The ## operator concatenates two tokens into one. For example, #define CONCAT(a,b) a##b creates CONCAT(hello, world) → helloworld.
Q.282Easy
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.283Easy
Which header file contains the definition of NULL macro?
Answer: C
NULL is defined in multiple headers including <stdio.h>, <stdlib.h>, <stddef.h>, and others. Any of these can be included to use NULL.
Q.284Easy
What does #undef directive do?
Answer: A
#undef cancels the definition of a macro, allowing it to be redefined later. After #undef, the macro name becomes undefined.
Q.285Easy
In C99, which of the following is a valid use of token pasting operator (##)?
Answer: D
Both options A and B are valid. The ## operator pastes two tokens together. Whitespace around ## doesn't matter. #define CONCAT(a,b) a##b could be used as CONCAT(var, 1) to create var1.
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Q.286Easy
Which header file is required to use assert() macro in C?
Answer: A
assert.h contains the assert() macro which is useful for debugging. It tests a condition and aborts the program if the condition is false. This macro is only active when NDEBUG is not defined.
Q.287Easy
What is the difference between #include <file.h> and #include "file.h"?
Answer: B
#include <file.h> searches in standard system include directories. #include "file.h" searches in the current working directory first, then system directories. This allows you to override system headers with local versions.
Q.288Easy
What happens when #if 0...#endif wraps a block of code?
Answer: B
#if 0...#endif is used to disable code during preprocessing. The enclosed code is skipped entirely and doesn't appear in the compiled output. This is more efficient than C-style comments for large blocks and maintains nested comment compatibility.
Q.289Easy
What is the purpose of defined() operator in preprocessor conditionals?
Answer: A
defined(MACRO) is used in #if and #elif directives to test if a macro is defined. Example: #if defined(DEBUG) ... #endif. It's equivalent to #ifdef but can be used in expressions with logical operators like && and ||.
Q.290Easy
What is the primary purpose of the C preprocessor?
Answer: A
The preprocessor is a text processing tool that processes source code before actual compilation, handling directives like #define, #include, and macros.
Q.291Easy
Which directive is used to conditionally compile code based on a logical condition?
Answer: B
#if evaluates constant expressions for conditional compilation. #ifdef checks if a macro is defined, #define creates macros, and #pragma provides compiler-specific directives.
