C questions in placement tests and university exams are rarely about syntax alone. Most of what you will find here is output prediction, pointer arithmetic, arrays and strings, structures and unions, storage classes, recursion, and file handling. The explanations walk through memory behaviour step by step, which is usually where the confusion sits rather than in the code itself.
What happens when you access a union member that wasn't the last one to be written?
Answer: C
All union members share the same memory. Accessing a member not recently written gives bitwise interpretation of that memory.
Q.22Medium
When comparing nested structures, which access method is INVALID? struct Address { char city[20]; }; struct Person { Address addr; }; Person p;
Answer: C
p is a structure variable (not pointer), so arrow operator (->) cannot be used. Only dot operator works.
Q.23Medium
Which structure feature is most suitable for implementing a state machine with limited states?
Answer: A
Bit fields efficiently store boolean/limited-range states using minimal memory, ideal for state machines.
Q.24Medium
What is the primary use case for unions in embedded systems?
Answer: A
Unions enable type punning and efficient hardware register representation where different interpretations of same memory are needed.
Q.25Medium
What is the size of the following structure on a 32-bit system? struct Point { char c; int x; };
Answer: B
Due to structure padding/alignment, char (1 byte) + 3 bytes padding + int (4 bytes) = 8 bytes total.
Q.26Medium
Which feature of structures makes them suitable for implementing linked lists?
Answer: A
Self-referential pointers (a structure containing a pointer to itself) are essential for creating linked list nodes.
Q.27Medium
What happens if you attempt to assign one structure variable to another of the same type?
Answer: B
In C, you can assign one structure variable to another of the same type, and all members are copied (shallow copy).
Q.28Medium
Which of the following is a valid declaration of nested structure?
Answer: C
Both nested structure definitions (defining B inside A and declaring B as member) are valid in C.
Q.29Medium
What is the primary advantage of using unions in embedded systems?
Answer: A
Unions are primarily used in embedded systems to conserve memory by allowing multiple members to share the same memory location.
Q.30Medium
How can you create a structure that can hold either an integer or a floating-point number, but not both simultaneously?
Answer: B
A union is the appropriate choice when you need to store different data types but only one value at a time.
Q.31Medium
What is the difference between 'struct' and 'typedef struct' in C?
Answer: B
typedef struct creates a type alias, allowing you to declare variables without using the 'struct' keyword each time.
Q.32Medium
What will be the output if you read a union member that was not last written to?
Answer: A
Reading a union member that wasn't last written gives unpredictable results (garbage value or leftover bits from previous member).
Q.33Medium
In a nested structure scenario, how do you access a member of the inner structure?
Answer: C
Use dot operator (.) for direct structure variables and arrow operator (->) for pointers, or combinations for nested access.
Q.34Medium
What is the size of the following structure on a 32-bit system? struct Point { char c; int x; double y; }
Answer: D
char(1) + padding(3) + int(4) + double(8) = 16 bytes minimum, but alignment to double boundary makes it 24 bytes.
Q.35Medium
What will be the output of this code? union Data { int a; char b; }; union Data d; d.a = 257; printf("%d %c", d.a, d.b);
Answer: C
Union shares memory. 257 in int format overwrites the char. On a little-endian system, only the lower byte (1) is stored in the char portion.
Q.36Medium
What is the size of the following union? union Test { int a[5]; char b[10]; double c; }
Answer: B
Union size equals the largest member. int[5] = 20 bytes, char[10] = 10 bytes, double = 8 bytes. Maximum is 20 bytes.
Q.37Medium
Consider a structure with bit fields: struct Flags { unsigned int flag1 : 1; unsigned int flag2 : 3; unsigned int flag3 : 4; }; What is the minimum size of this structure?
Answer: C
Bit fields must fit within an underlying integer type. Total bits = 8, which requires at least 4 bytes (one int on most systems).
Q.38Medium
What will happen if you try to compare two structures using the == operator directly?
Answer: B
C does not support direct comparison of structures using ==. You must compare members individually or use memcmp().
Q.39Medium
Which of the following is a valid way to pass a structure to a function?
Answer: C
Structures can be passed by value (copying the entire structure) or by pointer (passing address) for efficiency.
Q.40Medium
In a nested structure scenario like struct A contains struct B, how do you access a member of B from A's variable?
Answer: C
If B is embedded in A: a_var.b_var.member. If B is a pointer in A: a_var->b_var->member or a_var.b_ptr->member.
