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Date: Mon, 20 Jan 2020 19:39:22 +0900
From: Olaf Meeuwissen <>
Subject: [BUG] ioctl: overflow in implicit constant conversion

Hi all,

I've been asked[1] to upstream an issue I initially submitted[2] to the
Alpinelinux project.


You can find all the details in that issue[2], but to save you the trip,
here's the gist of it.

  I get `overflow in implicit constant conversion` compiler warnings on
  some `ioctl()` calls in my code.

  I found

  ``` c
  int ioctl (int, int, ...)

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

  ``` c
  #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.

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

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

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

This happens when I compile with `-Wall -pedantic` on x86_64/amd64.
When compiling without `-pedantic` the warning goes away.

Since my initial report (against musl-1.1.16) the warning has changed to
something like

  overflow in conversion from 'long unsigned int' to 'int' changes value
  from '3221771554' to '-1073195742' [-Woverflow]

and I have not checked what happens when `-pedantic` is not given.  Nor
have I checked whether my `unsigned int` still “fixes” this.

I still see this with 1.1.24.

I think this is a bug and would like to see this fixed.

Hope this helps,
Olaf Meeuwissen, LPIC-2            FSF Associate Member since 2004-01-27
 GnuPG key: F84A2DD9/B3C0 2F47 EA19 64F4 9F13  F43E B8A4 A88A F84A 2DD9
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