setlocale does not accept a string returned by setlocale as argument

[bhaible]

POSIX ​specifies that "The string returned by setlocale() is such that a subsequent call with that string and its associated category shall restore that part of the global locale."

This is meant to hold also for LC_ALL, and is the primary means of being able to switch to a different locale and then come back to the previous locale. Also, this rule is the basis of POSIX ​getlocalename_l.

This rule holds on all operating systems, except Haiku.

How to reproduce: Compile the attached program.

gcc -Wall foo.c

Run it.

./a.out en_US.UTF-8 de_DE.UTF-8 es_ES.UTF-8

Expected output:

LC_CTYPE    -> en_US.UTF-8
LC_NUMERIC  -> de_DE.UTF-8
LC_COLLATE  -> es_ES.UTF-8
LC_ALL -> LC_COLLATE=es_ES.UTF-8;LC_CTYPE=en_US.UTF-8;LC_MESSAGES=en_US.UTF-8;LC_MONETARY=en_US.UTF-8;LC_NUMERIC=de_DE.UTF-8;LC_TIME=en_US.UTF-8
After calling setlocale with this value:
LC_CTYPE    -> en_US.UTF-8
LC_NUMERIC  -> de_DE.UTF-8
LC_COLLATE  -> es_ES.UTF-8
Successful.

Actual output:

LC_CTYPE    -> en_US.UTF-8
LC_NUMERIC  -> de_DE.UTF-8
LC_COLLATE  -> es_ES.UTF-8
LC_ALL -> LC_COLLATE=es_ES.UTF-8;LC_CTYPE=en_US.UTF-8;LC_MESSAGES=en_US.UTF-8;LC_MONETARY=en_US.UTF-8;LC_NUMERIC=de_DE.UTF-8;LC_TIME=en_US.UTF-8
Switching to the mixed locale failed!

Seen in Haiku hrev57823 (from 2024-07-15).

[bhaible]

test case foo.c