Predefined macros - Step-by-Step Execution

Concept Flow - Predefined macros

Start Compilation

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Compiler Inserts Predefined Macros

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Code Uses Macros like __FILE__, __LINE__

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Macros Expand to Values

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Compilation Continues with Expanded Code

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Program Runs with Macro Values Embedded

During compilation, the compiler automatically inserts predefined macros with specific values, which the code can use and expand before running.

Execution Sample

C

#include <stdio.h>

int main() {
    printf("File: %s\n", __FILE__);
    printf("Line: %d\n", __LINE__);
    return 0;
}

This code prints the current file name and the line number where the macros are used.

Execution Table

Step	Code Line	Macro Used	Macro Expansion	Output Produced
1	printf("File: %s\n", __FILE__);	__FILE__	"example.c"	File: example.c
2	printf("Line: %d\n", __LINE__);	__LINE__	6	Line: 6
3	return 0;	None	None	Program ends

💡 All macros expanded by compiler, program outputs file name and line number, then ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
__FILE__	Not set	"example.c"	"example.c"	"example.c"
__LINE__	Not set	Not set	6	6

Key Moments - 2 Insights

Why does __LINE__ print 5 instead of the line where the program runs?

Because __LINE__ expands to the line number in the source code where it appears, not the runtime line. See execution_table row 2.

Is __FILE__ a string or a number?

__FILE__ expands to a string literal with the current file name, as shown in execution_table row 1.

Visual Quiz - 3 Questions

Look at the execution_table, what value does __FILE__ expand to at step 1?

A 7

B "main.c"

C "example.c"

D Undefined

💡 Hint

Check the 'Macro Expansion' column in execution_table row 1.

At which step does __LINE__ get expanded?

A Step 1

B Step 2

C Step 3

D Never

💡 Hint

Look at the 'Macro Used' column in execution_table to find when __LINE__ is used.

If you add a new line before printf(__LINE__), what happens to the output?

A The printed line number increases by 1

B The printed line number stays the same

C The program crashes

D The file name changes

💡 Hint

Refer to variable_tracker for __LINE__ changes when code lines change.

Concept Snapshot

Predefined macros are special names the compiler replaces with info during compilation.
Common macros:
- __FILE__: current file name as string
- __LINE__: current line number as int
They help track code location without manual input.
Used inside code, expanded before running.

Full Transcript

Predefined macros in C are special names that the compiler automatically replaces with useful information during compilation. For example, __FILE__ becomes the current file name as a string, and __LINE__ becomes the line number in the source code where it appears. In the example code, when printf uses __FILE__, it prints the file name, and when it uses __LINE__, it prints the line number. These macros are expanded before the program runs, so the output shows the actual file name and line number. If you add or remove lines before the macro, the line number changes accordingly. This helps programmers know exactly where in the code something happens without typing it manually.