C Refresher

What is . and -> || a->b === (*a).b

If you have a variable of a structure, use . to access its elements.

In case you have a pointer to the structure, use -> to access the elements in the structure stored at the loaction of that pointer. (eg: foo->bar)

foo->bar is the same as (*foo).bar.

The parenthesizes above are necessary because of the binding strength of the * and . operators.

*foo.bar() wouldn’t work because Dot (.) operator is evaluated first (see operator precedence)

The Dot (.) operator can’t be overloaded, arrow (->) operator can be overloaded.

The Dot (.) operator can’t be applied to pointers.

struct foo
{
  int x;
  float y;
};

struct foo var;
struct foo* pvar;

var.x = 5;
(&var)->y = 14.3;
pvar->y = 22.4;
(*pvar).x = 6;
// That's it!

Reference: 1, 2

String Input

scanf ("%[^\n]s", name) is got to be the best way to take input beyond a space. [^\n] tells to take input while it is not a newline ('\n').

When and why to use malloc answer by me

Why should I use malloc when I can normally allocate variable size array

There are at least five benefits to using malloc over variable length arrays.

  1. Most notably, objects created with malloc persist after execution of the current block ends. This means that such objects can be returned (by pointer) to callers of functions. This use is frequent in real-world applications. Arrays created as variable-length arrays cease to exist when execution of their block ends.
  2. Arrays created with malloc can be resized with realloc. Variable-length arrays cannot be resized.
  3. As of the 2011 C standard, variable-length arrays are optional for C implementations to support. A general-purpose C implementation of any quality will support them, but the fact they are optional means code that is intended to be portable must either not use variable-length arrays or must guard against the lack of support by testing the preprocessor macro STDC_NO_VLA and providing alternate code.
  4. Commonly, variable-length arrays are much more limited in size than arrays allocated with malloc. Variable-length arrays are generally implemented using stack space, and stacks are typically limited to some not-large number of mebibytes (although that can generally be increased when building an executable). For objects created with malloc, gibibytes of memory may be available in modern systems.
  5. If creation of an array does fail with malloc, NULL will be returned, and the programmer can easily write code to detect that and deal with it. If creation of a variable-length array fails, the common behavior is for the operating system to terminate the program with some memory error. (Various C implementations may provide means to intercept this error, but it is considerably more of a nuisance than testing the malloc return value for NULL, and it is not portable.) Ref

Why use malloc in general?

char *some_memory = "Hello World";

is creating a pointer to a string constant. That means the string “Hello World” will be somewhere in the read-only part of the memory and you just have a pointer to it. You can use the string as read-only. You cannot make changes to it. Example:

some_memory[0] = 'h';

Is asking for trouble.

On the other hand

some_memory = (char *)malloc(size_to_allocate);

is allocating a char array ( a variable) and some_memory points to that allocated memory. Now this array is both read and write. You can now do:

some_memory[0] = 'h';

and the array contents change to “hello World”

Ref

typedef

General: typedef <existing_name> <alias_name> Example: typedef unsigned long ulong;

Example using snippets of Linked List Code

typedef void (*callback)(node* data);

...

void traverse(node* head,callback f)
{
    node* cursor = head;
    while(cursor != NULL)
    {
        f(cursor);
        cursor = cursor->next;
    }
}

...

callback disp = display;
traverse(head,disp);

Explanation: typedef void (*callback)(node* data); defines *callback which is pointer to a function. It is pointer to a function which returns void and takes node* data as arguments. This is different from the general convention use of typedef

References: 1, 2

Linked List

http://www.zentut.com/c-tutorial/c-linked-list/

cdecl

cdecl> explain int (*FUNC_TYPE_1)(void)
declare FUNC_TYPE_1 as pointer to function (void) returning int
cdecl> explain double (*FUNC_TYPE_2)(void)
declare FUNC_TYPE_2 as pointer to function (void) returning double
cdecl> declare FUNC_TYPE_3 as pointer to function (pointer to function (void) returning double) returning pointer to function (void) returning int
int (*(*FUNC_TYPE_3)(double (*)(void )))(void )

Miscellaneous References

  1. Markdown Cheatsheet
  2. mycodeschool YouTube… haven’t seen but got the link from somewhere. MIGHT be interesting to check out
Written on December 30, 2018