Linking multiple files in C - Step-by-Step Execution

Concept Flow - Linking multiple files

Write code in file1.c

↓

Write code in file2.c

↓

Compile file1.c and file2.c separately

↓

Link object files together

↓

Create executable

↓

Run executable

Code is written in separate files, compiled into object files, then linked together to form one executable program.

Execution Sample

C

#include <stdio.h>

// file1.c
void greet();

int main() {
    greet();
    return 0;
}

// file2.c
#include <stdio.h>
void greet() {
    printf("Hello from greet!\n");
}

Two files: one with main calling greet(), the other defining greet(). They are compiled and linked to run together.

Execution Table

Step	Action	File	Result/Output
1	Write main() calling greet()	file1.c	main() calls greet()
2	Write greet() function	file2.c	greet() prints message
3	Compile file1.c	file1.c	Creates file1.o (object file)
4	Compile file2.c	file2.c	Creates file2.o (object file)
5	Link file1.o and file2.o	linker	Creates executable program
6	Run executable	executable	Output: Hello from greet!
7	Program ends	executable	Return code 0

💡 Program ends after printing message and returning 0

Variable Tracker

Variable/Function	Start	After Step 1	After Step 2	After Step 6	Final
main()	Not defined	Defined in file1.c	Defined in file1.c	Called and runs	Ends
greet()	Not defined	Declared in file1.c	Defined in file2.c	Called by main(), prints message	Ends
Output	None	None	None	"Hello from greet!\n"	Printed

Key Moments - 3 Insights

Why does main() know about greet() even though greet() is in another file?

Because main() has a declaration of greet() (a promise) and the linker connects main() with greet() from the other file as shown in steps 3-5 in the execution_table.

What happens if we forget to link the object files together?

The linker will give an error saying it cannot find greet(), because the function is defined in another file and linking is needed to combine them (see step 5).

Why do we compile files separately before linking?

Compiling separately creates object files for each source file, which the linker then combines. This allows modular code and faster builds (steps 3 and 4).

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the executable created?

A Step 5

B Step 3

C Step 6

D Step 4

💡 Hint

Check the 'Action' column for linking object files into executable (step 5).

According to variable_tracker, what is the state of greet() after step 2?

A Not declared

B Declared in file1.c only

C Defined in file2.c

D Called and runs

💡 Hint

Look at the 'greet()' row under 'After Step 2' in variable_tracker.

If we skip compiling file2.c, what will happen during linking?

A Linker will succeed with no errors

B Linker will fail due to missing greet() definition

C Program will run but greet() prints nothing

D Compilation will fail

💡 Hint

Refer to key_moments about linking errors when definitions are missing.

Concept Snapshot

Linking multiple files in C:
- Write code in separate .c files
- Declare functions in one file, define in another
- Compile each file to object files (.o)
- Link object files to create executable
- Run executable to see combined program output

Full Transcript

Linking multiple files means writing code in separate C files and combining them to run as one program. First, you write main() in one file and a function like greet() in another. You compile each file separately into object files. Then, the linker combines these object files into one executable. When you run the executable, main() calls greet(), and the program prints the message. This process allows you to organize code better and reuse functions across files. If you forget to link, the program won't find functions defined elsewhere and will give errors.