![Recursively defined structures
When two structures refer to each other, one must be
declared in incomplete (prototype) fashion
struct HUMAN;
struct PET {
char name[NAME_LIMIT];
char species[NAME_LIMIT];
struct HUMAN *owner;
} fido = {″Fido″, ″Canis lupus familiaris″};
struct HUMAN {
char name[NAME_LIMIT];
We can’t initialize the owner member
struct PET pets[PET_LIMIT]; at this point,
} sam = {″Sam″, {fido}}; since it hasn’t been declared yet
6 CS 3090: Safety Critical Programming in C](https://image.slidesharecdn.com/cstructuresandunions-090704223526-phpapp02/85/C-Structures-And-Unions-6-320.jpg)
![Member access
Direct access operator s.m
subscript and dot operators have same precedence and
associate left-to-right, so we don’t need parentheses for
sam.pets[0].species
Indirect access s->m: equivalent to (*s).m
Dereference a pointer to a structure, then return a member of
that structure
Dot operator has higher precedence than indirection operator
, so parentheses are needed in (*s).m
(*fido.owner).name or fido.owner->name
. evaluated first: access owner member . and -> have equal precedence
* evaluated next: dereference pointer to HUMAN and associate left-to-right
7 CS 3090: Safety Critical Programming in C](https://image.slidesharecdn.com/cstructuresandunions-090704223526-phpapp02/85/C-Structures-And-Unions-7-320.jpg)
![Memory layout
struct COST { int amount;
char currency_type[2]; }
struct PART { char id[2];
struct COST cost;
int num_avail; }
layout of struct PART:
currency_type
id amount num_avail
cost
Here, the system uses 4-byte alignment of integers,
so amount and num_avail must be aligned
Four bytes wasted for each structure!
8 CS 3090: Safety Critical Programming in C](https://image.slidesharecdn.com/cstructuresandunions-090704223526-phpapp02/85/C-Structures-And-Unions-8-320.jpg)
![Memory layout
A better alternative (from a space perspective):
struct COST { int amount;
char currency_type; }
struct PART { struct COST cost;
char id[2];
int num_avail;
}
currency_type
amount id num_avail
cost
9 CS 3090: Safety Critical Programming in C](https://image.slidesharecdn.com/cstructuresandunions-090704223526-phpapp02/85/C-Structures-And-Unions-9-320.jpg)
C Structures And Unions
- 1. C structures and unions 1
- 2. C structures: aggregate, yet scalar aggregate in that they hold multiple data items at one time named members hold data items of various types like the notion of class/field in C or C++ – but without the data hiding features scalar in that C treats each structure as a unit as opposed to the “array” approach: a pointer to a collection of members in memory entire structures (not just pointers to structures) may be passed as function arguments, assigned to variables, etc. Interestingly, they cannot be compared using == (rationale: too inefficient) 2
- 3. Structure declarations Combined variable and type declaration struct tag {member-list} variable-list; Any one of the three portions can be omitted struct {int a, b; char *p;} x, y; /* omit tag */ variables x, y declared with members as described: int members a, b and char pointer p. x and y have same type, but differ from all others – even if there is another declaration: struct {int a, b; char *p;} z; /* z has different type from x, y */ 3 CS 3090: Safety Critical Programming in C
- 4. Structure declarations struct S {int a, b; char *p;}; /* omit variables */ No variables are declared, but there is now a type struct S that can be referred to later struct S z; /* omit members */ Given an earlier declaration of struct S, this declares a variable of that type typedef struct {int a, b; char *p;} S; /* omit both tag and variables */ This creates a simple type name S (more convenient than struct S) 4 CS 3090: Safety Critical Programming in C
- 5. Recursively defined structures Obviously, you can’t have a structure that contains an instance of itself as a member – such a data item would be infinitely large But within a structure you can refer to structures of the same type, via pointers struct TREENODE { char *label; struct TREENODE *leftchild, *rightchild; } 5 CS 3090: Safety Critical Programming in C
- 6. Recursively defined structures When two structures refer to each other, one must be declared in incomplete (prototype) fashion struct HUMAN; struct PET { char name[NAME_LIMIT]; char species[NAME_LIMIT]; struct HUMAN *owner; } fido = {″Fido″, ″Canis lupus familiaris″}; struct HUMAN { char name[NAME_LIMIT]; We can’t initialize the owner member struct PET pets[PET_LIMIT]; at this point, } sam = {″Sam″, {fido}}; since it hasn’t been declared yet 6 CS 3090: Safety Critical Programming in C
- 7. Member access Direct access operator s.m subscript and dot operators have same precedence and associate left-to-right, so we don’t need parentheses for sam.pets[0].species Indirect access s->m: equivalent to (*s).m Dereference a pointer to a structure, then return a member of that structure Dot operator has higher precedence than indirection operator , so parentheses are needed in (*s).m (*fido.owner).name or fido.owner->name . evaluated first: access owner member . and -> have equal precedence * evaluated next: dereference pointer to HUMAN and associate left-to-right 7 CS 3090: Safety Critical Programming in C
- 8. Memory layout struct COST { int amount; char currency_type[2]; } struct PART { char id[2]; struct COST cost; int num_avail; } layout of struct PART: currency_type id amount num_avail cost Here, the system uses 4-byte alignment of integers, so amount and num_avail must be aligned Four bytes wasted for each structure! 8 CS 3090: Safety Critical Programming in C
- 9. Memory layout A better alternative (from a space perspective): struct COST { int amount; char currency_type; } struct PART { struct COST cost; char id[2]; int num_avail; } currency_type amount id num_avail cost 9 CS 3090: Safety Critical Programming in C
- 10. Bit fields Bit field members must be ints If space is a serious concern, you can select the number of bits used for each member struct CHAR { unsigned ch: 7; Note: This won’t work on unsigned font: 6; machines with 16-bit ints unsigned size: 19; }; Layout possibilities (machine-dependent): ch font size size font ch 10 CS 3090: Safety Critical Programming in C
- 11. Bit fields Portability is an issue: Do any bit field sizes exceed the machine’s int size? Is there any pointer manipulation in your code that assumes a particular layout? Bit fields are “syntactic sugar” for more complex shifting/ masking e.g. to get font value, mask off the ch and size bits, then shift right by 19 This is what actually happens in the object code – bit fields just make it look simpler at the source level 11 CS 3090: Safety Critical Programming in C
- 12. Structures as function arguments Structures are scalars, so they can be returned and passed as arguments – just like ints, chars struct BIG changestruct(struct BIG s); Call by value: temporary copy of structure is created Caution: passing large structures is inefficient – involves a lot of copying avoid by passing a pointer to the structure instead: void changestruct(struct BIG *s); What if the struct argument is read-only? Safe approach: use const void changestruct(struct BIG const *s); 12 CS 3090: Safety Critical Programming in C
- 13. Unions Like structures, but every member occupies the same region of memory! Structures: members are “and”ed together: “name and species and owner” Unions: members are “xor”ed together union VALUE { float f; int i; char *s; }; /* either a float xor an int xor a string */ 13 CS 3090: Safety Critical Programming in C
- 14. Unions Up to programmer to determine how to interpret a union (i.e. which member to access) Often used in conjunction with a “type” variable that indicates how to interpret the union value enum TYPE { INT, FLOAT, STRING }; struct VARIABLE { enum TYPE type; Access type to determine union VALUE value; how to interpret value }; 14 CS 3090: Safety Critical Programming in C
- 15. Unions Storage size of union is the size of its largest member avoid unions with widely varying member sizes; for the larger data types, consider using pointers instead Initialization Union may only be initialized to a value appropriate for the type of its first member 15 CS 3090: Safety Critical Programming in C