ioctl: overflow in implicit constant conversion

I’m compiling with -Wall on 3.6 and get overflow in implicit constant conversion compiler warnings on some ioctl() calls in my code.

Upon investigation, I found

int ioctl (int, int, ...)

in /usr/include/sys/ioctl.h and the following in /usr/include/bits/ioctl.h (included by the former)

#define _IOC(a,b,c,d) ( ((a)<<30) | ((b)<<8) | (c) | ((d)<<16) )
#define _IOC_NONE  0U
#define _IOC_WRITE 1U
#define _IOC_READ  2U

#define _IO(a,b) _IOC(_IOC_NONE,(a),(b),0)
#define _IOW(a,b,c) _IOC(_IOC_WRITE,(a),(b),sizeof(c))
#define _IOR(a,b,c) _IOC(_IOC_READ,(a),(b),sizeof(c))
#define _IOWR(a,b,c) _IOC(_IOC_READ|_IOC_WRITE,(a),(b),sizeof(c))

If I am not mistaken and assuming a 32-bit int, that means that any second argument passed to ioctl() that is created using the _IOR or _IOWR macros will be an unsigned 32-bit integer with its most significant bit set and will trigger the warning I observe.

I subscribe to AWARE (All Warnings Are Really Errors) and normally also set -Werror so I really would like to get this fixed.

BTW, most other distributions I compile on define ioctl() as

int ioctl (int, unsigned long, ...)

and chaging your declaration to take an unsigned int “fixes” this for me.

(from redmine: issue id 7580, created on 2017-07-21)