UNIX for Dummies Questions & Answers

Can someone tell me how to do this?

Just a thought that entered my mind when learning about structs.
First thought was:

struct one
{
struct two;
}

struct two
{
three;
}

one->two->three

would this be how you would access "three"?

Well kinda, but no, this is incorrect in some ways.

A struct adds labels to a group of related data in C. The group of data usually corresponds to a single record of structured data of some type, such as:

Code:

struct customer {
 int account_number;
 character *first_name;
 character *last_name;
} ;

It is important to realize that the definition of a structure is often different than the allocation of memory to use it. In the example above, the customer record above is defined by a structure but the allocation of memory to that structure is a separate step. No memory was allocated to 'customer' in the struct statement and usually this is done with an automatic array, a single automatic variable or a call to malloc().

This following statements creates a 3 integer struct and creates the associated memory (in this case on the stack)

Code:

struct point_s {
   int x;
   int y;
   int z;
} ;  /* define the record */

struct point_s point; /* one element of that record */

struct point_s *PteToPoint = &point;  /* a pointer pointing to that record */

/* set x, y, z of point in 3 different ways: */

point.x=1;
(*PtrToPoints).y=2;
PtrToPoints->z=3;   /* idiomatic C pointer to a struct member */

In this case, the data structure for a point in 3d space is created. Next, the memory for 1 element is created in one step. You can create an array of points similarly.

The example you have of one->two->three would be used in the case of a structure containing a pointer. Each application of '->' simply means 'what that points to' So you data structure would need to be:

Code:

struct s1 {
   [whatever data type] three;
};

struct s2 {
   [whatever other data];
   struct s1*two;
};

So one->two->three would be the data in a struct defined by s1 pointed to by a pointer inside s2...

You can, however, have nested structures such as:

Code:

struct first_type
{
  int i_member;
  float f_member;
};

struct second_type
{
  double d_member;
  struct first_type s_member;
};

struct second_type structure;

structure.d_member = 12345.6789;
structure.s_member.i_member = 5;
structure.s_member.f_member = 1023.17;

The cycle for using a struct is usually along these lines:

Code:

  1) Define the record (using struct statements, union, etc); 
  2) Allocate memory for the data (by a named variable, call to malloc, creation of an array);
  3) Fill the records (using C statements); 
  4) Access the data (using var.member directly or (*var).member or var->member with pointers).

http://en.wikipedia.org/wiki/Struct_...mming_language)