C Structure and Union
Q. What is a structure?
Ans: Structure constitutes a super data type which represents several different data types in a single unit. A structure can be initialized if it is static or global.
Ans: Structure constitutes a super data type which represents several different data types in a single unit. A structure can be initialized if it is static or global.
Q. What is a union?
Ans: Union is a collection of heterogeneous data type but it uses efficient memory utilization technique by allocating enough memory to hold the largest member. Here a single area of memory contains values of different types at different time. A union can never be initialized.
Ans: Union is a collection of heterogeneous data type but it uses efficient memory utilization technique by allocating enough memory to hold the largest member. Here a single area of memory contains values of different types at different time. A union can never be initialized.
Q. What are the differences between structures and union?
Ans: A structure variable contains each of the named members, and its size is large enough to hold all the members. Structure elements are of same size.
A union contains one of the named members at a given time and is large enough to hold the largest member. Union element can be of different sizes.
Ans: A structure variable contains each of the named members, and its size is large enough to hold all the members. Structure elements are of same size.
A union contains one of the named members at a given time and is large enough to hold the largest member. Union element can be of different sizes.
Q. What are the differences between structures and arrays?
Ans: Structure is a collection of heterogeneous data type but array is a collection of homogeneous data types.
Array
1-It is a collection of data items of same data type.
2-It has declaration only
3-.There is no keyword.
4- array name represent the address of the starting element.
Structure
1-It is a collection of data items of different data type.
2- It has declaration and definition
3- keyword struct is used
4-Structure name is known as tag it is the short hand notation of the declaration.
Ans: Structure is a collection of heterogeneous data type but array is a collection of homogeneous data types.
Array
1-It is a collection of data items of same data type.
2-It has declaration only
3-.There is no keyword.
4- array name represent the address of the starting element.
Structure
1-It is a collection of data items of different data type.
2- It has declaration and definition
3- keyword struct is used
4-Structure name is known as tag it is the short hand notation of the declaration.
Q. What the advantages of using Unions?
Ans: When the C compiler is allocating memory for unions it will always reserve enough room for the largest member.
Ans: When the C compiler is allocating memory for unions it will always reserve enough room for the largest member.
Q. How are Structure passing and returning implemented by the complier?
Ans: When structures are passed as argument to functions, the entire structure is typically pushed on the stack. To avoid this overhead many programmer often prefer to pass pointers to structure instead of actual structures. Structures are often returned from functions in a location pointed to by an extra, compiler-supported ‘hidden’ argument to the function.
Ans: When structures are passed as argument to functions, the entire structure is typically pushed on the stack. To avoid this overhead many programmer often prefer to pass pointers to structure instead of actual structures. Structures are often returned from functions in a location pointed to by an extra, compiler-supported ‘hidden’ argument to the function.
Q. what is the similarity between a Structure, Union and enumeration?
Ans: All of them let the programmer to define new data type.
Ans: All of them let the programmer to define new data type.
Q. Can a Structure contain a Pointer to itself?
Ans: Yes such structures are called self-referential structures.
Ans: Yes such structures are called self-referential structures.
Q. Why can’t we compare structures?
Ans:
There is no single, good way for a compiler to implement structure comparison which is consistent with C’s low-level flavor. A simple byte-by-byte comparison could founder on random bits present in unused “holes” in the structure (such padding is used to keep the alignment of later fields correct). A field-by-field comparison might require unacceptable amounts of repetitive code for large structures.
Ans:
There is no single, good way for a compiler to implement structure comparison which is consistent with C’s low-level flavor. A simple byte-by-byte comparison could founder on random bits present in unused “holes” in the structure (such padding is used to keep the alignment of later fields correct). A field-by-field comparison might require unacceptable amounts of repetitive code for large structures.
Q. How are structure passing and returning implemented?
Ans: When structures are passed as arguments to functions, the entire structure is typically pushed on
the stack, using as many words as are required. Some compilers merely pass a pointer to the structure, though they may have to make a local copy to preserve pass-by-value semantics.
Structures are often returned from functions in a location pointed to by an extra,compiler-supplied “hidden” argument to the function. Some older compilers used a special,static location for structure returns, although this made structure-valued functions non-reentrant, which ANSI C disallows.
Ans: When structures are passed as arguments to functions, the entire structure is typically pushed on
the stack, using as many words as are required. Some compilers merely pass a pointer to the structure, though they may have to make a local copy to preserve pass-by-value semantics.
Structures are often returned from functions in a location pointed to by an extra,compiler-supplied “hidden” argument to the function. Some older compilers used a special,static location for structure returns, although this made structure-valued functions non-reentrant, which ANSI C disallows.
Q. What are bit fields? What is the use of bit fields in a Structure declaration?
Ans: A bit field is a set of adjacent bits within a single implementation based storage unit that we
will call a “word”.
The syntax of field definition and access is based on structure.
Struct {
unsigned int k :1;
unsigned int l :1;
unsigned int m :1;
}flags;
the number following the colon represents the field width in bits.Flag is a variable that contains three bit fields.
Ans: A bit field is a set of adjacent bits within a single implementation based storage unit that we
will call a “word”.
The syntax of field definition and access is based on structure.
Struct {
unsigned int k :1;
unsigned int l :1;
unsigned int m :1;
}flags;
the number following the colon represents the field width in bits.Flag is a variable that contains three bit fields.
Q.Can we initialize unions?
Ans: ANSI Standard C allows an initializer for the first member of a union. There is no standard way
of initializing any other member (nor, under a pre-ANSI compiler, is there generally any way of
initializing a union at all).
Ans: ANSI Standard C allows an initializer for the first member of a union. There is no standard way
of initializing any other member (nor, under a pre-ANSI compiler, is there generally any way of
initializing a union at all).
Q. What’s the difference between these two declarations?
Ans: struct x1 { … };
typedef struct { … } x2;
The first form declares a structure tag; the second declares a typedef. The main difference is that the second declaration is of a slightly more abstract type.its users don’t necessarily know that it is a structure, and the keyword struct is not used when declaring instances of it.
Ans: struct x1 { … };
typedef struct { … } x2;
The first form declares a structure tag; the second declares a typedef. The main difference is that the second declaration is of a slightly more abstract type.its users don’t necessarily know that it is a structure, and the keyword struct is not used when declaring instances of it.
Q.Why doesn’t struct x { … };
x thestruct;
work?
Ans:
C is not C++. Typedef names are not automatically generated for structure tags.
x thestruct;
work?
Ans:
C is not C++. Typedef names are not automatically generated for structure tags.
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