There are three files that govern userspace struct timespec on glibc:
1. bits/wordsize.h, defining:
(a) __WORDSIZE to 32 on ILP32 and 64 on LP64
(b) on x32: __SYSCALL_WORDSIZE to 64
2. bits/timesize.h, defining
(a) __TIMESIZE to __WORDSIZE, except on x32 where it's 64
3. bits/types/struct_timespec.h, declaring struct timespec as:
struct timespec
{
__time_t tv_sec; /* Seconds. */
#if __WORDSIZE == 64 \
|| (defined __SYSCALL_WORDSIZE && __SYSCALL_WORDSIZE == 64) \
|| __TIMESIZE == 32
__syscall_slong_t tv_nsec; /* Nanoseconds. */
#else
# if __BYTE_ORDER == __BIG_ENDIAN
int: 32; /* Padding. */
long int tv_nsec; /* Nanoseconds. */
# else
long int tv_nsec; /* Nanoseconds. */
int: 32; /* Padding. */
# endif
#endif
};
this has two side-effects: struct timespec
(a) is always sizeof==time_t+8, and
(b) has tv_nsec as __syscall_slong_t
*and* !is_same<__syscall_slong_t, long>
if using LP64 syscalls on an ILP32 system, i.e. on x32.
This means, that the simplified
struct timespec {
time_t tv_sec; /* Seconds */
long tv_nsec; /* Nanoseconds [0 .. 999999999] */
};
declaration is *invalid* for x32,
where struct timespec::tv_nsec is an int64_t (long long).
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@xxxxxxxxxxxxxxxxxx>
---
The reasoning is explained in the commit message,
but I elucidated it in a more approachable way in the Notes,
which also include the link from Jakub Wilk upthread.
Nevertheless, log2(10^(3*3) - 1) <= 31 is a good point!
I also added this as a slight portability guide toward the end.
This replaces 3/4 and 4/4.
man7/system_data_types.7 | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)
diff --git a/man7/system_data_types.7 b/man7/system_data_types.7
index 1e6a3f74c..cce17fc3e 100644
--- a/man7/system_data_types.7
+++ b/man7/system_data_types.7
@@ -1553,6 +1553,36 @@ Describes times in seconds and nanoseconds.
.IR "Conforming to" :
C11 and later; POSIX.1-2001 and later.
.PP
+.IR Notes :
+.I tv_nsec
+is the
+.I syscall
+long, though this affects only fringe architectures like X32,
+which is ILP32, but uses the LP64 AMD64 syscall ABI.
+.br
+In reality, the field ends up being defined as:
+.EX
+.in +4
+#if !(__x86_64__ && __ILP32__ /* == x32 */)
+ long tv_nsec;
+#else
+ long long tv_nsec;
+#endif
+.in
+.EE
+.PP
+This is a long-standing and long-enshrined
+.UR https://sourceware.org/bugzilla/show_bug.cgi?id=16437
+glibc bug
+.I #16437
+.UE ,
+an incompatible extension to the standards;
+however, as even a 32-bit
+.I long
+can hold the entire
+.I tv_nsec
+range, it's safe to forcibly down-cast it to the standard type.
+.PP
.IR "See also" :
.BR clock_gettime (2),
.BR clock_nanosleep (2),
--
2.30.2
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