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 is the primary difference between a structure and a union in C?
Answer: A
In structures, each member gets its own memory space. In unions, all members share the same memory location, so only one member can hold a value at a time.
Q.2Easy
What will be the output of sizeof(union test) if union has members: int a, char b, float c?
Answer: A
Union size equals the size of its largest member. Here, both int and float are 4 bytes (largest), so sizeof(union test) = 4 bytes.
Q.3Easy
Which keyword is used to define a structure in C?
Answer: B
The 'struct' keyword is used to define structures in C. 'class' is used in C++ and Java.
Q.4Easy
What is the size of the following structure? struct point { int x; int y; float z; };
Answer: B
int x = 4 bytes, int y = 4 bytes, float z = 4 bytes. Total = 12 bytes (no padding in this case on most systems).
Q.5Easy
Can we pass a structure variable to a function in C?
Answer: C
Structures can be passed by value (copy of structure) or by reference (using pointers to structure).
Q.6Medium
What happens when you initialize a structure variable without assigning values?
Answer: B
Uninitialized local structure variables contain garbage values. Global structures are automatically initialized to zero.
Q.7Medium
Which of the following correctly declares a pointer to a structure?
Answer: A
Correct syntax is 'struct typename *pointerName;'. Option A follows proper declaration syntax.
Q.8Medium
What is the difference between struct and typedef struct?
Answer: B
typedef struct creates an alias, so you can declare variables directly using the alias name without repeating 'struct'.
Q.9Medium
What will be printed? struct s { int a; char b; int c; }; printf("%lu", sizeof(struct s));
Answer: B
Due to memory alignment/padding: int a(4) + char b(1) + 3 padding bytes + int c(4) = 12 bytes.
Q.10Medium
How do you access a member of a structure using a pointer?
Answer: B
The arrow operator (->) is used to access structure members through a pointer. Dot (.) is for direct variables.
Q.11Medium
What is the purpose of padding in structures?
Answer: B
Padding ensures that structure members are aligned in memory according to their natural boundaries, improving CPU access speed.
Q.12Medium
Consider: union u { int x; double y; }; What is sizeof(u)?
Answer: B
The size of a union is equal to its largest member. double is typically 8 bytes, which is larger than int (4 bytes).
Q.13Hard
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.14Hard
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.15Hard
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.16Hard
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.17Hard
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.
Q.18Easy
Which of the following is a key difference between struct and union in C?
Answer: B
In unions, all members share the same memory location, so only one member can hold a value at a time. In structs, each member has its own memory space.
Q.19Easy
What will be the output of sizeof() for the following union? union data { int a; float b; double c; }
Answer: B
The size of a union is the size of its largest member. Here, double (8 bytes) is the largest, so sizeof(union data) = 8 bytes.
Q.20Easy
In C, what is a self-referential structure?
Answer: A
A self-referential structure contains a pointer to its own type, commonly used in creating linked lists, trees, and other data structures.
