What does the g++ -D flag do?
Solution 1
It is equivalent to adding the statement #define LINUX 1
in the source code of the file that is being compiled. It does not have any effect on other compilation flags. The reason for this is it's an easy way to enable #ifdef
statements in the code. So you can have code that says:
#ifdef LINUX
foo;
#endif
It will only be enabled if that macro is enabled which you can control with the -D
flag. So it is an easy way to enable/disable conditional compilation statements at compile time without editing the source file.
Solution 2
It doesn't have anything to do with -O3
. Basically, it means the same as
#define LINUX 1
at the beginning of the compiled file.
Related videos on Youtube
Sterling
Updated on July 18, 2022Comments
-
Sterling almost 2 years
I am looking at a CFLAGS of -
CFLAGS=-g -w -D LINUX -O3 -fpermissive
in a Makefile. What does the -D flag do? I see on the man page that
-D name Predefine name as a macro, with definition 1.
but I don't know how to interpret that. My interpretation is...its making LINUX a macro and only doing -03 and -fpermissive when in a linux environment. Is that right? If not, then what? Thanks for any help
-
Basile Starynkevitch about 12 yearsOften, the
-D
is glued to the following (defined) name, e.g.-DLINUX
or-DFOO=BAR
-
-
jpm about 12 yearsI would +1, but the interpretation is only correct up to a point. The
-D
flag doesn't affect the following flags (e.g. making them conditional or some such crazy talk), and that's a fairly important distinction. -
Gabriel Southern about 12 years@jpm that's a good point I had not interpreted the question that way when I first read it, but now I see you are correct. I edited my answer to clarify that point.
-
jpm about 12 years
-O3
sets the compiler's optimization level to 3.