一本介绍C指针的书--指针和结构体5.1

As you may know, we can declare the form of a block of data containing different data
types by means of a structure declaration. For example, a personnel file might contain
structures which look something like:  
    struct tag {
        char lname[20];        /* last name */
        char fname[20];        /* first name */
        int age;               /* age */
        float rate;            /* e.g. 12.75 per hour */
    };
Let's say we have a bunch of these structures in a disk file and we want to read each one
out and print out the first and last name of each one so that we can have a list of the
people in our files. The remaining information will not be printed out. We will want to do
this printing with a function call and pass to that function a pointer to the structure at
hand. For demonstration purposes I will use only one structure for now. But realize the
goal is the writing of the function, not the reading of the file which, presumably, we
know how to do.  
For review, recall that we can access structure members with the dot operator as in:  
 
--------------- program 5.1 ------------------
 
/* Program 5.1 from PTRTUT10.HTM     6/13/97 */
 
 
#include <stdio.h>
#include <string.h>
 
struct tag {
    char lname[20];      /* last name */
    char fname[20];      /* first name */
    int age;             /* age */
    float rate;          /* e.g. 12.75 per hour */
};
 
struct tag my_struct;       /* declare the structure my_struct */
 
int main(void)
{
    strcpy(my_struct.lname,"Jensen");
    strcpy(my_struct.fname,"Ted");
    printf("\n%s ",my_struct.fname);
    printf("%s\n",my_struct.lname);
    return 0;
}
 
-------------- end of program 5.1 --------------
 

Now, this particular structure is rather small compared to many used in C programs. To
the above we might want to add:  
 
    date_of_hire;                  (data types not shown)
    date_of_last_raise;
    last_percent_increase;
    emergency_phone;
    medical_plan;
    Social_S_Nbr;
    etc.....
If we have a large number of employees, what we want to do is manipulate the data in
these structures by means of functions. For example we might want a function print out
the name of the employee listed in any structure passed to it. However, in the original C
(Kernighan & Ritchie, 1st Edition) it was not possible to pass a structure, only a pointer
to a structure could be passed. In ANSI C, it is now permissible to pass the complete
structure. But, since our goal here is to learn more about pointers, we won't pursue that.  
Anyway, if we pass the whole structure it means that we must copy the contents of the
structure from the calling function to the called function. In systems using stacks, this is
done by pushing the contents of the structure on the stack. With large structures this
could prove to be a problem. However, passing a pointer uses a minimum amount of
stack space.  
In any case, since this is a discussion of pointers, we will discuss how we go about
passing a pointer to a structure and then using it within the function.  
Consider the case described, i.e. we want a function that will accept as a parameter a
pointer to a structure and from within that function we want to access members of the
structure. For example we want to print out the name of the employee in our example
structure.