Which statement is true about nested structures in C?
Answer: B
C allows structures to have other structures as members. This is called nested structures and is a common practice.
Q.2Hard
What happens if you assign a union member and then access another member?
struct u { int a; char b; }; u.a = 257; printf("%d", u.b);
Answer: B
In a union, both members share memory. 257 in binary is 100000001. u.b (char) reads only the LSB, which is 1.
Q.3Hard
How can you initialize a structure array of 10 elements partially in C?
Answer: D
Both {0} and {} will initialize all members to zero. These are equivalent in C99 and later standards.
Q.4Hard
What is the difference between self-referential and recursive structures?
Answer: B
Self-referential structures contain pointers to the same structure type (like linked list nodes). Recursion refers to function calls, not structures.
Q.5Hard
Given union test { int x; char y[4]; }; If you set y[0]=65, y[1]=66, what happens to x?
Answer: C
Since union members share memory, setting y[0]=65 and y[1]=66 overwrites the int x value. The exact value depends on endianness.
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Q.6Hard
Which approach is more memory efficient for storing 100 flags: array of char or bit fields in a structure?
Answer: B
Bit fields can pack 8 flags per byte, so 100 flags need ~13 bytes. Char array needs 100 bytes. Bit fields are more memory-efficient for flag storage.
Q.7Hard
In nested structures with pointers, which access method is correct? struct outer { struct inner *ptr; } *o; Accessing inner's member x:
Answer: D
Both o->ptr->x and (*o).ptr->x are equivalent. Arrow operator can chain for pointers, and (*pointer).member is equivalent to pointer->member.
Q.8Hard
What is the relationship between structure alignment and padding in modern C compilers?
Answer: B
Compilers add padding bytes between structure members to align them on boundaries (usually power of 2), improving memory access performance on the target architecture.
Q.9Hard
In a structure with flexible array members, which statement is INCORRECT?
struct FlexArray { int len; int arr[]; };
Answer: B
Flexible array members require dynamic allocation; the structure itself cannot be allocated on the stack with a defined size for the array.
Q.10Hard
What is the output of the following?
struct S { int a:3; int b:3; int c:3; };
printf("%zu", sizeof(struct S));
Answer: D
Bit field packing is implementation-defined. Size depends on compiler's bit field allocation strategy.
Q.11Hard
In competitive programming, when should you use union instead of struct?
Answer: B
Unions are optimal for memory-constrained scenarios and hardware programming where type casting is needed.
Q.12Hard
What will be the output of this code?
struct X { int a; float b; };
struct Y { struct X x; int c; };
printf("%zu", sizeof(struct Y));
Answer: C
Nested struct X: int(4) + float(4) = 8 bytes. Y: struct X(8) + padding(0) + int(4) = 12, but with overall alignment it becomes 16 bytes.
Q.13Hard
In a structure, what does the #pragma pack(1) directive do?
Answer: A
#pragma pack(1) removes padding, making members occupy consecutive memory locations for compact storage.
Q.14Hard
If a structure contains a flexible array member, where must it be declared?
Answer: B
A flexible array member must be the last member of a structure. This is a C99 feature used for dynamic-sized arrays.
Q.15Hard
In competitive programming, when implementing a graph using adjacency lists, which data structure is most suitable?
Answer: B
An array of structures, where each structure contains a linked list (using self-referential pointers), is ideal for adjacency list representation.
Q.16Hard
If you have a structure with bit fields, which statement about their behavior is true?
Answer: A
Bit fields save memory by storing multiple values in single bytes, but behavior varies by compiler/platform, and you cannot take addresses of bit field members.
Q.17Hard
What is the output of this complex code?
struct S { char c; int i; } s;
printf("%lu", sizeof(s));
Answer: B
char(1 byte) + padding(3 bytes) + int(4 bytes) = 8 bytes due to alignment requirements.
Q.18Hard
In competitive programming, when you need a structure that can hold either an integer or a floating-point number (but not both simultaneously) in the most memory-efficient way, which should you use?
Answer: B
Union is memory-efficient as members share space. Only one can be accessed at a time, which suits the requirement.
Q.19Hard
What will be the output of the following code?
union Test {
int a;
char b;
};
union Test t;
t.a = 65;
printf("%d %c", t.a, t.b);
Answer: D
Since union members share memory, the output depends on the system's endianness. On little-endian systems, t.b would be 65 (or 'A'), but on big-endian systems, the result differs.
Q.20Hard
In competitive programming for GATE/ISRO 2025, which scenario would union be preferable over structure?
Answer: B
Unions are preferred when you have mutually exclusive data (only one member is used at a time) and memory is critical. A classic example is storing different data types in a tagged union pattern used in compilers and interpreters.
