make[1]: *** /lib/modules//build/: No such file or directory. Stop

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i was compiling a kernel module thermal.c at an ARCH linux distro using Makefile

Makefile:

obj-m += thermal.o

all:
    make -C /lib/modules/$(uname -r)/build/ M=$(PWD) modules

clean:
    make -C /lib/modules/$(uname -r)/build/ M=$(PWD) clean

the make command output is:

make -C /lib/modules//build/ M=/home/user/dir modules
make[1]: *** /lib/modules//build/: No such file or directory.  Stop.
make: *** [Makefile:4: all] Error 2

uname -r output : 5.6.8-arch1-1

archlinux kernel-module linux-kernel module makefile
1 Answers

make expands the recipes before passing them to the shell. In your case make replaced $(PWD) by the value of the make variable named PWD (correct) but it also replaced $(uname -r) by the value of the make variable named uname -r. As there was no such make variable defined, the result was the empty string. Using $$(uname -r) solves the problem because make expands it as $(uname -r), exactly what you want to pass to the shell.

In summary, you must escape the $ you want to preserve from the make expansion by doubling them:

all:
    make -C /lib/modules/$$(uname -r)/build/ M=$(PWD) modules

clean:
    make -C /lib/modules/$$(uname -r)/build/ M=$(PWD) clean
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