memory allocation

[rithish]

what is the use of malloc,calloc,realloc ?? i studied i didnt get proper knowledge .why instead of this why cant we use array???

[VernonDozier]

what is the use of malloc,calloc,realloc ?? i studied i didnt get proper knowledge .why instead of this why cant we use array???

I can't explain malloc, calloc, and realloc better than all of the tutorials.
cplusplus.com is my go-to site.

http://www.cplusplus.com/reference/clibrary/cstdlib/malloc/

As to why you can't use an array, the question shows the misunderstanding. You use malloc to allocate memory so you can use an array.

int* array = (int*) malloc(100 * sizeof(int));
// now I can use it.  Without malloc, I couldn't.
array[32] = 7;

[deceptikon]

why instead of this why cant we use array???

And if you don't know the size of the array at compile time?

[irre]

you need it for dynamic memory allocation. just think of a program where sb has to input a n-dimensional matrix of unkown n. let the user input n and then go forward to get the necessary memory space.
next advantage of malloc is that all the memory-places allocated stick next to each other. if you would set up static variables, their locations would be at random places. for example you want to allocate a dynamic nxn-matrix:

double *matrix;
int n;
printf("input dimension:");
scanf("%d",&n);
matrix=(double*)malloc(n*sizeof(double));

malloc is a void function so you have to bring it into a any pointer type like (int*),(float*),or(double*)
matrix then represents the start address of your memoryspace allocated.
You can access these spaces like following:

scanf("%lf",matrix[i]);  // i=0,1,2...,n-1
//that should be the same as
scanf("%lf",matrix)          //for i=0
//or
scanf("%d",matrix+3);        //for i=3

you wont need the address operator "&" cause pointers are adresses.

[deceptikon]

malloc is a void function so you have to bring it into a any pointer type like (int*),(float*),or(double*)

That's a confusing statement, and it's also incorrect. When people say "void function" they typically mean a function that returns void, not a generic pointer to void. A more accurate, but still incorrect statement would be:

"malloc() returns a pointer to void, so you have to cast it into the destination pointer type."

But C allows an implicit conversion between void* and any non-function pointer type without a cast. In fact, adding the cast is dangerous because it can hide the warnings you would get by forgetting to include stdlib.h.

A good pattern for malloc() that I see a lot is this:

p = malloc(n * sizeof *p);

No cast is needed, and sizeof *p gets the size of the pointed-to type without having to name it explicitly. That way if you decide to change the type, you don't need to change the call to malloc(). So your code, with a few alterations, would look like this:

double *matrix;
int n;

printf("input dimension: ");
fflush(stdout); // Force the prompt to show before blocking

if (scanf("%d",&n) != 1) // Always check input for failure
{
    // Handle failure
}

matrix = malloc(n * sizeof *matrix);

if (!matrix) // Always check malloc() for failure too :)
{
    // Handle failure
}

One caveat to omitting the cast is that if you're writing code that needs to compile as both C and C++, the cast is required. C++ has different rules for converting from void*.