casaroli opened a new pull request, #21099:
URL: https://github.com/apache/nuttx/pull/21099
## Summary
Without the environment variable `TZ`, `tzset()` returned at once, so
`localtime()` always gave UTC: the tzcode default name `TZDEFAULT`
("localtime") was never used. Systems with the IANA TZ database take the local
zone from `/etc/localtime`, a link to the file of the zone.
This PR adds `CONFIG_LIBC_TZDEFAULT`: the time zone file that `tzset()`
loads when `TZ` is not set, an absolute path or a name in `CONFIG_LIBC_TZDIR`.
When the file cannot be read, the local time is UTC. As in the tzcode,
`g_lcl_isset` is -1 while this default zone is in use, so that a later
`tzset()` without `TZ` does not load the file again, and a `TZ` that is set
later still takes effect. The second commit describes the order (`TZ`, the
default file, UTC) and the settings for the IANA layout in
`Documentation/os/libs/libc/zoneinfo.rst`.
## Impact
The default of `CONFIG_LIBC_TZDEFAULT` is empty: without `TZ` the local time
stays UTC, so existing configurations do not change. Only builds with
`CONFIG_LIBC_LOCALTIME`; all build modes.
## Testing
Tested on master aeab6fbf989 and on this PR rebased on it.
Test program `tzdflt.c` on `qemu-armv8a:knsh` (QEMU with HVF) with
`CONFIG_LIBC_LOCALTIME=y`, `CONFIG_LIBC_TZDIR="/system/zoneinfo"` and
`CONFIG_LIBC_TZDEFAULT="/system/zoneinfo/Europe/Rome"` (the TZif file of the
host in the shared `/system` directory). 2026-07-01 12:00 UTC must be 14:00
CEST without `TZ`.
| Case | master | this PR |
|---|---|---|
| `TZ` not set | 12:00, `tzname` empty | 14:00, CET/CEST |
| `TZ=UTC0` | 12:00 | 12:00 |
| `TZ` not set again | 12:00 (UTC0 stays) | 14:00, CET/CEST |
| ostest, `qemu-armv8a:knsh` and `qemu-armv8a:nsh` | status 0 | status 0 |
Build with `CONFIG_LIBC_LOCALTIME` and
`CONFIG_LIBC_TZDEFAULT="/etc/localtime"`, each OK: `rv-virt:nsh64` (flat,
rv64), `rv-virt:pnsh64` (protected, rv64), `rv-virt:knsh64` (kernel, rv64); and
`qemu-armv8a:nsh` (flat, arm64), `qemu-armv8a:knsh` (kernel, arm64) with the
test settings above.
`tools/checkpatch.sh -c -u -m -g` passes.
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