Pointers in C

Introduction

  • This is a variable that stores the address of a memory location.
  • Can contain the address in memory of another variable, object or function.
  • Object is said to be memory allocated using one of the memory allocation functions such as malloc.
  • Gets its type from what it points to: such as pointer to a char.
  • Nothing inherent in the pointer indicated what type data pointer is referencing. Only contains address.
  • Key to comprehend pointers is understanding how memory is managed in a C program.
  • When a C program is compiled it works with 3 types of memory all differentiated by their scope and lifetime:
    • Static/Global - static vars and global vars use this.
    • Automatic - declared within a function.
    • Dynamic - allocated from the heap and released when necessary.
  • Most pointers are used to manipulate data in memory, hence need to understand how memory is partitioned and organised.
  • Faster and more efficient code can be written because pointers are closer to the hardware, compiler can easily translate operation into machine code.
  • Many data structures are more easily implemented using pointers. i.e linked list, arrays.

Uses of pointers

  • Creating fast and efficient code.
  • Convenient means for addressing many types of problems.
  • Support dynamic memory allocation
  • Make expression compact and succinct.
  • Reduce overhead by passing data structures by pointers.
  • Protecting data passed as a parameter to a function.
  • Multiple levels of indirection. i.e double pointer,
  • Constant pointers

Pointer hazards

  • Accessing arrays and other data structures beyond their bounds.
  • Referencing automatic variables after they have gone out of existence.
  • Referencing heap allocated memory after it has been released.
  • Dereferencing a pointer before memory has been allocated to it.

Declaring pointers

 int *num
 int* num

 const int *pci

// overloaded symbol as its also used for multiplication.

// The trick is to read the declaration backward.

// operator address

  num = 0;
  pi = #

// assignment of integer to pointer will cause a warning.
// good practice to initialize a pointer as soon as possible.

// indirection operator * is used to return the value pointed to by a pointer variable. deferencing
  int num = 0;
  int *pi = #
  
  printf("%p\n", *pi);
  
// use the result of dereference as lvalue hence assignable.
  • An invalid address is one that the program is not authorized to access.
  • Uninitialized memory refers to references to variables uninitialized thus contain garbage.
  • Virtual memory and pointers.
  • The size of a pointer depends on the machine in use and compiler.
  • On modern platforms, the size of a pointer is the same regardless of pointer type.
  • Predefined pointer data types.
  • Pointer operators
  • Pointer Arithmetic
    • Add an integer to a pointer.
    • Subtracting an integer from a pointer.
    • Subtracting two pointer from each other.(only useful when need to know the order of elements in an array)
    • Comparing pointers.