Strings.


Pointers to strings.

C does not have a "string" datatype. To create a string you have to use a char array or a char pointer. If you are not familur with char arrays I recomend that you read about them now.

To recap, a char pointer is defined like this:

 
     main()
     {
       char *Text;
     }
     

All this program does is reserve storage that will hold an address. At this point the address could be anything. To initalize Text you can code:

    
     main()
     {
       char *Text = "Thunder";
     }
     

Text now has the address of the first character in Thunder. Graphically, things look like this.


      (Address) (Data)
      
           ---- ----    
          | F1 | 00 <------- Text |----|----| (Data) (Adress) | F2 | 00 | ------------- |----|----| -------> 54 (T) | D1 |
	  | F3 | 00 |        |       |--------|----|
	  |----|----|  *Text |       | 68 (h) | D2 |
	  | F4 | D1 | -------        |--------|----|
	   ---------                 | 75 (u) | D3 |
	                             |--------|----|
				     | 6E (n) | D4 |
				     |--------|----|
                                     | 64 (d) | D5 |
                                     |--------|----|
                                     | 65 (e) | D6 |
                                     |--------|----|
                                     | 72 (r) | D7 |
                                     |--------|----|     
                                     | 00     | D8 |
                                      -------------

Please note the 00 at the end of Thunder. This is the NULL character and is used to mark the end of a string.

If we wanted to O/P the data pointed to by a char pointer we can code.

Source

     main()
     {
       char *Text1 = "Thunder";          /* Define and initalize */
       char *Text2;                      /* Define only          */
       
       Text2 = "Bird";                   /* Point to some text   */
       
       printf("%s%s\n", Text1, Text2);
     }
     
Result

      ThunderBird

This is all very well, but there is a MAJOR problem! Thunder and Bird are constants, they cannot be changed in anyway. We need a method of pointing to some storage that can be altered and true to form, C provides a function called malloc to do just that.


See Also:

VOID keyword.


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Martin Leslie