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.
Consider the following code: #define PI 3.14 #undef PI #define PI 3.14159 What is the value of PI after execution?
Answer: B
#undef removes the previous definition of PI. The subsequent #define redefines PI as 3.14159. This is valid C syntax and the final value of PI is 3.14159.
Q.42Medium
What happens when you use stringification operator (#) in a macro? #define STRINGIFY(x) #x
Answer: B
The # operator (stringification) converts its argument into a string literal. For example, STRINGIFY(hello) becomes "hello". This is useful for creating string representations of identifiers.
Q.43Medium
What is the token pasting operator (##) used for in C preprocessor?
Answer: B
The ## operator (token pasting) concatenates two tokens into a single token during preprocessing. For example, #define CONCAT(a,b) a##b creates CONCAT(var,1) as var1.
Q.44Medium
Which of the following macro definitions will correctly compute the absolute value? #define ABS(x) ((x)<0?-(x):(x))
Answer: A
The macro correctly uses the ternary operator with proper parenthesization. Each use of x is wrapped in parentheses to prevent precedence issues. This is a well-formed macro for computing absolute value.
Q.45Medium
What will be the result of this code? #define MAX(a,b) (a>b?a:b) int x = MAX(MAX(2,5), MAX(3,4));
Answer: A
Inner MAX calls: MAX(2,5)=5 and MAX(3,4)=4. Outer MAX call: MAX(5,4)=5. The macro correctly handles nested calls due to proper parenthesization.
Q.46Hard
How many times is the argument evaluated in this macro? #define CUBE(x) ((x)*(x)*(x)) int result = CUBE(a++);
Answer: C
The macro expands to ((a++)*(a++)*(a++)), so the argument a++ is evaluated 3 times. This demonstrates a critical problem with macros - side effects are repeated multiple times, leading to unexpected behavior.
Q.47Easy
Which header file must be included to use the NULL macro?
Answer: D
NULL is defined in multiple standard headers including <stdio.h>, <stdlib.h>, <stddef.h>, <string.h>, and others. Any of these can be included to use NULL.
Q.48Hard
What is a dangling macro problem in C?
Answer: C
A dangling macro typically refers to a macro that is defined but its usage may cause issues. More accurately, it's when a macro's scope or definition is unclear, especially in conditional compilation scenarios where #undef might cause problems.
Q.49Medium
What will happen when this code is compiled? #define SIZE 10 int arr[SIZE]; #undef SIZE int arr2[SIZE];
Answer: A
After #undef SIZE, the macro SIZE is no longer defined. Attempting to use SIZE in int arr2[SIZE] will cause a compilation error because SIZE is not recognized by the compiler.
Q.50Easy
Which of the following correctly demonstrates the use of conditional compilation?
Answer: A
Option A uses #ifdef to check if DEBUG is defined, which is correct for conditional compilation. Option B would check if DEBUG has a non-zero value (expression-based). Options C and D have invalid syntax.
Q.51Medium
What is the purpose of predefined macros like __LINE__ and __FILE__?
Answer: B
__LINE__ expands to the current line number and __FILE__ expands to the current filename. These are predefined macros that provide compile-time information useful for debugging and error reporting.
Q.52Hard
Consider the following macro: #define SWAP(a,b) {int temp=a; a=b; b=temp;} What issue might occur with this macro?
Answer: D
While the macro works for all types (a and b are copied), the curly braces can cause issues when used in if-else statements without proper syntax (like missing semicolon after if). Additionally, it's limited to specific types. The safest answer is D regarding syntactic issues with placement.
Q.53Easy
What is the output of the following code? #define PI 3.14 int main() { printf("%f", PI); return 0; }
Answer: C
PI is replaced by 3.14 during preprocessing. When printed with %f format specifier, it displays as 3.140000 (default 6 decimal places).
Q.54Medium
What is the output of: #define SQUARE(x) x*x int main() { int a = SQUARE(2+3); printf("%d", a); return 0; }
Answer: B
SQUARE(2+3) expands to 2+3*2+3 = 2+6+3 = 11 due to operator precedence. Proper macro should use parentheses: #define SQUARE(x) ((x)*(x))
Q.55Medium
Which of the following correctly defines a macro with multiple statements?
Answer: C
Option C using do-while(0) is the safest approach to avoid issues with semicolons in if-else statements. Option B can cause problems with control flow statements.
Q.56Easy
What does the ## operator in preprocessor do?
Answer: B
The ## operator concatenates two tokens into one. For example, #define CONCAT(a,b) a##b creates CONCAT(hello, world) → helloworld.
Q.57Medium
What is the output of: #define STR(x) #x int main() { printf("%s", STR(Hello)); return 0; }
Answer: A
The # operator converts the macro argument into a string literal. STR(Hello) becomes "Hello", and printf outputs Hello without quotes.
Q.58Easy
Which predefined macro gives the line number in the source file?
Answer: B
__LINE__ expands to the current line number. __FILE__ gives filename, __DATE__ gives compilation date, __STDC__ indicates C standard compliance.
Q.59Medium
What is the issue with this macro: #define MAX(a,b) a>b?a:b int x = MAX(2, 3); int y = MAX(++i, ++j);
Answer: B
In MAX(++i, ++j), both i and j are incremented twice due to double evaluation. Parentheses help but don't solve side effects completely.
Q.60Medium
What is the purpose of #pragma pack() directive?
Answer: B
#pragma pack() controls how compiler aligns structure members in memory. #pragma pack(1) minimizes padding, while #pragma pack() without arguments resets to default.
