are usually related with 32-bit system, Turbo C IDE in windows or GCC under Linux environment [not always].

For more such examples, click C_Q&A label.

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When does the compiler not implicitly generate the address of the first element of an array?

 

Whenever an array name appears in an expression such as

- array as an operand of the sizeof operator

- array as an operand of & operator

- array as a string literal initializer for a character array

 

Then the compiler does not implicitly generate the address of the address of the first element of an array.

 

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What is a null pointer?

 

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

 

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What is the difference between text and binary modes?

 

Streams can be classified into two types: text streams and binary streams. Text streams are interpreted, with a maximum length of 255 characters. With text streams, carriage return/line feed combinations are translated to the newline n character and vice versa. Binary streams are uninterrupted and are treated one byte at a time with no translation of characters. Typically, a text stream would be used for reading and writing standard text files, printing output to the screen or printer, or receiving input from the keyboard.

 

A binary text stream would typically be used for reading and writing binary files such as graphics or word processing documents, reading mouse input, or reading and writing to the modem.

 

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What is static memory allocation and dynamic memory allocation?

 

Static memory allocation: The compiler allocates the required memory space for a declared variable. By using the address of operator, the reserved address is obtained and this address may be assigned to a pointer variable. Since most of the declared variable have static memory, this way of assigning pointer value to a pointer variable is known as static memory allocation. memory is assigned during compilation time.

 

Dynamic memory allocation: It uses functions such as malloc( ) or calloc( ) to get memory dynamically. If these functions are used to get memory dynamically and the values returned by these functions are assigned to pointer variables, such assignments are known as dynamic memory allocation. memory is assigned during run time.

 

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When should a far pointer be used?

 

Sometimes you can get away with using a small memory model in most of a given program. There might be just a few things that don’t fit in your small data and code segments. When that happens, you can use explicit far pointers and function declarations to get at the rest of memory. A far function can be outside the 64KB segment most functions are shoehorned into for a small-code model. (Often, libraries are declared explicitly far, so they’ll work no matter what code model the program uses.) A far pointer can refer to information outside the 64KB data segment. Typically, such pointers are used with farmalloc() and such, to manage a heap separate from where all the rest of the data lives. If you use a small-data, large-code model, you should explicitly make your function pointers far.

 

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How are pointer variables initialized?

 

Pointer variable are initialized by one of the following two ways

- Static memory allocation

- Dynamic memory allocation

 

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Difference between arrays and pointers?

 

- Pointers are used to manipulate data using the address. Pointers use * operator to access the data pointed to by them

- Arrays use subscripted variables to access and manipulate data.

Array variables can be equivalently written using pointer expression.

 

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Is using exit() the same as using return?

 

No. The exit() function is used to exit your program and return control to the operating system. The return statement is used to return from a function and return control to the calling function. If you issue a return from the main() function, you are essentially returning control to the calling function, which is the operating system. In this case, the return statement and exit() function are similar.

 

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What is a method?

 

Method is a way of doing something, especially a systematic way; implies an orderly logical arrangement (usually in steps).

 

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What is indirection?

 

If you declare a variable, its name is a direct reference to its value. If you have a pointer to a variable or any other object in memory, you have an indirect reference to its value.

 

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…till next post, bye-bye & take care.

C Questions And Answers – 2018B3

There are many commonly asked questions regarding C programming language. Below are some collected such question-answer examples. The questions are usually related with 32-bit system, Turbo C IDE in windows or GCC under Linux environment [not always].

For more such examples, click C_Q&A label.

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What is the difference between strings and character arrays?

 

A major difference is: string will have static storage duration, whereas as a character array will not, unless it is explicitly 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.

 

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Difference between const char* p and char const* p

 

In const char* p, the character pointed by ‘p’ is constant, so u can’t change the value of character pointed by p but u can make ‘p’ refer to some other location.

 

in char const* p, the ptr ‘p’ is constant not the character referenced by it, so u can’t make ‘p’ to reference to any other location but u can change the value of the char pointed by ‘p’.

 

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What is hashing?

 

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.

 

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How can you determine the size of an allocated portion of memory?

 

You can’t, really. free() can , but there’s no way for your program to know the trick free() uses. Even if you disassemble the library and discover the trick, there’s no guarantee the trick won’t change with the next release of the compiler.

 

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Can static variables be declared in a header file?

 

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.

 

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Can a variable be both const and volatile?

 

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.

 

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Can include files be nested?

 

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.

 

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…till next post, bye-bye & take care.

C Questions And Answers – 2018B2

There are many commonly asked questions regarding C programming language. Below are some collected such question-answer examples. The questions are usually related with 32-bit system, Turbo C IDE in windows or GCC under Linux environment [not always].

For more such examples, click C_Q&A label.

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

 

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.

 

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"union" Data Type What is the output of the following program? Why?

 

#include

 

main() {

typedef union {

int a;

char b[10];

float c;

}

Union;

Union x,y = {100};

x.a = 50;

strcpy(x.b,"hello");

x.c = 21.50;

printf("Union x : %d %s %f n",x.a,x.b,x.c);

printf("Union y : %d %s %f n",y.a,y.b,y.c);

}

 

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What does static variable mean?

 

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 within the file

 

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Advantages of a macro over a function?

 

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

 

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What are the differences between malloc() and calloc()?

 

There are 2 differences.

 

First, is in the number of arguments. malloc() takes a single argument(memory required in bytes), while calloc() needs 2 arguments(number of variables to allocate memory, size in bytes of a single variable).

 

Secondly, malloc() does not initialize the memory allocated, while calloc() initializes the allocated memory to ZERO.

 

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What are the different storage classes in C?

 

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 static specifier also have static scope.

Memory obtained from calls to malloc(), alloc() or realloc() belongs to allocated storage class.

 

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…till next post, bye-bye & take care.

C Questions And Answers – 2018B1

There are many commonly asked questions regarding C programming language. Below are some collected such question-answer examples. The questions are usually related with 32-bit system, Turbo C IDE in windows or GCC under Linux environment [not always].

For more such examples, click C_Q&A label.

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What is C language?

 

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.

 

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printf() Function

What is the output of printf("%d")?

 

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 whatever 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(...).

 

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malloc() Function- What is the difference between "calloc(...)" and

"malloc(...)"?

 

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.

 

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printf() Function- What is the difference between "printf(...)" and "sprintf(...)"?

 

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

 

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Compilation How to reduce a final size of executable?

 

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

 

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…till next post, bye-bye & take care.

C Questions And Answers – 2018A24

There are many commonly asked questions regarding C programming language. Below are some collected such question-answer examples. The questions are usually related with Turbo C IDE in windows or GCC under Linux environment [not always].

For more such examples, click C_Q&A label.

 

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How to obtain a path of the given file?

 

Ans:

The function searchpath( ) searches for the specified file in the subdirectories of the current path.

 

Following program shows how to make use of the searchpath( ) function.

 

#include "dir.h"

 

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

{

char *

path ;

if ( path = searchpath ( argv[ 1 ] ) )

printf ( "Pathname : %s\n", path ) ;

else

printf ( "File not found\n" ) ;

}

 

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Can we get the process identification number of the current program?

 

Ans:

Yes! The macro getpid( ) gives us the process identification number of the program currently running. The process id. uniquely identifies a program. Under DOS, the getpid( ) returns the Program Segment Prefix as the process id.

 

Following program illustrates the use of this macro.

 

#include <stdio.h>

#include <process.h>

 

void main( )

{

printf ( "The process identification number of this program is %X\n",

getpid( ) ) ;

}

 

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How do I write a function that takes variable number of arguments?

 

Ans:

The following program demonstrates this.

 

#include <stdio.h>

#include <stdarg.h>

 

void main( )

{

int I = 10 ;

float f = 2.5 ;

char *str = "Hello!" ;

vfpf ( "%d %f %s\n", i, f, str ) ;

vfpf ( "%s %s", str, "Hi!" ) ;

}

 

void vfpf ( char *fmt, ... )

{

va_list argptr ;

va_start ( argptr, fmt ) ;

vfprintf ( stdout, fmt, argptr ) ;

va_end ( argptr ) ;

}

 

Here, the function vfpf( ) has called vfprintf( ) that take variable argument lists. va_list is an array that holds information required for the macros va_start and va_end. The macros va_start and va_end provide a portable way to access the variable argument lists. va_start would set up a pointer argptr to point to the first of the variable arguments being passed to the function. The macro va_end helps the called function to perform a normal return.

 

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Can we change the system date to some other date?

 

Ans:

Yes, We can! The function stime( ) sets the system date to the specified date. It also sets the system time. The time and date is measured in seconds from the 00:00:00 GMT, January 1, 1970.

 

The following program shows how to use this function.

 

#include <stdio.h>

#include <time.h>

void main( )

{

time_t tm ;

int d ;

tm = time ( NULL ) ;

printf ( "The System Date : %s", ctime ( &tm ) ) ;

printf ( "\nHow many days ahead you want to set the date : " ) ;

scanf ( "%d", &d ) ;

tm += ( 24L * d ) * 60L * 60L ;

stime ( &tm ) ;

printf ( "\nNow the new date is : %s", ctime ( &tm ) ) ;

}

 

In this program we have used function ctime( ) in addition to function stime( ). The ctime( ) function converts time value to a 26-character long string that contains date and time.

 

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How to use function strdup( ) in a program?

 

Ans:

The string function strdup( ) copies the given string to a new location. The function uses

malloc( ) function to allocate space required for the duplicated string. It takes one argument a pointer to the string to be duplicated. The total number of characters present in the given string plus one bytes get allocated for the new string. As this function uses malloc( ) to allocate memory, it is the programmer’s responsibility to deallocate the memory using free( ).

 

#include <stdio.h>

#include <string.h>

#include <alloc.h>

 

void main( )

{

char * str1, *str2 ="double";

str1 = strdup ( str2 ) ;

printf ( "%s\n", str1 ) ;

free ( str1 ) ;

}

 

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On including a file twice I get errors reporting redefinition of function.

How can I avoid duplicate inclusion?

 

Ans:

Redefinition errors can be avoided by using the following macro definition. Include this definition in the header file.

 

#if !defined filename_h

#define filename_h

/* function definitions */

#endif

 

Replace filename_h with the actual header file name. For example, if name of file to be included is 'goto.h' then replace filename_h with 'goto_h'.

 

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How to write a swap( ) function which swaps the values of the variables using bitwise operators.

 

Ans:

Here is the swap( ) function.

 

swap ( int *x, int *y )

{

*x ^= *y ;

*y ^= *x ;

*x ^= *y ;

}

 

The swap( ) function uses the bitwise XOR operator and does not require any temporary variable for swapping.

 

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…till next post, bye-bye & take care.