c++ "#include" output explanation

Question

Trying to understand how #include works. I'm reading that, during pre-procesing, it just replaces itself with the contents of the referenced file.

To verify, I create two files. A file named otherfile containing only the string 1234, and a file test.cpp which contains

#include otherfile
abcd

I run g++ -E test.cpp, and the output I get is

# 1 "test.cpp"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 373 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "test.cpp" 2
# 1 "./wtf" 1
1234
# 2 "test.cpp" 2
abcd

Where do the rest of the lines come from, and what do they mean?

Answer 1

Where do the rest of the lines come from

They are added by the pre-processor.

and what do they mean?

As per the documentation

Source file name and line number information is conveyed by lines of the form

# linenum filename flags

These are called linemarkers. They are inserted as needed into the output (but never within a string or character constant). They mean that the following line originated in file filename at line linenum. filename will never contain any non-printing characters; they are replaced with octal escape sequences.