The document discusses various user-defined data types in C programming including structures, unions, enumerated types, and type definitions. It provides details on declaring and defining structures and unions, accessing structure members, nested structures, arrays of structures, passing structures to functions, and bit fields. Enumerated types allow assigning names to integer values. Typedef allows abbreviating existing data type names. Structures and unions allow grouping related data as records while enumerated types and typedefs create new data types.

1. C Programming : User-defined data types
By
Mr.S.Selvaraj
Asst. Professor (SRG) / CSE
Kongu Engineering College
Perundurai, Erode, Tamilnadu, India
Thanks to and Resource from : Sumitabha Das, “Computer Fundamentals and C Programming”, 1st Edition, McGraw Hill, 2018.

2. 20CST11 – Problem Solving and Programming
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Syllabus

3. Contents
• There are 4 types of user-defined data types.
1. Structure (struct)
 Structure basics
 Declaring and defining a structure
 Attributes of structures
 Nested structures
 Arrays as structure members
 Arrays of structure
 Passing structures as arguments to functions
 Bit Fields
2. Unions (union)
3. Enumerated type (enum)
4. Type definition (typedef)
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4. Structure Basics
• C offers a basket of data types for handling different
levels of complexity in the organization of data.
• The primary data types (like int and float) are meant
for using simple and small amounts of data.
• The array is offered as a derived data type for handling
a group of data items of the same type.
• Real-life situations often involve the use of
heterogeneous data that need a mix of data types to
be handled as a single unit.
• Structures and unions meet this requirement perfectly.
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5. Structure Basics
• A group of data items are often logically related to one
another.
• For instance,
– the attributes of an employee include the name, address,
date of birth, salary, and so forth.
• A structure is a data type that is designed by the user.
• However, unlike the other data types, a structure
variable is based on a template that must be created
first.
• Each member of the structure is accessed by the
notation structure.member, and can be used in
practically the same way as a variable.
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6. Declaring and Defining a Structure
• a structure variable is created in two steps
which may or may not be combined:
– Declare the structure to create a template which
specifies the components of the structure.
• Declaration simply makes type information available
to the compiler. However, the compiler doesn’t
allocate memory by seeing the declaration.
– Define a variable that conforms to the template.
• The compiler allocates memory for the variable and
creates an instance of the template.
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7. Declaring and Defining a Structure
• Declaration uses the struct keyword.
• The following generalized syntax declares a structure
named struct_name.
• The syntax of the definition also specifies the keyword
struct, the structure tag (struct_name) and the
variable name (struct_var).
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8. Example
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9. Accessing Members of a Structure
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10. Combining Declaration, Definition and Initialization
• For an initialized structure, the variable name is followed by an = and a list of
comma-delimited values enclosed by curly braces.
• These initializers obviously must be provided in the right order.
• In Form 2, “Beethoven” is assigned to title and 3 to qty.
• You can create multiple variables and initialize them at the same time.
• Like with arrays, you can partially initialize a structure. If you initialize only
title, the remaining members, qty and price, are automatically assigned zeroes
• By default, uninitialized structures have junk values unless they are global.
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11. Declaring without Structure Tag
• If declaration and definition are combined, the structure
tag can be omitted.
• This is usually done when no more structure variables of
that type require to be defined later in the program.
• You can’t subsequently create any more variables of this
type.
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14. Memory Layout of Members of music_cd
• Note that sizeof computes the total memory
occupied by a structure.
• This is not necessarily the sum of the sizes of the
members.
• Thus, on this machine having a 4-byte word.
– title occupies 28 bytes (7 words) even though it uses
27 of them.
– qty and price individually occupy an entire word (4
bytes each), but qty uses two of these bytes.
– disk1 thus needs 33 bytes (27 + 2 + 4) but it actually
occupies 36 bytes.
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15. Memory Layout of Members of music_cd
• Alignment issues related to structures lead to
the creation of slack bytes or holes in the
allocated memory segment.
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16. Attributes Of Structures
• (1) There are no name conflicts between
structure templates, their instantiated
variables and members.
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17. Attributes Of Structures
• (2) The =, dot, -> and & are the only operators
that can be used on structures.
• (3) A structure can be copied to another
structure provided both objects are based on
the same template.
• This feature is not available with arrays; all
array elements have to be copied individually
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18. Attributes Of Structures
• (4) No arithmetic, relational or logical operations can be
performed on structures even when the operation logically makes
sense.
• (5) It is not possible to compare two structures using a single
operator even though such comparison could be logically
meaningful.
– Each member has to be compared individually as shown by the
following code.
– If a structure contains 20 members, you need to use 20 relational
expressions to test for equality.
– Unfortunately, C doesn’t support a better option.
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20. Attributes Of Structures
• (6) When a structure is passed by name as an
argument to a function, the entire structure is
copied inside the function.
• (7) A member of a structure can contain a
reference to another structure of the same
type.
– This property of self-referential structures is used
for creating linked lists.
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21. Nested Structures
• A structure member can have any data type—including
another structure (but not of the same type though).
• The inner structure can contain another structure and
so on.
• C supports nested structures and the following outer
structure named student contains an inner structure
named dt_birth as one of its four members.
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22. Example
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23. Initializing a Nested Structure
• When it comes to initialization, nested
structures resemble multi-dimensional arrays.
• For every level of nesting, a set of inner
braces have to be used for members of the
inner structure.
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24. Accessing Members
• A member of an inner structure is connected
by a dot to its immediately enclosing
structure, which is connected by another dot
to the outer structure.
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25. Arrays as structure members
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26. Arrays Of Structures
• We used two structure variables, stud1 and stud2, to
handle data of two students.
• This technique won’t work with 500 students.
• C supports an array of structures, whose definition
may or may not be combined with the declaration of
the structure.
• The following statement defines an array of type struct
student that can hold 50 sets (or records) of student
data.
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27. Arrays Of Structures
• Alternatively, you can separate the declaration and
definition.
• You can also use typedef to replace struct student with
STUDENT.
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28. Arrays Of Structures
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29. Structures in functions
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30. Structures in functions
• A function using a structure as argument
copies the entire structure.
• If the structure contains an array, it too will be
copied.
• A program may run out of memory when
using structures as arguments.
• A structure containing numerous members
and large arrays can lead to stack overflow
and cause the program to abort
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33. Bit fields
• Have our programs wasted memory?
– Yes, in many cases they have.
• We used 2-byte short integers to store each
component of a date when a few bits would
have sufficed.
• We also wasted 15 bits of the 16 available in
short to set a flag to 0 and 1
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34. Bit fields
• It’s time to know that C supports a memory saving
feature—the bit field.
• It can be used with the right number of bits, rather
than bytes, to store data.
• One or two bits are adequate to represent the sex.
• while six or seven bits are good enough to represent
the age.
• Bit fields can be used only as structure members.
• Properly sized bit fields prevent both data overflow
and memory wastage.
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35. Example
• The first two members, id and age, are bit fields.
• The declaration of a bit field specifies an int (signed or
unsigned), followed by the member name, a colon and the
number of allocated bits.
• Here, id and age occupy 10 and 6 bits, respectively.
• This means that the maximum values of id and age are
restricted to 1023 and 63, respectively.
• sizeof evaluates the size of emp to 32.
• A similar structure using two short variables for id and age
would occupy 34 bytes.
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36. Unions
• When searching a database using a person’s id, you must
have often wanted to search by name as well.
• That is possible if the two attributes are held in a union, a
data type that resembles a structure in form.
• A union comprises two or more members that share the
same memory, with the most recent assignment to a
member being active at any instant.
• You can define a union with many members, but only one
member can contain a value at any given time.
• Unions provide an efficient way of using the same
memory location for multiple purposes.
• Consider the following declaration and definition of a union
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37. Unions
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• A union is declared, defined and accessed exactly
like a structure.
• The previous statement declares a two-member
union named uemp and simultaneously defines
the union variable u.
• The members are accessed as u.emp_id and
u.name.
• However, both members share a single memory
region and you can correctly access only one
member at a time—the one that was last
assigned.

38. Unions
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39. Unique Attributes of Unions
• (1) All members of a union can’t be initialized
simultaneously.
• (2) The size of a union is determined by the size of the
member having the largest footprint in memory. (For
uemp, this would be 30, the size of the name field).
• (3) The compiler doesn’t output an error when you
access a member that was not the last one to be
assigned. A programmer must keep track of the
member that is active at any instant.
• (4) You can use a union to assign a value to a member
and read it back using a different member.
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40. Unions
• A union can easily determine whether a
machine is little-endian or big-endian.
• In a big-endian machine, the most significant
bit is stored first while the reverse is true for
little-endian machines.
• However, unions are mainly used as memory-
saving objects.
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41. Example
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42. Example
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43. Difference between Structure and Union
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44. The enumerated type (enum)
• We have used symbolic constants to represent
integers using the #define feature of the
preprocessor.
• C also supports the user-defined enumeration
data type that assigns names to a set of
integer values called enumerators.
• For instance, you can create a set of named
constants using the enum keyword.
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45. • This statement creates a set of enumerated
integer values which, by default, begin with 0.
• YES evaluates to 0, while NO and CANT_SAY
have the values 1 and 2, respectively.
• If this declaration is made before main, you
can use NO to represent 1 anywhere in the
program.
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46. • You can now create one or more enum
variables of type eday.
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48. Abbreviating a Data Type : The typedef Feature
• The typedef keyword is used to abbreviate
the names of data types.
• Using A Data type LL instead of long long
involves less typing.
• The syntax of typedef is simple; follow
typedef with the existing data type and its
proposed synonym.
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49. Example
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50. Thank you
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