C language interview questions

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.

1.What are the advantages of the functions?
- Debugging is easier
- It is easier to understand the logic involved in the program
- Testing is easier
- Recursive call is possible
- Irrelevant details in the user point of view are hidden in functions
- Functions are helpful in generalizing the program

2.Is NULL always defined as 0?
NULL is defined as either 0 or (void*)0. These values are almost identical; either a literal zero or a void pointer is converted automatically to any kind of pointer, as necessary, whenever a pointer is needed (although the compiler can’t always tell when a pointer is needed).
3.What is the difference between NULL and NUL?
NULL is a macro defined in for the null pointer.
NUL is the name of the first character in the ASCII character set. It corresponds to a zero value. There’s no standard macro NUL in C, but some people like to define it.
The digit 0 corresponds to a value of 80, decimal. Don’t confuse the digit 0 with the value of ‘’ (NUL)! NULL can be defined as ((void*)0), NUL as ‘’.
4.Can the sizeof operator be used to tell the size of an array passed to a function?
No. There’s no way to tell, at runtime, how many elements are in an array parameter just by looking at the array parameter itself. Remember, passing an array to a function is exactly the same as passing a pointer to the first element.
5.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.
6.Can math operations be performed on a void pointer?
No. Pointer addition and subtraction are based on advancing the pointer by a number of elements. By definition, if you have a void pointer, you don’t know what it’s pointing to, so you don’t know the size of what it’s pointing to. If you want pointer arithmetic to work on raw addresses, use character pointers.
7.Can the size of an array be declared at runtime?
No. In an array declaration, the size must be known at compile time. You can’t specify a size that’s known only at runtime. For example, if i is a variable, you can’t write code like this:
char array[i]; /* not valid C */
Some languages provide this latitude. C doesn’t. If it did, the stack would be more complicated, function calls would be more expensive, and programs would run a lot slower. If you know that you have an array but you won’t know until runtime how big it will be, declare a pointer to it and use malloc() or calloc() to allocate the array from the heap.
8.Can you add pointers together? Why would you?
No, you can’t add pointers together. If you live at 1332 Lakeview Drive, and your neighbor lives at 1364 Lakeview, what’s 1332+1364? It’s a number, but it doesn’t mean anything. If you try to perform this type of calculation with pointers in a C program, your compiler will complain.
The only time the addition of pointers might come up is if you try to add a pointer and the difference of two pointers.

9.Are pointers integers?
No, pointers are not integers. A pointer is an address. It is merely a positive number and not an integer.
10.How do you redirect a standard stream?
Most operating systems, including DOS, provide a means to redirect program input and output to and from different devices. This means that rather than your program output (stdout) going to the screen; it can be redirected to a file or printer port. Similarly, your program’s input (stdin) can come from a file rather than the keyboard. In DOS, this task is accomplished using the redirection characters, < and >. For example, if you wanted a program named PRINTIT.EXE to receive its input (stdin) from a file named STRINGS.TXT, you would enter the following command at the DOS prompt:
C:> PRINTIT <STRINGS.TXT
Notice that the name of the executable file always comes first. The less-than sign (<) tells DOS to take the strings contained in STRINGS.TXT and use them as input for the PRINTIT program.
The following example would redirect the program’s output to the prn device, usually the printer attached on LPT1:
C :> REDIR > PRN
Alternatively, you might want to redirect the program’s output to a file, as the following example shows:
C :> REDIR > REDIR.OUT
In this example, all output that would have normally appeared on-screen will be written to the file
REDIR.OUT.
Redirection of standard streams does not always have to occur at the operating system. You can redirect a standard stream from within your program by using the standard C library function named freopen(). For example, if you wanted to redirect the stdout standard stream within your program to a file named OUTPUT.TXT, you would implement the freopen() function as shown here:
... freopen(output.txt, w, stdout);
...
Now, every output statement (printf(), puts(), putch(), and so on) in your program will appear in the file OUTPUT.TXT.
11.What is a method?
Method is a way of doing something, especially a systematic way; implies an orderly logical arrangement (usually in steps).
12.What is the easiest searching method to use?
Just as qsort() was the easiest sorting method, because it is part of the standard library, bsearch() is the easiest searching method to use. If the given array is in the sorted order bsearch() is the best method.
Following is the prototype for bsearch():
void *bsearch(const void *key, const void *buf, size_t num, size_t size, int (*comp)(const void *, const void*));
Another simple searching method is a linear search. A linear search is not as fast as bsearch() for searching among a large number of items, but it is adequate for many purposes. A linear search might be the only method available, if the data isn’t sorted or can’t be accessed randomly. A linear search starts at the beginning and sequentially compares the key to each element in the data set.
13.Is it better to use a pointer to navigate an array of values, or is it better to use a subscripted array name?
It’s easier for a C compiler to generate good code for pointers than for subscripts.
14.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.
15.How are portions of a program disabled in demo versions?
If you are distributing a demo version of your program, the preprocessor can be used to enable or disable portions of your program. The following portion of code shows how this task is accomplished, using the preprocessor directives #if and #endif:
int save_document(char* doc_name)
{
#if DEMO_VERSION
printf(Sorry! You can’t save documents using the DEMO version of this programming);
return(0);
#endif
...
}

16.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 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.
17.Write down the equivalent pointer expression for referring the same element a[i][j][k][l]?
a[i] == *(a+i)
a[i][j] == *(*(a+i)+j)
a[i][j][k] == *(*(*(a+i)+j)+k)
a[i][j][k][l] == *(*(*(*(a+i)+j)+k)+l)
18.Which bit wise operator is suitable for checking whether a particular bit is on or off?
The bitwise AND operator. Here is an example:enum {
KBit0 = 1,
KBit1,
…
KBit31,
};

if ( some_int & KBit24 )
printf ( “Bit number 24 is ON\n” );
else
printf ( “Bit number 24 is OFF\n” );
19.Which bit wise operator is suitable for turning off a particular bit in a number?
The bitwise AND operator, again. In the following code snippet, the bit number 24 is reset to zero.
some_int = some_int & ~KBit24;
20.Which bit wise operator is suitable for putting on a particular bit in a number?
The bitwise OR operator. In the following code snippet, the bit number 24 is turned ON:
some_int = some_int | KBit24;