Makefile Error - Linker input file unused

Question

I'm trying to run a simple makefile that looks like this:

T=-ansi -pedantic -Wall -Werror
a.out: test.o extra.o
    gcc $(T) -c test.o extra.o

test.o: test.c test.h 
    gcc $(T) -c test.c

extra.o: extra.c extra.h 
    gcc $(T) -c extra.c

clean:
    rm *.o a.out

But I seem to be getting warnings telling me that "linker input file unused because linking not done"

I tried removing the "-c" from the a.out directive, after the gcc, but that produced to give me more problems. I'm not sure how to go about proceeding from here, any ideas/input would be much appreciated.

EDIT: I'm running the program by "make -T", also removing the -c from a.out, causes the error" invalid symbol index"

Sorry I'm running it like "make -T", also if I remove the -c, I get the problem "invalid symbol index" — user3739406
– user3739406, Commented Oct 15, 2015 at 0:33
See answer...but make -T is odd; what does that do? The 'invalid symbol index' suggests that you should remove all the .o files and run from the start again. — Jonathan Leffler
– Jonathan Leffler, Commented Oct 15, 2015 at 0:34
My Assigment specifies to use the makefile which is build with the T command (First line in the makefile). — user3739406
– user3739406, Commented Oct 15, 2015 at 0:35
make -T doesn't do that...whatever else it does. The T macro is fine; the usage of it is fine. Drop the -T from the command line. (I note that GNU make 3.81 does not recognize -T as a valid option. Either it is new or you are using a different variant of make.) — Jonathan Leffler
– Jonathan Leffler, Commented Oct 15, 2015 at 0:36

Jonathan Leffler · Accepted Answer · 2015-10-15 00:38:42Z

You need to remove the -c from the a.out command:

a.out: test.o extra.o
    gcc $(T) test.o extra.o

or, better:

a.out: test.o extra.o
    gcc $(T) -o $@ test.o extra.o

or, still better:

extra: test.o extra.o
    gcc $(T) -o $@ test.o extra.o

The error message is normally because you specify something like -lm on a command line with -c, but you're not doing that here. OTOH, you are listing object files with the -c option — that'll generate the warning:

gcc -ansi -pedantic -Wall -Werror -c test.o extra.o
                                   ^ this one, here

Those .o files are the linker inputs, but you're not linking. Drop the -c and you will generate a.out. That should proceed OK.

I think this is the first time I've seen a makefile used to build a.out. It is unusual in the extreme — not precisely wrong, but definitely not the way you normally use make. It has built-in rules to build programs from single source files, such as example from example.c. Normally, you give a program a meaningful name based on one of the source files. Note that creating a program test is usually a bad idea; there is a standard test command built into the typical shell and confusion is rampant.

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