一本介绍C指针的书--指针和字符串3.1

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's first. Then, strA has the first 42
characters initialized to the string in quotes.  
Now, moving into the code, we declare two character pointers and show the string on the
screen. We then "point" the pointer pA at strA. That is, by means of the assignment
statement we copy the address of strA[0] into our variable pA. We now use puts() to
show that which is pointed to by pA on the screen. Consider here that the function
prototype for puts() is:  
    int puts(const char *s);  
For the moment, ignore the const. The parameter passed to puts() is a pointer, that is the
value of a pointer (since all parameters in C are passed by value), and the value of a
pointer is the address to which it points, or, simply, an address. Thus when we write
puts(strA); as we have seen, we are passing the address of strA[0].  
Similarly, when we write puts(pA); we are passing the same address, since we have set
pA = strA;  
 
Given that, follow the code down to the while() statement on line A. Line A states:  
While the character pointed to by pA (i.e. *pA) is not a nul character (i.e. the terminating
'\0'), do the following:  
Line B states: copy the character pointed to by pA to the space pointed to by pB, then
increment pA so it points to the next character and pB so it points to the next space.  
When we have copied the last character, pA now points to the terminating nul character
and the loop ends. However, we have not copied the nul character. And, by definition a
string in C must be nul terminated. So, we add the nul character with line C.  
It is very educational to run this program with your debugger while watching strA, strB,
pA and pB and single stepping through the program. It is even more educational if
instead of simply defining strB[] as has been done above, initialize it also with something
like:  
    strB[80] = "12345678901234567890123456789012345678901234567890"
where the number of digits used is greater than the length of strA and then repeat the
single stepping procedure while watching the above variables. Give these things a try!  
Getting back to the prototype for puts() for a moment, the "const" used as a parameter
modifier informs the user that the function will not modify the string pointed to by s, i.e.
it will treat that string as a constant.