C Programming: May 2011

Thursday, May 26, 2011

C provides rectangular multi-dimensional arrays, in practice they are much less used than array of pointers
In C, a two dimensional array is really a one dimensional array, each of whose elements is an array. Hence subscripts are written as:

An array is initialized by a list of initializers in braces
If a two-dimensional array is to be passed to a function, the parameter declaration in the function must include the number of columns; the number of rows is irrelevant

Each line of character array can be accessed by a pointer to its first character and pointers themselves can be stored in an array
When two out-of-order lines have to be exchanged, the pointers in the pointer array are exchanged, not the text lines themselves
These prevent complicated storage management and high overhead of moving the lines themselves

C's integration of pointers, arrays and address arithmetic is one of the strengths of the language
alloc(n), returns a pointer p to n-consecutive character positions, which can be used by the caller of alloc for storing character
afree(n) releases the storage thus acquired so it can be re-used later
Storage managed by alloc and afree is a stack or last-in, first-out list
In general, a pointer can be initialized to zero or an expression involving the addresses of previously defined data of appropriate type
C guarantees that zero is never a valid address of data, so a return value of zero signal an abnormal event
Pointers and integers are not interchangeable
Zero is sole exception: the constant zero may be assigned to a pointer, and a pointer may be compared with the constant zero
The construction p+n means the address of the n-th object beyond the one p currently points to
n is scaled according to the size of the objects p points to, which is determined by the declaration of p.
The valid pointer operations are assignment of pointers of the same type, adding or subtracting a pointer and an integer, subtracting or comparing two pointers to members of the same array, and assigning or comparing to zero

a[i] can also be written as *(a+i); &a[i] = a+i
A pointer is a variable, so pa = a and pa++ is legal
An array name is not a variable, so a = pa and a++ is illegal
When an array name is passed to a function, what is passed is the location of the initial element
Within the called function, this argument is a local variable, and so an array name parameter is a pointer(variable containing an address)
As formal parameters in a function definition, char s[] and char *s are equivalent
It is possible to pass part of an array to a function, by passing a pointer to the beginning of the subarray

Since C passes arguments to functions by value, there is no direct way for the called function to alter a variable in the calling function
Because of call-by-value, function below only swaps copies of a and b:

The way to obtain the desired effect is for calling program to pass pointers to the value to be changed:

Pointer arguments enable a function to access and change objects in the function that called it

A typical machine has an array of consecutively numbered or addressed memory cells that may be manipulated individually or in contiguous groups
The unary operator & gives the address of an object
& operator only applies to objects in memory: variables and array elements
The unary operator * is the indirection or dereferencing operator; when applied to a pointer, it accesses the object the pointer points to
Example: