Predefined macros - Mini Project: Build & Apply

Using Predefined Macros in C

📖 Scenario: You are writing a simple C program that needs to display information about when and where it was compiled. This is useful for debugging and tracking versions.

🎯 Goal: Build a C program that uses predefined macros to show the current date, time, and file name of the source code.

📋 What You'll Learn

Use the predefined macro __DATE__ to get the compilation date

Use the predefined macro __TIME__ to get the compilation time

Use the predefined macro __FILE__ to get the source file name

Print these values clearly labeled

💡 Why This Matters

🌍 Real World

Developers often use predefined macros to embed build information in programs for debugging and version tracking.

💼 Career

Knowing how to use predefined macros is useful for software developers working on C projects, especially in embedded systems and system programming.

1

Create a C program skeleton

Write the initial C program with #include <stdio.h> and an empty main function that returns 0.

C

#include 

int main() {
    // Your code here
    return 0;
}

Hint

Start by including the standard input-output header and create the main function.

2

Add variables for predefined macros

Inside main, create three const char* variables named compile_date, compile_time, and source_file. Assign them the values __DATE__, __TIME__, and __FILE__ respectively.

C

#include 

int main() {
    // Your code here
    return 0;
}

Hint

Use the predefined macros exactly as shown to assign the variables.

3

Print the predefined macro values

Use printf to display the labels and values of compile_date, compile_time, and source_file on separate lines.

C

#include 

int main() {
    const char* compile_date = __DATE__;
    const char* compile_time = __TIME__;
    const char* source_file = __FILE__;
    // Your code here
    return 0;
}

Hint

Use printf with %s to print each string variable with a label.

4

Run and display the output

Run the program and print the output showing the compilation date, time, and source file name.

C

#include 

int main() {
    const char* compile_date = __DATE__;
    const char* compile_time = __TIME__;
    const char* source_file = __FILE__;
    printf("Compilation Date: %s\n", compile_date);
    printf("Compilation Time: %s\n", compile_time);
    printf("Source File: %s\n", source_file);
    // Your code here
    return 0;
}

Hint

Run the program to see the compilation details printed.

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