C Interview Questions Answers for Experienced

  1. 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.
  2. 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 NULL 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.
  3. What is the difference between const char* p and char const* p?
    In const char* p, the character pointed by ‘p’ is constant, so u cant 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 cant make ‘p’ to reference to any other location but u can change the value of the char pointed by ‘p’.
  4. 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.
  5. 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.
  6. 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.
  7. 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.
  8. 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.
  9. 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.
  10. 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:
    To stop indirection in a recursive data structure
    As an error value
    As a sentinel value

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