c interview answers

C interview answers

C interview question1: 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.

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" );

C interview question2 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’

C interview question3: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.

C interview question4: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.

C interview question5: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.

C interview question6: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.

C interview question7: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.


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