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pointers
summary
This subchapter looks at pointers.
The educational goal of this subchapter is for the student to learn what pointers are and how to use them.
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This subchapter is a stub section. It will be filled in with instructional material later. For now it serves the purpose of a place holder for the order of instruction.
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pointers
This subchapter looks at pointers.
quotations about pointers
“Pointers also require sensible notation. np is just as mnemonic as nodepointer if you consistently use a naming convention from which np means “node pointer” is easily derived.” —Rob Pike, Notes on Programming in C, February 21, 1989
“Of course Java has pointers. In fact, just about everything in Java is implicitly a pointer. They just call them references. There are advantages to having pointers implicit as well as disadvantages. Separately,there are advantages to having true local objects (as in C++) as well as disadvantages.” —Bjarne Stroustrup, Masterminds of Progtramming, Chapter One, 2009
“What Java doesn’t have—and good for Java for that—is C and C++’sability to misuse pointers through pointer arithmetic. Well-written C++ doesn’t suffer from that problem either: people use higher-level abstractions, such as iostreams, containers, and algorithms, rather than fiddling with pointers. Essentially all arrays and most pointers belong deep in implementations that most programmers don’t have to see. Unfortunately, there is also lots of poorly written and unnecessarily low-level C++ around.” —Bjarne Stroustrup, Masterminds of Progtramming, Chapter One, 2009
JOVIAL
The following material is from the unclassified Computer Programming Manual for the JOVIAL (J73) Language, RADC-TR-81-143, Final Technical Report of June 1981.
The kinds of values provided by JOVIAL reflect the applications
of the language; they are oriented toward engineering and contrl
programming rather than, for example, commercial and business
programming. The JOVIAL values are:
7. Pointer values, which are data addresses, meaningful
only within the program. They are used to locate data
indirectly. For example, a list of items can use
pointers to connect each item to the next item in the
list.
Chapter 1 INTRODUCTION, page 3
ITEM HEAD P DATA; A pointer item, whose value is the
address of a data object of type DATA.
Chapter 1 INTRODUCTION, page 5
1.1.5 Built-In Functions
The JOVIAL built-in functions provide advanced, specialized
operations that are not covered by the JOVIAL operators.
LOC(x) A pointer to the object referenced by r
NEXT(p,i) A pointer to the i'th data object after
the one selected by p
Chapter 1 Introduction, page 9
C
Stanford C essentials
Stanford CS Education Library This [the following section until marked as end of Stanford University items] is document #101, Essential C, in the Stanford CS Education Library. This and other educational materials are available for free at http://cslibrary.stanford.edu/. This article is free to be used, reproduced, excerpted, retransmitted, or sold so long as this notice is clearly reproduced at its beginning. Copyright 1996-2003, Nick Parlante, nick.parlante@cs.stanford.edu.
Pointers
A pointer is a value which represents a reference to another value sometimes known as the pointer’s “pointee”. Hopefully you have learned about pointers somewhere else, since the preceding sentence is probably inadequate explanation. This discussion will concentrate on the syntax of pointers in C -- for a much more complete discussion of pointers and their use see http://cslibrary.stanford.edu/102/, Pointers and Memory.
Syntax
Syntactically C uses the asterisk or “star” (*) to indicate a pointer. C defines pointer types based on the type pointee. A char* is type of pointer which refers to a single char. A struct fraction* is type of pointer which refers to a struct fraction.
int* intPtr; // declare an integer pointer variable intPtr
char* charPtr; // declares a character pointer --
// a very common type of pointer
// Declare two struct fraction pointers
// (when declaring multiple variables on one line, the *
// should go on the right with the variable)
struct fraction *f1, *f2;The Floating “*”
In the syntax, the star is allowed to be anywhere between the base type and the variable name. Programmer’s have their own conventions-- I generally stick the * on the left with the type. So the above declaration of intPtr could be written equivalently…
int *intPtr; // these are all the same
int * intPtr;
int* intPtr;Pointer Dereferencing
We’ll see shortly how a pointer is set to point to something -- for now just assume the pointer points to memory of the appropriate type. In an expression, the unary * to the left of a pointer dereferences it to retrieve the value it points to. The following drawing shows the types involved with a single pointer pointing to a struct fraction.
struct fraction* f1;
Expression Type f1 struct fraction* *f1 struct fraction (*f1).numerator int There’s an alternate, more readable syntax available for dereferencing a pointer to a struct. A “->” at the right of the pointer can access any of the fields in the struct. So the reference to the numerator field could be written f1->numerator.
Here are some more complex declarations…
struct fraction** fp; // a pointer to a pointer to a struct fraction
struct fraction fract_array[20]; // an array of 20 struct fractions
struct fraction* fract_ptr_array[20]; // an array of 20 pointers to
// struct fractionsOne nice thing about the C type syntax is that it avoids the circular definition problems which come up when a pointer structure needs to refer to itself. The following definition defines a node in a linked list. Note that no preparatory declaration of the node pointer type is necessary.
struct node {
int data;
struct node* next;
};The & Operator
The & operator is one of the ways that pointers are set to point to things. The & operator computes a pointer to the argument to its right. The argument can be any variable which takes up space in the stack or heap (known as an “LValue” technically). So &i and &(f1->numerator) are ok, but &6 is not. Use & when you have some memory, and you want a pointer to that memory.
void foo() {
int* p; // p is a pointer to an integer
int i;; // i is an integer
p = &i; // Set p to point to i
*p = 13; // Change what p points to -- in this case i -- to 13
// At this point i is 13. So is *p. In fact *p is i.
}
When using a pointer to an object created with &, it is important to only use the pointer so long as the object exists. A local variable exists only as long as the function where it is declared is still executing (we’ll see functions shortly). In the above example, i exists only as long as foo() is executing. Therefore any pointers which were initialized with &i are valid only as long as foo() is executing. This “lifetime” constraint of local memory is standard in many languages, and is something you need to take into account when using the & operator.
NULL
A pointer can be assigned the value 0 to explicitly represent that it does not currently have a pointee. Having a standard representation for “no current pointee” turns out to be very handy when using pointers. The constant NULL is defined to be 0 and is typically used when setting a pointer to NULL. Since it is just 0, a NULL pointer will behave like a boolean false when used in a boolean context. Dereferencing a NULL pointer is an error which, if you are lucky, the computer will detect at runtime -- whether the computer detects this depends on the operating system.
Pitfall -- Uninitialized Pointers
When using pointers, there are two entities to keep track of. The pointer and the memory it is pointing to, sometimes called the “pointee”. There are three things which must be done for a pointer/pointee relationship to work…
(1) The pointer must be declared and allocated
(2) The pointee must be declared and allocated
(3) The pointer (1) must be initialized so that it points to the pointee (2)
The most common pointer related error of all time is the following: Declare and allocate the pointer (step 1). Forget step 2 and/or 3. Start using the pointer as if it has been setup to point to something. Code with this error frequently compiles fine, but the runtime results are disastrous. Unfortunately the pointer does not point anywhere good unless (2) and (3) are done, so the run time dereference operations on the pointer with * will misuse and trample memory leading to a random crash at some point.
{
int* p;
*p = 13; // NO NO NO p does not point to an int yet
asdf
// this just overwrites a random area in memory
}
Of course your code won’t be so trivial, but the bug has the same basic form: declare a pointer, but forget to set it up to point to a particular pointee.
Using Pointers
Declaring a pointer allocates space for the pointer itself, but it does not allocate space for the pointee. The pointer must be set to point to something before you can dereference it.
Here’s some code which doesn’t do anything useful, but which does demonstrate (1) (2) (3) for pointer use correctly…
int* p; // (1) allocate the pointer
int i; // (2) allocate pointee
struct fraction f1; // (2) allocate pointee
p = &i; // (3) setup p to point to i
*p = 42; // ok to use p since it's setup
p = &(f1.numerator); // (3) setup p to point to a different int
*p = 22;
p = &(f1.denominator); // (3)
*p = 7;So far we have just used the & operator to create pointers to simple variables such as i. Later, we’ll see other ways of getting pointers with arrays and other techniques.
See the discussion on C strings in the Stanford CS Education information in the subchapter on strings
Stanford CS Education Library This [the above section] is document #101, Essential C, in the Stanford CS Education Library. This and other educational materials are available for free at http://cslibrary.stanford.edu/. This article is free to be used, reproduced, excerpted, retransmitted, or sold so long as this notice is clearly reproduced at its beginning. Copyright 1996-2003, Nick Parlante, nick.parlante@cs.stanford.edu.
end of Stanford C essentials
Ada
“36 Access types allow the construction of linked data structures. A value of an access type represents a reference to an object declared as aliased or to an object created by the evaluation of an allocator. Several variables of an access type may designate the same object, and components of one object may designate the same or other objects. Both the elements in such linked data structures and their relation to other elements can be altered during program execution. Access types also permit references to subprograms to be stored, passed as parameters, and ultimately dereferenced as part of an indirect call.” —:Ada-Europe’s Ada Reference Manual: Introduction: Language Summary See legal information
assembly language instructions
address registers
Address registers store the addresses of specific memory locations. Often many integer and logic operations can be performed on address registers directly (to allow for computation of addresses).
Sometimes the contents of address register(s) are combined with other special purpose registers to compute the actual physical address. This allows for the hardware implementation of dynamic memory pages, virtual memory, and protected memory.
The number of bits of an address register (possibly combined with information from other registers) limits the maximum amount of addressable memory. A 16-bit address register can address 64K of physical memory. A 24-bit address register can address address 16 MB of physical memory. A 32-bit address register can address 4 GB of physical memory. A 64-bit address register can address 1.8446744 x 1019 of physical memory. Addresses are always unsigned binary numbers. See number of bits.
- MIX: one jump registers; named J-register; two bytes and sign is always positive
- Motorola 680x0, 68300: 8 longword (32 bit) address registers; named A0, A1, A2, A3, A4, A5, A6, and A7 (also called the stack pointer)
index registers
Index registers are used to provide more flexibility in addressing modes, allowing the programmer to create a memory address by combining the contents of an address register with the contents of an index register (with displacements, increments, decrements, and other options). In some processors, there are specific index registers (or just one index register) that can only be used only for that purpose. In some processors, any data register, address register, or general register (or some combination of the three) can be used as an index register.
- IBM 360/370: any of the 16 general purpose registers may be used as an index register
- Intel 80x86: 7 of the 8 general purpose registers may be used as an index register (the ESP is the exception)
- MIX: five index registers; named I-registers I1, I2, I3, I4, and I5; five bytes plus sign
- Motorola 680x0, 68300: any of the 8 data registers or the 8 address registers may be used as an index register
base registers
Base registers or segment registers are used to segment memory. Effective addresses are computed by adding the contents of the base or segment register to the rest of the effective address computation. In some processors, any register can serve as a base register. In some processors, there are specific base or segment registers (one or more) that can only be used for that purpose. In some processors with multiple base or segment registers, each base or segment register is used for different kinds of memory accesses (such as a segment register for data accesses and a different segment register for program accesses).
- IBM 360/370: any of the 16 general purpose registers may be used as a base register
- Intel 80x86: 6 dedicated segment registers: CS (code segment), SS (stack segment), DS (data segment), ES (extra segment, a second data segment register), FS (third data segment register), and GS (fourth data segment register)
- Motorola 680x0, 68300: any of the 8 address registers may be used as a base register
See also Registers
address movement
Address movement instructions move addresses from one location to another. The source and destination locations are determined by the addressing modes, and can be registers or memory. Address movement instructions can come in a variety of sizes. Address movement instructions destroy the previous contents of the destination. Address movement instructions typically do not modify processor flags. When the destination is a register and the address is smaller than the full register size, the data might be placed only in the low order bits (leaving high order bits unchanged), or might be zero- or sign-extended to fill the entire register (some processors only use one choice, others permit the programmer to choose how this is handled).
- MOVEA.W Move Address (Word); Motorola 680x0, Motorola 68300; move an address word (16 bits) as sign-extended data (32 bits); memory to address register or register to address register; does not modify flags
- MOVEA.L Move Address (Longword); Motorola 680x0, Motorola 68300; move an address longword (32 bits); memory to address register or register to address register; does not modify flags
- LEA Load Effective Address; Motorola 680x0, Motorola 68300; computes an effective address and loads the result into an address register
- LA Load Address; RX format; IBM 360/370; computes an effective address and loads the 24-bit result (zero extended to 32-bits) into a general purpose register; does not affect condition code
- ENTA Enter A-register; MIX; move word or partial word field contents of index register to A-register (accumulator)
- ENTX Enter X-register; MIX; move word or partial word field contents of index register to X-register (extension)
- ENTi Enter I-register; MIX; move word or partial word field contents of index register to designated index register
- ENNA Enter Negative A-register; MIX; move word or partial word field contents of index register to A-register (accumulator), opposite sign loaded
- ENNX Enter Negative X-register; MIX; move word or partial word field contents of index register to X-register (extension), opposite sign loaded
- ENNi Enter Negative I-register; MIX; move word or partial word field contents of index register to designated index register, opposite sign loaded
- INCA Increase A-register; MIX; add word or partial word field contents of memory to A-register (accumulator), overflow toggle possibly set
- INCX Increase X-register; MIX; add word or partial word field contents of memory to X-register (extension), overflow toggle possibly set
- INCi Increase I-register; MIX; add word or partial word field contents of memory to designated index register, overflow toggle possibly set
- DECA Decrease A-register; MIX; subtract word or partial word field contents of memory from A-register (accumulator), overflow toggle possibly set
- DECX Decrease X-register; MIX; subtract word or partial word field contents of memory from X-register (extension), overflow toggle possibly set
- DECi Decrease I-register; MIX; subtract word or partial word field contents of memory from designated index register, overflow toggle possibly set
- PEA Push Effective Address; Motorola 680x0, Motorola 68300; computes an effective address and pushes the result onto a stack (predecrementing an address register acting as a stack pointer)
- LINK Link Stack; Motorola 680x0, Motorola 68300
- UNLK Unlink Stack; Motorola 680x0, Motorola 68300
See also Data Movement Instructions in Assembly Language
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