sizeof(struct timeval)

[Tony Houghton]

In <DcmdnRkLxJg4gYvZRVnyrw at pipex.net>,
Big and Blue <No_4 at dsl.pipex.com> wrote:

> Tony Houghton wrote:
>> 
>> How safe would I be assuming that 
>> 
>>     sizeof(struct timeval) == 2 * sizeof(long)
>> 
>> is always true on Linux on different architectures? 
>
>     Based on what I was looking at today (well, yesterday now), you might 
> be wrong.
>
>     I do know that the size of a struct utmp differs between a Linux 
> Itanium system and a Linux x86_64 system, and I seem to recall it migh be 
> related to timeval.  I could be wrong - I wasn't interested in the timeval 
> part - but this part of the bits/utmp.h include file indicates the issue:

So is __WORDSIZE_COMPAT32 defined for one of Itanium or x86_64 but not
the other? The comment in the code below implies the opposite of what
you're saying: the size in 64-bit mode has to be the same as in 32-bit
mode, and as both CPUs have the same 32-bit architecture they should
therefore both use the same sized struct in 64-bit mode.

In any case, it does imply that timeval can be relied on to be 2 *
32-bits (2 * long) in 32-bit architectures and something else in 64-bit
architectures - where long is 64-bit.

> /* The ut_session and ut_tv fields must be the same size when compiled
>     32- and 64-bit.  This allows data files and shared memory to be
>     shared between 32- and 64-bit applications.  */
> #if __WORDSIZE == 64 && defined __WORDSIZE_COMPAT32
>    int32_t ut_session;           /* Session ID, used for windowing.  */
>    struct
>    {
>      int32_t tv_sec;             /* Seconds.  */
>      int32_t tv_usec;            /* Microseconds.  */
>   } ut_tv;                      /* Time entry was made.  */
> #else
>    long int ut_session;          /* Session ID, used for windowing.  */
>    struct timeval ut_tv;         /* Time entry was made.  */
> #endif

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