-fPIC ignored for target (all code is position independent), useless warning

Question

When I compile my library I have switched on -fPIC because I want to be able to compile it as a shared and as a static library.

Using gcc 3.4.4 on cygwin I get this warning on all source files:

-fPIC ignored for target (all code is position independent)

And I really wonder what's the point of it. It tells me that I use a switch which has no effect because what the switch should achieve is already accomplished. Well, it means it's redundant, fine. But what's the point of it and how can I suppress it?

I'm not talking about why using PIC or not, just why it generates that IMO useless warning.

Answer 1

and how can I suppress it?

Not only a useless warning, but a distraction that makes it difficult to follow other warnings and errors.

Given my make output consistently showed 3 related lines, I opted to filter out the 3 "useless" lines using the following:

make 2>&1 | sed '/PIC ignored/{N;N;d;}'

I realize this is not the ideal way to suppress the noise, but maybe it will help to some degree. Be advised that I'm cutting 3 lines where other situations may only require removal of just one line. Note that I'm also routing stderr to stdout.

Here's a snipit of make output without the sed filter:

libavcodec/x86/mlpdsp.c:51:36: warning: taking address of expression of type 'void'
                                    &ff_mlp_iirorder_4 };
                                    ^
CC      libavcodec/x86/motion_est_mmx.o
libavcodec/x86/motion_est_mmx.c:1:0: warning: -fPIC ignored for target (all code is position independent)
 /* */ 
 ^
CC      libavcodec/x86/mpegaudiodec_mmx.o
libavcodec/x86/mpegaudiodec_mmx.c:1:0: warning: -fPIC ignored for target (all code is position independent)
 /* */ 
 ^
CC      libavcodec/x86/mpegvideo_mmx.o
libavcodec/x86/mpegvideo_mmx.c:1:0: warning: -fPIC ignored for target (all code is position independent)
 /* */ 
 ^

And the same with the sed filter:

                                                    ^
libavcodec/x86/mlpdsp.c:51:36: warning: taking address of expression of type 'void'
                                    &ff_mlp_iirorder_4 };
                                    ^
CC      libavcodec/x86/motion_est_mmx.o
CC      libavcodec/x86/mpegaudiodec_mmx.o
CC      libavcodec/x86/mpegvideo_mmx.o
CC      libavcodec/x86/proresdsp-init.o

Answer 2

Personally, I'd just add os detection to the makefile. Something along the lines of

TARGET_TRIPLE := $(subst -, ,$(shell $(CC) -dumpmachine))
TARGET_ARCH   := $(word 1,$(TARGET_TRIPLE))
TARGET_OS     := $(word 3,$(TARGET_TRIPLE))

ifeq      ($(TARGET_OS),mingw32)
else ifeq ($(TARGET_OS),cygwin)
else
CFLAGS += -fPIC
endif