Bad interaction between signal constants and -Wsign-conversion

["Zack Weinberg" <zack@owlfolio.org>]

This test program does something perfectly normal.  Well, it's cut down
from a program that does something perfectly normal.  SA_RESETHAND is
one of the bit flags intended to be used with the sa_flags field of
struct sigaction.

#include <signal.h>

struct sigaction sa = { 
  .sa_flags = SA_RESETHAND,
};

However ... on Linux, SA_RESETHAND happens to have the value 0x80000000,
and therefore, the type of that constant is 'unsigned int'.  And the
type of sa_flags is 'int'.  So if you compile this test program with
-Wconversion you get a warning:

$ gcc -fsyntax-only -Wconversion test.c
test.c:4:15: warning: signed conversion from ‘unsigned int’ to ‘int’
   changes value from ‘2147483648’ to ‘-2147483648’ [-Wsign-conversion]
    4 |  .sa_flags = SA_RESETHAND,
      |              ^~~~~~~~~~~~

This strikes me as a (minor) bug in our headers.  Given the ABI constraints
(this is a flags word passed to the kernel), I think the most straightforward
fix is probably to change the #define in bits/sigaction.h to

# define SA_RESETHAND ((int)0x80000000) /* Reset to SIG_DFL on entry to handler.  */

Anyone got a better idea?

zw