strerror_r: provide POSIX implementation

* libc/include/string.h (strerror_r): Update declaration.
* libc/string/strerror.c (strerror): Update documentation.
* libc/string/strerror_r.c (strerror_r): Always return
NUL-terminated string; don't overwrite too-short buf.
* libc/string/xpg_strerror_r.c (__xpg_strerror_r): Implement POSIX
variant.
* libc/string/Makefile.am (GENERAL_SOURCES): Build new file.
* libc/string/Makefile.in: Regenerate.

1 files changed, 14 insertions, 0 deletions

diff --git a/newlib/libc/string/strerror.c b/newlib/libc/string/strerror.c
index 043763141..61e40ab7a 100644
--- a/newlib/libc/string/strerror.c
+++ b/newlib/libc/string/strerror.c
@@ -301,6 +301,17 @@ declares that subsequent calls to <<strerror>> may overwrite the
 result string; therefore portable code cannot depend on the reentrancy
 of this subroutine.
 
+Although this implementation of <<strerror>> guarantees a non-null
+result with a NUL-terminator, some implementations return <<NULL>>
+on failure.  Although POSIX allows <<strerror>> to set <<errno>>
+to EINVAL on failure, this implementation does not do so (unless
+you provide <<_user_strerror>>).
+
+POSIX recommends that unknown <[errnum]> result in a message
+including that value, however it is not a requirement and this
+implementation does not provide that information (unless you
+provide <<_user_strerror>>).
+
 This implementation of <<strerror>> provides for user-defined
 extensibility.  <<errno.h>> defines <[__ELASTERROR]>, which can be
 used as a base for user-defined error values.  If the user supplies a
@@ -313,6 +324,9 @@ character pointer.  If <[errnum]> is unknown to <<_user_strerror>>,
 <<_user_strerror>> returns <[NULL]>.  The default <<_user_strerror>>
 returns <[NULL]> for all input values.
 
+Note that <<_user_sterror>> must be thread-safe and not alter <<errno>>
+if <<strerror_r>> is to comply with POSIX.
+
 <<strerror>> requires no supporting OS subroutines.
 
 QUICKREF