svn commit: r295768 - head/usr.sbin/iostat

[Alan Somers]

On Fri, Feb 19, 2016 at 5:24 AM, Sergey Kandaurov <pluknet at gmail.com> wrote:
> On 18 February 2016 at 23:08, Alan Somers <asomers at freebsd.org> wrote:
>> Author: asomers
>> Date: Thu Feb 18 20:08:01 2016
>> New Revision: 295768
>> URL: https://svnweb.freebsd.org/changeset/base/295768
>>
>> Log:
>>   Fix compiler warnings in iostat
>
>> Modified: head/usr.sbin/iostat/iostat.c
>> ==============================================================================
>> --- head/usr.sbin/iostat/iostat.c       Thu Feb 18 19:37:39 2016        (r295767)
>> +++ head/usr.sbin/iostat/iostat.c       Thu Feb 18 20:08:01 2016        (r295768)
>> @@ -117,30 +117,34 @@
>>  #include <termios.h>
>>  #include <unistd.h>
>>
>> -struct nlist namelist[] = {
>> +static struct nlist namelist[] = {
>>  #define X_TTY_NIN      0
>> -       { "_tty_nin" },
>> +       { .n_name = "_tty_nin",
>> +         .n_type = 0, .n_other = 0, .n_desc = 0, .n_value = 0 },
>>  [...]
>
> You unlikely need this excessive explicit zeroization.
> In this case it is implicitly prezeroed.
>
> Consider these two cases:
>
> : #include <nlist.h>
> :
> : int main(void) {
> :   struct nlist namelist[2] = {{ .n_type = 0x42 }};
> :   return sizeof(namelist);
> : }
>
> (__TEXT,__text) section
> _main:
> 0000000000000000    pushq    %rbp
> 0000000000000001    movq    %rsp, %rbp
> 0000000000000004    leaq    -0x30(%rbp), %rdx
> 0000000000000008    movl    $0x0, %eax
> 000000000000000d    movl    $0x6, %ecx
> 0000000000000012    movq    %rdx, %rdi
> 0000000000000015    rep
> 0000000000000016    stosq
> 0000000000000018    movb    $0x42, -0x28(%rbp)
> 000000000000001c    movl    $0x30, %eax
> 0000000000000021    popq    %rbp
> 0000000000000022    retq
>
> rep stosq does zero 48 bytes, that is namelist[].
>
> Or, if it is static.
>
> : #include <nlist.h>
> :
> : int main(void) {
> :   static struct nlist namelist[2] = {{ .n_type = 0x42 }};
> :   return sizeof(namelist);
> : }
>
> (__DATA,__data) section
> 0000000000000020    00 00 00 00 00 00 00 00 42 00 00 00 00 00 00 00
> 0000000000000030    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> 0000000000000040    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

Yeah, it was being implicitly zeroized before.  But Clang complained
about the structures being only partially initialized.  Since the
whole point of my commit was to increase the WARNS level, I explicitly
zeroed the zero fields to silence Clang.

-Alan