Unable to link math lib using a makefile [duplicate] - c

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Linker errors when compiling against glib…?
Okay, I know this may be a duplicate, but I can't find any other answer to my problem.
I am trying to install Pintos and when I run 'make' in the src/utils directory, I get the error that there is an undefined reference to 'floor'. I checked the makefile and here's what I got:
all: setitimer-helper squish-pty squish-unix
# 2207718881418
CC = gcc
CFLAGS = -Wall -W
LDFLAGS = -lm
setitimer-helper: setitimer-helper.o
squish-pty: squish-pty.o
squish-unix: squish-unix.o
clean:
rm -f *.o setitimer-helper squish-pty squish-unix
I tried adding LIBS = -lm but that didn't help.
Output of make:
gcc -lm setitimer-helper.o -o setitimer-helper
setitimer-helper.o: In function `main':
setitimer-helper.c:(.text+0xbb): undefined reference to `floor'
collect2: ld returned 1 exit status
make: *** [setitimer-helper] Error 1
Any solutions to this dilemma?

Answer 1

Your original makefile defines a bunch of variables
CC = gcc
# etc
and lists some dependencies
setitimer-helper: setitimer-helper.o
# etc
but doesn't have any recipes giving the exact commands to be used to remake the targets, except for the clean rule. This means that built-in implicit rules will be used; e.g., to link setitimer-helper the following built-in rule will be used:
$(CC) $(LDFLAGS) $^ $(LDLIBS) -o $#
For setitemer-helper, the automatic variables are filled in using the relevant dependency:
$(CC) $(LDFLAGS) setitimer-helper.o $(LDLIBS) -o setitimer-helper
and from this you can see how the remaining variables -- $(CC), $(LDFLAGS), and $(LDLIBS) -- were filled in to give the output of make that you saw.
As various people have noted, you need to make sure that -lm goes at the end of the link command so that it can be used to satisfy references to library functions like floor(). At the moment, your makefile sets $(LDFLAGS) to -lm, but that variable is used at the start of the link command.
The conventional variables are set up in this built-in rule so that LDFLAGS can be used for options (a.k.a. "flags") that (historically) need to be at the start of the link command, and LDLIBS can be used for libraries that need to be specified after the *.o object files.
So to fix this in terms of the makefile you are using, you need to remove -lm from the LDFLAGS variable that is defined, and instead add another variable definition for LDLIBS:
LDLIBS = -lm
(I'm oversummarising slightly: the built-in rule also contains $(TARGET_ARCH) and $(LOADLIBES), but those aren't of interest here.)

Answer 2

It's compiled in wrong order, the way to proceed is:
CC = gcc
CFLAGS = -Wall -W
LDFLAGS = -lm
myprog: myprog.o more_code.o
${CC} ${CFLAGS} myprog.o more_code.o ${LDFLAGS} -o myprog
myprog.o: myprog.c
${CC} ${CFLAGS} -c myprog.c
more_code.o: more_code.c
${CC} ${CFLAGS} -c more_code.c
clean:
\rm myprog.o more_code.o myprog
More info: http://www.physics.utah.edu/~p5720/rsrc/make.html
Can you show me in terms of the original makefile?
I can try :)
CC = gcc
CFLAGS = -Wall -W
LDFLAGS = -lm
OBJECTS = setitimer-helper.o squish-pty.o squish-unix.o
all: setitimer-helper
setitimer-helper: $(OBJECTS)
${CC} ${CFLAGS} $(OBJECTS) ${LDFLAGS} -o setitimer-helper
setitimer-helper.o: setitimer-helper.c
${CC} ${CFLAGS} -c setitimer-helper.c
squish-pty.o: squish-pty.c
${CC} ${CFLAGS} -c squish-pty.c
squish-unix.o: squish-unix.c
${CC} ${CFLAGS} -c squish-unix.c
And since you are new to Makefile, it's a good idea to add -Wextra -pedantic to CFLAGS