Translation Unit in C

A Translation Unit (TU) is a source file that is given to the compiler to compile into an object file.
Each TU can include any number of files using the #include directive. The included file can be any file with any extension (e.g., *.c, *.h, *.cpp) as long as it contains C source code that the compiler can understand.
In the end, all object files generated for each TU will be linked by the linker to produce an executable.

Example

Snippet 1:

File: print.exe

#include <stdio.h>

void print_exe() {
    printf("print-exe\n");
}

File: main.c

#include "print.exe"
#include <stdio.h>

int main() {
    printf("print-main\n");
    print_exe();
    return 0;
}

Compile:

gcc main.c -o main.exe
./main.exe

Snippet 1 stdout:

print-main
print-exe

In this example, we have one TU i.e, main.c which includes other source file print.exe, even though the extension is *.exe it still works as expected. Output prints both the print statements from main.c and print.exe file.