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pointer                     C Language                    pointer




A pointer is an object whose  value is the address of another ob-
ject.  The  name ``pointer'' derives from the  fact that its con-
tents ``point  to'' another object.   A pointer may  point to any
type, complete or  incomplete, including another pointer.  It may
also point to a function, or to nowhere.

The term pointer type refers to the object of a pointer.  The ob-
ject to  which a  pointer points  is called the  referenced type.
For example, an int * (``pointer to iinntt'') is a pointer type; the
referenced  type is  int.   Constructing a  pointer  type from  a
referenced type is called pointer type derivation.

***** The Null Pointer *****

A pointer  that points to  nowhere is a null  pointer.  The macro
NULL, which  is defined in  the header stdio.h,  defines the null
pointer.  The null pointer  is an integer constant with the value
zero.  It  compares unequal to  a pointer to any  object or func-
tion.

***** Declaring a Pointer *****

To declare a pointer,  use the indirection operator `*'.  For ex-
ample, the declaration


        int *pointer;


declares that  the variable pointer holds the  address of an iinntt-
length object.  Likewise, the declaration


        int **pointer;


declares that  pointer holds the address of  a pointer whose con-
tents, in turn, point to an iinntt-length object.

Failure to declare a  function that returns a pointer will result
in that function being  implicitly declared as an int.  This will
not  cause an  error on  microprocessors  in which  an int  and a
pointer have the same  size; however, transporting this code to a
microprocessor in which an int  consists of 16 bits and a pointer
consists of  32 bits will result in  the pointers being truncated
to 16 bits and the program probably failing.

C allows  pointers and  integers to  be compared or  converted to
each other  without restriction.   The COHERENT C  compiler flags
such conversions with the strict message






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pointer                     C Language                    pointer



        integer pointer pun


and comparisons with the strict message


        integer pointer comparison


These problems  should be corrected  if you want your  code to be
portable to other computing environments.

See declarations for more information.

***** Wild Pointers *****

Pointers  are omnipresent  in  C.  C  also  allows you  to use  a
pointer to read or write  the object to which the pointer points;
this  is called  pointer  dereferencing.  Because  a pointer  can
point to any place within memory,  it is possible to write C code
that  generates unpredictable results,  corrupts itself,  or even
obliterates the operating  system if running in unprotected mode.
A pointer that aims where it ought not is called a wild pointer.

When a  program declares a pointer, space is  set aside in memory
for it.  However, this space has not yet been filled with the ad-
dress of  an object.  To fill  a pointer with the  address of the
object  you wish  to access  is called  initializing it.   A wild
pointer,  as  often as  not,  is  one that  is  not properly  in-
itialized.

Normally,  to  initialize  a pointer  means  to  fill  it with  a
meaningful  address.  For  example, the  following  initializes a
pointer:


        int number;
        int *pointer;
           . . .
        pointer = &number;


The address  operator `&' specifies that you  want the address of
an object rather than its contents.  Thus, pointer is filled with
the address of number, and it  can now be used to access the con-
tents of number.

The initialization of a string is somewhat different than the in-
itialization of a pointer to an integer object.  For example,








COHERENT Lexicon                                           Page 2




pointer                     C Language                    pointer



        char *string = "This is a string."


declares that string is a pointer  to a char.  It then stores the
string literal  This is a string in memory  and fills string with
the address of its first character.  string can then be passed to
functions  to access  the  string, or  you can  step through  the
string  by incrementing  string until its  contents point  to the
null character at the end of the string.

Another way  to initialize a pointer  is to fill it  with a value
returned by a function  that returns a pointer.  For example, the
code


        extern char *malloc(size_t variable);
        char *example;
           . . .
        example = malloc(50);


uses the  function malloc to allocate 50  bytes of dynamic memory
and then initializes example to the address that malloc returns.

***** Reading What a Pointer Points To *****

The indirection  operator `*' can  be used to read  the object to
which a pointer points.  For example,


        int number;
        int *pointer;
           . . .
        pointer = &number;
           . . .
        printf("%d\n", *pointer);


uses pointer to access the contents of number.

When a  pointer points  to a  structure, the elements  within the
structure can  be read by using the  structure offset operator `-
>'.  See the entry for -> for more information.

***** Pointers to Functions *****

A pointer  can also contain  the address of a  function.  For ex-
ample,


        char *(*example)();


declares example  to be  a pointer to  a function that  returns a
pointer to a char.


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pointer                     C Language                    pointer




This declaration is quite different from:


        char **different();


The latter  declares that different is a  function that returns a
pointer to a pointer to a char.

The following demonstrates how to call a function via a pointer:


        (*example)(_a_r_g_1, _a_r_g_2);


_H_e_r_e, _t_h_e  `*' _t_a_k_e_s _t_h_e  _c_o_n_t_e_n_t_s _o_f _t_h_e _p_o_i_n_t_e_r,  _w_h_i_c_h _i_n _t_h_i_s
_c_a_s_e _i_s  _t_h_e _a_d_d_r_e_s_s  _o_f _t_h_e _f_u_n_c_t_i_o_n,  _a_n_d _u_s_e_s _t_h_a_t  _a_d_d_r_e_s_s _t_o
_p_a_s_s _t_o _a _f_u_n_c_t_i_o_n _i_t_s _l_i_s_t _o_f _a_r_g_u_m_e_n_t_s.

_A _p_o_i_n_t_e_r _t_o  _a _f_u_n_c_t_i_o_n _c_a_n _b_e _p_a_s_s_e_d _a_s  _a_n _a_r_g_u_m_e_n_t _t_o _a_n_o_t_h_e_r
_f_u_n_c_t_i_o_n.   _T_h_e _f_u_n_c_t_i_o_n_s  _b_s_e_a_r_c_h _a_n_d  _q_s_o_r_t _b_o_t_h  _t_a_k_e _f_u_n_c_t_i_o_n
_p_o_i_n_t_e_r_s  _a_s _a_r_g_u_m_e_n_t_s.   _A  _p_r_o_g_r_a_m _m_a_y  _a_l_s_o _u_s_e  _o_f _a_r_r_a_y_s  _o_f
_p_o_i_n_t_e_r_s _t_o _f_u_n_c_t_i_o_n_s.

***** _P_o_i_n_t_e_r _C_o_n_v_e_r_s_i_o_n *****

_O_n_e _t_y_p_e  _o_f _p_o_i_n_t_e_r _m_a_y _b_e _c_o_n_v_e_r_t_e_d, _o_r  _c_a_s_t, _t_o _a_n_o_t_h_e_r.  _F_o_r
_e_x_a_m_p_l_e, _a _p_o_i_n_t_e_r _t_o _a _c_h_a_r  _m_a_y _b_e _c_a_s_t _t_o _a _p_o_i_n_t_e_r _t_o _a_n _i_n_t,
_a_n_d _v_i_c_e _v_e_r_s_a.

_P_o_i_n_t_e_r_s _t_o  _d_i_f_f_e_r_e_n_t _d_a_t_a _t_y_p_e_s _a_r_e  _c_o_m_p_a_t_i_b_l_e _i_n _e_x_p_r_e_s_s_i_o_n_s,
_b_u_t _o_n_l_y _i_f _t_h_e_y _a_r_e _c_a_s_t _a_p_p_r_o_p_r_i_a_t_e_l_y.  _U_s_i_n_g _t_h_e_m _w_i_t_h_o_u_t _c_a_s-
_t_i_n_g _p_r_o_d_u_c_e_s _a _p_o_i_n_t_e_r-_t_y_p_e _m_i_s_m_a_t_c_h.

***** _P_o_i_n_t_e_r _A_r_i_t_h_m_e_t_i_c *****

_A_r_i_t_h_m_e_t_i_c  _m_a_y _b_e  _p_e_r_f_o_r_m_e_d _o_n  _a_l_l  _p_o_i_n_t_e_r_s _t_o  _s_c_a_l_a_r _t_y_p_e_s,
_i._e.,  _p_o_i_n_t_e_r_s _t_o  cchhaarrs or  iinntt.   Pointer arithmetic  is quite
limited and consists of the following:

11. One pointer may be subtracted from another.

22. An int or a long, either variable or constant, may be added to
   a pointer or subtracted from it.

33. The operators ++ or -- may be used to increment or decrement a
   pointer.

No other  pointer arithmetic is permitted.   No arithmetic can be
performed on  pointers to  non-scalar objects, e.g.,  pointers to
functions.





COHERENT Lexicon                                           Page 4




pointer                     C Language                    pointer



***** i8086 Pointers *****

Intel designed  the i8086 to use  a segmented architecture.  This
means that  the i8086  divides memory into  64-kilobyte segments.
To  program the  i8086 requires  that you  use a  specific memory
model, which  describes how the segments of memory  are to be or-
ganized.

***** See Also *****

C language, data formats, operators, portability














































COHERENT Lexicon                                           Page 5

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