Pointers

Pointer & Array

Pointers and arrays are very closely linked in C.

Hint: Think of array elements arranged in consecutive memory locations.

Consider the following:

int a[10], x;

int *pa;

pa = &a[0]; /* pa pointer to address of a[0] */

x = *pa;

/* x = contents of pa (a[0] in this case) */

Warning: There is no bound checking of arrays and pointers so you can easily go beyond array memory and overwrite other things.

C however is much more subtle in its link between arrays and pointers.

For example we can just type:

pa = a;

instead of

pa = &a[0]

and

a[i] can be written as *(a + i).

i.e. &a[i] =a + i.

We also express pointer addressing like this:

pa[i] =*(pa + i).

However pointers and arrays are different:

A pointer is a variable. We can do pa = a and pa++.

An Array is not a variable. a = pa and a++ ARE ILLEGAL

This stuff is very important. Make sure you understand it. We will see a lot more of this. We can now understand how arrays are passed to functions.

When an array is passed to a function what is actually passed is its initial element location in memory

So: strlen(s) strlen(&s[0])

This is why we declare the function:

int strlen(char s[]);

An equivalent declaration is:

int strlen(char *s);

since char s[] is equivalent to char *s.

strlen () is a standard library function that returns the length of a string.

Let's look at how we may write a function:

int strlength(char *s)

{

char *p = s;

while (*p != '\0');

p++;

return p-s;

}

Now let’s write a function to copy a string to another string. strcpy () is a standard library function that does this:

void strcopy (char *s, char *t)

{ while ( (*s++ = *t++) != `\0' );}

This uses pointers and assignment by value.

Note: Uses of Null statements with while.

Malloc Library Function

Function: Allocates main memory

Syntax: void*malloc(size_t size);

Prototype in: stdlib.h, alloc.h

Remarks: malloc allocates a block of size bytes from the C heap memory. It allows a program to allocate memory explicitly, as it is needed and in the exact amounts needed.

Calloc Library Function

Function: Allocates main memory

Syntax: void*calloc(size_t n size);

Prototype in: stdlib.h, alloc.h

Remarks: Calloc provides access to the C heap memory . Calloc allocates a block of size n items of x size. The block is cleared to 0.