The macros do not make the field names part of the structure. They are separate
aliases for various relative locations reserved by the structure declaration.
In normal practice the distinction is inconsequential, but as Philip G. points
out, in the absence of explicit allowance for storage overlap 'no named field
shall be part of a union' is what I also see as the basic intent of all
structure declarations in the standard. What individual platforms may elect to
put in a union is fields they add to the minimum required by the standard for
internal or extension's use. This is due to the scoping of name reservations.
Macros as translation unit level identifiers have more restrictions to ensure
uniqueness than the stricter scope of a structure typedef. Once typedef'd they
can't be #undef'd either.
What would be conforming is defining separate labels for the fields relevant to
particular signals, e.g. #define _si_poll_band si_band, to emphasize the
field's usage context, or an entire separate structure for internal use, such
as _siginfo_t_. That is where the leading '_' is expected to show it's a
platform specific identifier conforming code can safely ignore.
On Monday, November 14, 2016 Scott Lurndal <sc...@lurndal.org> wrote:
On Mon, Nov 14, 2016 at 04:04:04PM -0500, Shware Systems wrote:
> Technically, it doesn't
> match. The #defines after provide matching labels for symbolic references, so
> portable source most likely will compile, but using a field not listed for an
> si_code value (with an intent of temporary storage, perhaps, rather than
> increase the frame size for automatic variables) risks overwriting values
> that are pertinent to that code, or reading invalid values. The method can
> save space for packed allocations, usually "a good thing", at the expense of
> precluding some forms of algorithm optimizations.
The spec says that all members must be part of the structure. It does't mean
that they each need unique storage _at the same time_. Only the fields relevent
to the signal that caused siginfo_t to be presented to the application need be
valid for that signal.
Note that Unix and linux systems have used a union to represent this for
many years.
scott
>
> On Monday, November 14, 2016 FELLIN, JEFF K (JEFF) (JEFF)
> <j...@research.att.com> wrote:
>
> As I read this information from signal.h :
>
> ??
>
> [CX] The <signal.h> header shall define the siginfo_t type as a structure,
> which shall include at least the following members:
>
> [CX]
> int???????????????????? si_signo ??Signal number.
> int???????????????????? si_code?? ??Signal code.
>
> [XSI]
> int???????????????????? si_errno ??If non-zero, an errno value associated
> with
> ????????????????????????????????????????????????this signal, as described in
> <errno.h>.
>
> [CX]
> pid_t???????????????? si_pid???? ??Sending process ID.
> uid_t???????????????? si_uid???? ??Real user ID of sending process.
> void???????????????? *si_addr?? ??Address of faulting instruction.
> int???????????????????? si_status Exit value or signal.
>
> [OB XSR]
> long?????????????????? si_band?? ??Band event for SIGPOLL.
>
> [CX]
> union sigval?? si_value ??Signal value.
>
> ??
> ??
>
> All of the above members MUST be available in all occurrences of siginfo_t,
> whether or not they are applicable on a specific signal.
>
> ??
>
> So can someone tell me how this siginfo_t definition matches the minimum set
> of members for ALL instances of siginfo_t?
>
> ??
>
> typedef struct siginfo
>
> ?? {
>
> ?????? int si_signo;???????????????????????????? /* Signal number.?? */
>
> ?????? int si_errno;???????????????????????????? /* If non-zero, an errno
> value associated with
>
> ???????????????????????????????????????????????????????????????????? this
> signal, as defined in <errno.h>.?? */
>
> ?????? int si_code;?????????????????????????????? /* Signal code.?? */
>
> ??
>
> ?????? union
>
> ?????????? {
>
> ?????????????? int _pad[__SI_PAD_SIZE];
>
> ??
>
> ???????????????? /* kill().?? */
>
> ?????????????? struct
>
> ?????????????????? {
>
> ?????????????????????? __pid_t si_pid;???????? /* Sending process ID.?? */
>
> ?????????????????????? __uid_t si_uid;???????? /* Real user ID of sending
> process.?? */
>
> ?????????????????? } _kill;
>
> ??
>
> ?????????????? /* POSIX.1b timers.?? */
>
> ?????????????? struct
>
> ?? ????????????????{
>
> ?????????????????????? int si_tid;???????????????? /* Timer ID.?? */
>
> ?????????????????????? int si_overrun;???????? /* Overrun count.?? */
>
> ?????????????????????? sigval_t si_sigval; /* Signal value.?? */
>
> ?????????????????? } _timer;
>
> ??
>
> ?????????????? /* POSIX.1b signals.?? */
>
> ?????????????? struct
>
> ?????????????????? {
>
> ???????????????? ??????__pid_t si_pid;???????? /* Sending process ID.?? */
>
> ?????????????????????? __uid_t si_uid;???????? /* Real user ID of sending
> process.?? */
>
> ?????????????????????? sigval_t si_sigval; /* Signal value.?? */
>
> ?????????????????? } _rt;
>
> ??
>
> ?????????????? /* SIGCHLD.?? */
>
> ?????????????? struct
>
> ?????????????????? {
>
> ?????????????????????? __pid_t si_pid;???????? /* Which child.?? */
>
> ?????????????????????? __uid_t si_uid;???????? /* Real user ID of sending
> process.?? */
>
> ?????????????????????? int si_status;?????????? /* Exit value or signal.?? */
>
> ?????????????????????? __clock_t si_utime;
>
> ?????????????????????? __clock_t si_stime;
>
> ???????????? ??????} _sigchld;
>
> ???????????? /* SIGILL, SIGFPE, SIGSEGV, SIGBUS.?? */
>
> ?????????????? struct
>
> ?????????????????? {
>
> ?????????????????????? void *si_addr;?????????? /* Faulting insn/memory
> ref.?? */
>
> ?????????????????? } _sigfault;
>
> ??
>
> ?????????????? /* SIGPOLL.?? */
>
> ?????????????? struct
>
> ?????????????????? {
>
> ?????????????????????? long int si_band;???? /* Band event for SIGPOLL.?? */
>
> ?????????????????????? int si_fd;
>
> ?????????????????? } _sigpoll;
>
> ?????????? } _sifields;
>
> ?? } siginfo_t;
>
> ??
>
> ??
>
> /* X/Open requires some more fields with fixed names.?? */
>
> # define si_pid???????????????? _sifields._kill.si_pid
>
> # define si_uid???????????????? _sifields._kill.si_uid
>
> # define si_timerid???????? _sifields._timer.si_tid
>
> # define si_overrun???????? _sifields._timer.si_overrun
>
> # define si_status?????????? _sifields._sigchld.si_status
>
> # define si_utime???????????? _sifields._sigchld.si_utime
>
> # define si_stime???????????? _sifields._sigchld.si_stime
>
> # define si_value???????????? _sifields._rt.si_sigval
>
> # define si_int???????????????? _sifields._rt.si_sigval.sival_int
>
> # define si_ptr???????????????? _sifields._rt.si_sigval.sival_ptr
>
> # define si_addr?????????????? _sifields._sigfault.si_addr
>
> # define si_band?????????????? _sifields._sigpoll.si_band
>
> # define si_fd?????????????????? _sifields._sigpoll.si_fd
>
> ??
>
> Jeff Fellin
>
