0 Pointer & Array-Pointers

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.

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