tm.3type: describe tm_zone, tm_gmtoff

FreeBSD timezone(3) is V7 char *timezone(int zone, int dst),
our documentation would imply it's impossible to service this type of
system portably

Indeed, glibc defines them both, and they make more sense for most
use-cases than trying to use the globals

glibc cited for __USE_MISC, tm_zone invalidation is Debian 2.33-7
POSIX cited for XSI marking is Issue 7 TC2
CSRG CD #2 contains 4.3BSD-Tahoe with both members

We currently ship an outdated (and factually misleading) version
of this description in ctime.3

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>

1 files changed, 51 insertions, 0 deletions

diff --git a/man3/tm.3type b/man3/tm.3type
index 4117b36a0..4b98819f9 100644
--- a/man3/tm.3type
+++ b/man3/tm.3type
@@ -26,8 +26,26 @@ Standard C library
 .BR "    int         tm_yday;" \
 "   /* Day of the year  [" 0 ", " 365 "] (Jan/01 = " 0 ") */"
 .BR "    int         tm_isdst;" "  /* Daylight savings flag */"
+.PP
+.BR "    long        tm_gmtoff;" " /* Seconds East of UTC */"
+.BR "    const char *tm_zone;" "   /* Timezone abbreviation */"
 .B };
 .fi
+.PP
+.RS -4
+Feature Test Macro Requirements for glibc (see
+.BR feature_test_macros (7)):
+.RE
+.PP
+.IR tm_gmtoff ,
+.IR tm_zone :
+.nf
+.\" Guarded with __USE_MISC:
+    Since glibc 2.20:
+        _DEFAULT_SOURCE
+    Glibc 2.20 and earlier:
+        _BSD_SOURCE
+.fi
 .SH DESCRIPTION
 Describes time, broken down into distinct components.
 .PP
@@ -36,6 +54,17 @@ describes wether daylight saving time is in effect at the time described.
 The value is positive if daylight saving time is in effect,
 zero if it is not,
 and negative if the information is not available.
+.PP
+.I tm_gmtoff
+is the difference, in seconds,
+of the timezone represented by this broken-down time and UTC
+(this is the additive inverse of
+.BR timezone (3)).
+.PP
+.I tm_zone
+is the equivalent of
+.BR tzname (3)
+for the timezone represented by this broken-down time.
 .SH VERSIONS
 In C90,
 .I tm_sec
@@ -49,10 +78,32 @@ in C99.
 .SH CONFORMING TO
 C90 and later;
 POSIX.1-2001 and later.
+.PP
+.I tm_gmtoff
+and
+.I tm_zone
+originate from 4.3BSD-Tahoe (where
+.I tm_zone
+is a
+.IR "char *" ).
 .SH NOTES
 .I tm_sec
 can represent a leap second with the value
 .BR 60 .
+.PP
+.BR timezone (3),
+as a variable, is an XSI extension: some systems provide the V7-compatible
+.\" FreeBSD
+.BR timezone (3)
+function.
+The
+.I tm_gmtoff
+field provides an alternative (with the opposite sign) for those systems.
+.PP
+.I tm_zone
+points to static storage and may be overriden on subsequent calls to
+.BR localtime (3)
+and similar functions (however, this never happens under glibc).
 .SH SEE ALSO
 .BR ctime (3),
 .BR strftime (3),