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, 38 insertions, 8 deletions

diff --git a/newlib/libc/string/strerror_r.c b/newlib/libc/string/strerror_r.c
index be5358f3e..c2057b7f0 100644
--- a/newlib/libc/string/strerror_r.c
+++ b/newlib/libc/string/strerror_r.c
@@ -1,3 +1,4 @@
+/* GNU variant of strerror_r. */
 /*
 FUNCTION
 	<<strerror_r>>---convert error number to string and copy to buffer
@@ -7,7 +8,11 @@ INDEX
 
 ANSI_SYNOPSIS
 	#include <string.h>
+	#ifdef _GNU_SOURCE
 	char *strerror_r(int <[errnum]>, char *<[buffer]>, size_t <[n]>);
+	#else
+	int strerror_r(int <[errnum]>, char *<[buffer]>, size_t <[n]>);
+	#endif
 
 TRAD_SYNOPSIS
 	#include <string.h>
@@ -19,35 +24,60 @@ TRAD_SYNOPSIS
 DESCRIPTION
 <<strerror_r>> converts the error number <[errnum]> into a
 string and copies the result into the supplied <[buffer]> for
-a length up to <[n]>, including the NUL terminator. The value of 
-<[errnum]> is usually a copy of <<errno>>.  If <<errnum>> is not a known 
+a length up to <[n]>, including the NUL terminator. The value of
+<[errnum]> is usually a copy of <<errno>>.  If <<errnum>> is not a known
 error number, the result is the empty string.
 
 See <<strerror>> for how strings are mapped to <<errnum>>.
 
 RETURNS
-This function returns a pointer to a string.  Your application must
-not modify that string.
+There are two variants: the GNU version always returns a NUL-terminated
+string, which is <[buffer]> if all went well, but which is another
+pointer if <[n]> was too small (leaving <[buffer]> untouched).  If the
+return is not <[buffer]>, your application must not modify that string.
+The POSIX version returns 0 on success, <[EINVAL]> if <<errnum>> was not
+recognized, and <[ERANGE]> if <[n]> was too small.  The variant chosen
+depends on macros that you define before inclusion of <<string.h>>.
 
 PORTABILITY
-<<strerror_r>> is a GNU extension.
+<<strerror_r>> with a <[char *]> result is a GNU extension.
+<<strerror_r>> with an <[int]> result is required by POSIX 2001.
+This function is compliant only if <<_user_strerror>> is not provided,
+or if it is thread-safe and does not modify <<errno>>.
+
+POSIX states that the contents of <[buf]> are unspecified on error,
+although this implementation guarantees a NUL-terminated string for
+all except <[n]> of 0.
+
+POSIX recommends that unknown <[errnum]> result in a message including
+that value, however it is not a requirement and this implementation
+provides only an empty string (unless you provide <<_user_strerror>>).
+POSIX also recommends that unknown <[errnum]> fail with EINVAL even
+when providing such a message, however it is not a requirement and
+this implementation will return success if <<_user_strerror>> provided
+a non-empty alternate string.
 
 <<strerror_r>> requires no supporting OS subroutines.
 
 */
 
 #undef __STRICT_ANSI__
+#define _GNU_SOURCE
 #include <errno.h>
 #include <string.h>
+#undef strerror_r
 
+/* For backwards-compatible linking, this must be the GNU signature;
+   see xpg_strerror_r.c for the POSIX version.  */
 char *
 _DEFUN (strerror_r, (errnum, buffer, n),
 	int errnum _AND
 	char *buffer _AND
 	size_t n)
 {
-  char *error;
-  error = strerror (errnum);
+  char *error = strerror (errnum);
 
-  return strncpy (buffer, (const char *)error, n);
+  if (strlen (error) >= n)
+    return error;
+  return strcpy (buffer, error);
 }