How to use make and compile as C99?

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.