Question

Based on gcc: Do I need -D_REENTRANT with pthreads?

> echo | g++          -E -dM -c - > singlethreaded
> echo | g++ -pthread -E -dM -c - > multithreaded
> diff singlethreaded multithreaded
39a40
> #define _REENTRANT 1

The author uses the above command to export the options used by the compiler.
I want to know why such a command works in detail:

> echo | g++ -E -dM -c - > singlethreaded

I understand the following parts:

> singlethread # means to redirect the results to the file singlethread

Answer 1

The -dM option:

Instead of the normal output, generate
a list of #define directives for all
the macros defined during the
execution of the preprocessor,
including predefined macros. This
gives you a way of finding out what is
predefined in your version of the
preprocessor.

The -E option:

Stop after the preprocessing stage; do
not run the compiler proper. The
output is in the form of preprocessed
source code, which is sent to the
standard output.

The -c option causes the compiler to compile but not link. This is superfluous with the -E option.

The solitary - tells the compiler to read its input from stdin.

Thus, the command echoes an empty stream to stdout, pipes that empty stream to g++, which then reads the empty stream but runs only the preprocessor, which has been told to ignore the input (empty) and output the complete list of defined preprocessor macros.