How to use make and compile as C99? - linux

Question

I'm trying to compile a linux kernel module using a Makefile:
obj-m += main.o
all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
Which gives me:
main.c:54: warning: ISO C90 forbids mixed declarations and code
I need to switch to C99. After reading I noticed I need to add a flag -std=c99, not sure where it suppose to be added.
How do I change the Makefile so it will compile as C99?

Answer 1

The correct way to add compiler flags when compiling modules is by setting the ccflags-y variable. Like this:
ccflags-y := -std=gnu99
See Documentation/kbuild/makefiles.txt in the kernel tree for more information.
Note that I'm using the gnu99 standard instead of c99 since the Linux kernel heavily relies on GNU extensions.

Answer 2

You could just add
CFLAGS=-std=c99
To the top of your makefile, or you can make the code compliant with C90 (as LukeN suggests.)

Answer 3

It's got nothing to do with the makefile. ISO C90 forbids declaring variables anywhere but in the beginning of a block or the file - like this
int main(int argc, char **argv) {
int a; /* Ok */
int b = 3; /* Ok */
printf("Hello, the magic number is %d!\n", b);
int c = 42; /* ERROR! Can only declare variables in the beginning of the block */
printf("I also like %d.. but not as much as %d!\n", c, b);
return 0;
}
Thus it has to be modified to this...
int main(int argc, char **argv) {
int a; /* Ok */
int b = 3; /* Ok */
int c = 42; /* Ok! */
printf("Hello, the magic number is %d!\n", b);
printf("I also like %d.. but not as much as %d!\n", c, b);
return 0;
}
You can only "fix" that in the source code, not in the makefile.
This rule has been relaxed in C99, but in my opinion it's a good idea to separate variable definitions, declarations and initializations from the code below it :)
So to change your makefile to make it compile with C99, you need to change the Makefile in the "build" directory that your makefile is referencing, and add the "-std=c99" at the "gcc" line compiling the source file.