CHAPTER 3: Pointers and Strings

The study of strings is useful to further tie in the relationship between pointers and arrays. It also makes it easy to illustrate how some of the standard C string functions can be implemented. Finally it illustrates how and when pointers can and should be passed to functions.
In C, strings are arrays of characters. This is not necessarily true in other languages. In BASIC, Pascal, Fortran and various other languages, a string has its own data type. But in C it does not. In C a string is an array of characters terminated with a binary zero character (written as '\0'). To start off our discussion we will write some code which, while preferred for illustrative purposes, you would probably never write in an actual program. Consider, for example:

      char my_string[40];
      my_string[0]     =  'T';
      my_string[1]     =  'e';
      my_string[2]     =  'd':
      my_string[3]     =  '\0';

While one would never build a string like this, the end result is a string in that it is an array of characters terminated with a nul character. By definition, in C, a string is an array of characters terminated with the nul character. Be aware that "nul" is not the same as "NULL". The nul refers to a zero as defined by the escape sequence '\0'. That is it occupies one byte of memory. NULL, on the other hand, is the name of the macro used to initialize null pointers. NULL is #defined in a header file in your C compiler, nul may not be #defined at all.

Since writing the above code would be very time consuming, C permits two alternate ways of achieving the same thing. First, one might write:

      char my_string[40] = {'T', 'e', 'd', '\0',};

But this also takes more typing than is convenient. So, C permits:

      char my_string[40] = "Ted";

When the double quotes are used, instead of the single quotes as was done in the previous examples, the nul character ( '\0' is automatically appended to the end of the string.

In all of the above cases, the same thing happens. The compiler sets aside an contiguous block of memory 40 bytes long to hold characters and initialized it such that the first 4 characters are Ted\0.

Now, consider the following program:
------------------program 3.1-------------------------------------
/* Program 3.1 from PTRTUT10.HTM                6/13/97 */
#include <stdio.h>
char strA[80] = "A string to be used for demonstration purposes";
char strB[80];
int main(void)
{
     char *pA;          /* a pointer to type character */
     char *pB;          /* another pointer to type character */
     puts(strA);        /* show string A */
     pA = strA;         /* point pA at string A */
     puts(pA);          /* show what pA is pointing to */
     pB = strB;         /* point pB at string B */
     putchar('\n');               /* move down one line on the screen */
     while(*pA != '\0')           /* line A (see text) */
     {
           *pB++ = *pA++;         /* line B (see text) */
     }
     *pB = '\0';                  /* line C (see text) */
     puts(strB);                  /* show strB on screen */
     return 0;
}
--------- end program 3.1 -------------------------------------

In the above we start out by defining two character arrays of 80 characters each. Since these are globally defined, they are initialized to all '\0' first. Then, strA has the first 42 characters initialized to the string in quotes.