In a Makefile of a library I am trying to build, there are a few lines specify the options to gcc:
CFLAGS += -I$(CURDIR) -pedantic -std=c89 -O3
CFLAGS += -Wall -Wno-unused-function -Wno-long-long
CFLAGS += $(if $(DEBUG), -O0 -g)
If DEBUG exists, there will be both -O3 and -O0 -g in CFLAGS. But -O0 and -O3 cannot be used at the same time. Will the one specified later supersede the one earlier?
Thanks and regards!
From the manpage:
If you use multiple -O options, with or without level numbers, the
last such option is the one that is effective.
Related
The CFLAGS part that I don't understand is
CFLAGS = -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP
Can not get answer from gcc --help.
"Options starting with -g, -f, -m, -O, -W, or --param are automatically
passed on to the various sub-processes invoked by gcc. In order to pass
other options on to these processes the -W options must be used."
Does -Wextra also enable -Wall, and does -Wall also enable -W in GCC?
-Wextra is a replacement to the old -W flag. It enables some extra warnings which are not enabled by -Wall, and does not enable -Wall by itself - if you want both, you'll have to use -Wall -Wextra.
I have the following in my GNU makefile:
# CXXFLAGS ?= -DNDEBUG -g2 -O3
CXXFLAGS ?=
# Add -DNDEBUG if nothing specified
ifeq ($(filter -DDEBUG -DNDEBUG,$(CXXFLAGS)),)
$(info Adding -DNDEBUG to CXXFLAGS)
CXXFLAGS += -DNDEBUG
endif
# Add a symolize if nothing specified
ifeq ($(filter -g -g1 -g2 -g3 -Oz,$(CXXFLAGS)),)
$(info Adding -g2 to CXXFLAGS)
CXXFLAGS += -g2
endif
# Add an optimize if nothing specified
$(info Adding -O3 to CXXFLAGS)
ifeq ($(filter -O -O0 -O1 -O2 -O3 -Og -Os -Oz -Ofast,$(CXXFLAGS)),)
CXXFLAGS += -O3
endif
When I run it:
$ make CXXFLAGS="-g3"
Adding -DNDEBUG to CXXFLAGS
Adding -O3 to CXXFLAGS
g++ -g3 -c foo.cpp
...
In fact, if I uncomment the CXXFLAGS ?= -DNDEBUG ..., then I can append again. But that's not very helpful since I'm trying to make arguments optional (but with sane defaults).
And if I type just make, then it works (-fPIC -march=native -Wall -Wextra -pipe is added later by the same makefile, and it has always worked):
$ make
Adding -DNDEBUG to CXXFLAGS
Adding -g2 to CXXFLAGS
Adding -O3 to CXXFLAGS
g++ -DNDEBUG -g2 -O3 -fPIC -march=native -Wall -Wextra -pipe -c serpent.cpp
...
According to the manual and 6.6 Appending More Text to Variables:
Often it is useful to add more text to the value of a variable already defined. You do this with a line containing ‘+=’, like this:
objects += another.o
Why is make not adding the values to the variable? How can I achieve the behavior I want?
By passing a variable via command-line, you're telling make that you are overriding any definitions in the file, which allows a user to compile as they intend rather than as you intend. Ignoring the restriction of user freedom, you can use the override directive:
To append more text to a variable defined on the command line, use:
override variable += more text
Variable assignments marked with the override flag have a higher priority
than all other assignments, except another override. Subsequent
assignments or appends to this variable which are not marked override
will be ignored.
I would discourage you from using override when possible because it's annoying to realize that -O0 was needed to disable the optimizations that you enabled when I don't want them enabled (after all, I specify my own flags for a reason). Of course, if no flags were specified at all, then defaults are perfectly reasonable. In fact, Automake projects seem to default to -g -O2 when no compilation flags are specified.
There are exceptions to this advice of course, such as adding a directory to search for includes/libs or preprocessor definitions for compiling a conditional section of code on a certain platform.
I am trying to run program from the Learn C Hard Way book
I need to pass the library filename 'build/liblcthw.a' as the last parameter.
For eg :
Doesnt Work on Ubuntu :
gcc -g -O2 -Wall -Wextra -Isrc -rdynamic -DNDEBUG build/liblcthw.a tests/list_tests.c -o tests/list_tests
Works on Ubuntu :
gcc -g -O2 -Wall -Wextra -Isrc -rdynamic -DNDEBUG tests/list_tests.c -o tests/list_tests build/liblcthw.a
How do I handle this in Makefile ? CFLAGS will only add it before the source filename and it doesnt work. How do I force CFALGS to add the library filename at the end of the command ?
CFLAGS are flags for the C compiler. Libraries typically go into a variable called LDLIBS. Set LDLIBS=build/liblcthw.a and see if that works.
The first invocation doesn't succeed because the order of sources and libraries in the command line is wrong. The correct order is source files, then object files, followed by static libraries followed by dynamic libraries.
I am compiling some C/C++ files using gcc.
I noticed today a bug that caused my app to crash. It was caused by the fact that my function didn't return any value (see below). Do you know if there is some flag in gcc enforcing these kind of checking or why the compiler is not warning me about this?
I am compiling C files into object files with a basic -g -D_GNU_SOURCE -o outObjectFile -c myFile.c option.
//.c file
int
myFunc(){
...do something
..without return statement
}
//.h file
extern int myFun();
When using GCC, always compile with:
-std=c99 -pedantic -Wall -Wextra -Wwrite-strings for C
-ansi -pedantic -Wall -Wextra -Weffc++ for C++