Predefined macros - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a predefined macro in C?

A predefined macro is a special identifier that the C compiler automatically defines before compilation. It provides useful information like the current file name, line number, or compilation date.

beginner

What does the predefined macro __FILE__ represent?

The __FILE__ macro expands to the name of the current source file as a string literal.

beginner

What value does the predefined macro __LINE__ hold?

The __LINE__ macro expands to the current line number in the source file as an integer constant.

intermediate

What information does the predefined macro __DATE__ provide?

The __DATE__ macro expands to a string literal representing the date when the source file was compiled, in the format "Mmm dd yyyy".

intermediate

How can predefined macros help in debugging C programs?

Predefined macros like __FILE__ and __LINE__ can be used to print the current file name and line number, helping programmers quickly find where an error or message originated.

Which predefined macro gives the current line number in a C source file?

A __TIME__

B __FILE__

C __DATE__

D __LINE__

The macro __LINE__ expands to the current line number in the source file.

What does the predefined macro __FILE__ return?

A The current line number

B The name of the current source file

C The compilation date

D The compilation time

The macro __FILE__ expands to the name of the current source file as a string.

Which predefined macro provides the date when the program was compiled?

A __DATE__

B __TIME__

C __FILE__

D __LINE__

The macro __DATE__ expands to the compilation date as a string.

How can predefined macros be useful in debugging?

A They can print the current file and line number

B They can optimize code speed

C They can change program logic

D They can store variable values

Predefined macros like __FILE__ and __LINE__ help identify where in the code an error or message occurs.

Which of these is NOT a standard predefined macro in C?

A __TIME__

B __DATE__

C __COMPILER__

D __FILE__

__COMPILER__ is not a standard predefined macro in C.

Explain what predefined macros are in C and list at least three common examples with their purpose.

Think about what information the compiler automatically provides during compilation.

Describe how you can use predefined macros to help find bugs in your C program.

Consider how knowing the exact file and line number can speed up fixing problems.

Practice

1. Which of the following predefined macros in C gives the current line number in the source code?

easy

A. __LINE__

B. __FILE__

C. __DATE__

D. __TIME__

Solution

1. Step 1: Understand predefined macros purpose

   Predefined macros provide info about the code or environment automatically.

2. Step 2: Identify macro for line number

   __LINE__ gives the current line number in the source file.

3. Final Answer:

   __LINE__ -> Option A

4. Quick Check:

   Line number macro = __LINE__ [OK]

Hint: Line number macro always has LINE in its name [OK]

Common Mistakes:

• Confusing __FILE__ with line number
• Using __DATE__ or __TIME__ for line info
• Thinking __LINE__ is a function

2. Which of the following is the correct syntax to print the current file name using a predefined macro in C?

easy

A. printf("File: %s", FILE);

B. printf("File: %s", __FILE__);

C. printf("File: %s", _FILE_);

D. printf("File: %s", file);

Solution

1. Step 1: Recall predefined macro for file name

   The macro __FILE__ holds the current source file name as a string.

2. Step 2: Check correct syntax usage

   Using printf("File: %s", __FILE__); correctly prints the file name.

3. Final Answer:

   printf("File: %s", __FILE__); -> Option B

4. Quick Check:

   File name macro syntax = __FILE__ [OK]

Hint: Predefined macros always have double underscores before and after [OK]

Common Mistakes:

• Omitting underscores in __FILE__
• Using FILE without underscores
• Using lowercase file instead of macro

3. What will be the output of the following C code snippet?

#include <stdio.h>
int main() {
    printf("Date: %s\n", __DATE__);
    printf("Time: %s\n", __TIME__);
    return 0;
}

medium

A. Compilation error due to undefined macros

B. Date: __DATE__\nTime: __TIME__

C. Date: current date at compilation\nTime: current time at compilation

D. Date: 0000-00-00\nTime: 00:00:00

Solution

1. Step 1: Understand __DATE__ and __TIME__ macros

   These macros expand to strings representing the date and time when the code was compiled.

2. Step 2: Predict output of printf statements

   The program prints the compilation date and time as strings, not the macro names or errors.

3. Final Answer:

   Date: current date at compilation\nTime: current time at compilation -> Option C

4. Quick Check:

   __DATE__ and __TIME__ show compile date/time [OK]

Hint: __DATE__ and __TIME__ show compile time info as strings [OK]

Common Mistakes:

• Expecting runtime current date/time
• Thinking macros print their own names
• Assuming macros are undefined

4. Consider this C code snippet:

#include <stdio.h>
int main() {
    printf("Line: %d\n", LINE);
    return 0;
}

What is the problem with this code?

medium

A. LINE is not a predefined macro; it should be __LINE__

B. Missing semicolon after printf

C. printf format specifier %d is incorrect for LINE

D. No problem; code runs fine

Solution

1. Step 1: Identify the macro usage error

   The code uses LINE instead of the correct predefined macro __LINE__.

2. Step 2: Understand consequences

   Since LINE is undefined, this causes a compilation error.

3. Final Answer:

   LINE is not a predefined macro; it should be __LINE__ -> Option A

4. Quick Check:

   Predefined macros need double underscores [OK]

Hint: Predefined macros always have double underscores before and after [OK]

Common Mistakes:

• Using LINE without underscores
• Thinking %d is wrong for line number
• Ignoring missing macro error

5. You want to log the current file name, line number, and compilation date in your C program for debugging. Which code snippet correctly uses predefined macros to do this?

hard

A. printf("File: %s, Line: %d, Date: %s\n", _FILE_, _LINE_, _DATE_);

B. printf("File: %s, Line: %d, Date: %s\n", FILE, LINE, DATE);

C. printf("File: %s, Line: %d, Date: %s\n", "__FILE__", "__LINE__", "__DATE__");

D. printf("File: %s, Line: %d, Date: %s\n", __FILE__, __LINE__, __DATE__);

Solution

1. Step 1: Identify correct predefined macros

   The macros __FILE__, __LINE__, and __DATE__ provide file name, line number, and compile date respectively.

2. Step 2: Check correct usage in printf

   Using them directly without quotes and with correct underscores prints the desired info.

3. Final Answer:

   printf("File: %s, Line: %d, Date: %s\n", __FILE__, __LINE__, __DATE__); -> Option D

4. Quick Check:

   Use double underscore macros directly in printf [OK]

Hint: Use __FILE__, __LINE__, __DATE__ directly without quotes [OK]

Common Mistakes:

• Using macros without underscores
• Putting macros inside quotes (prints names, not values)
• Using single underscores instead of double