Allow strerror(0) and strerror_r(0, ...).

Of course, strerror_r() may still fail with ERANGE.

Although the POSIX specification said this could fail with EINVAL and
doing this likely indicates invalid use of errno, most other
implementations permitted it, various POSIX testsuites require it to
work (matching the older sys_errlist array) and apparently some
applications depend on it.

PR:		standards/151316
MFC after:	1 week

lib/libc/gen/errlst.c

@@ -36,7 +36,7 @@ __FBSDID("$FreeBSD$");
 #include <stdio.h>
 
 const char *const sys_errlist[] = {
-	"Undefined error: 0",			/*  0 - ENOERROR */
+	"No error: 0",				/*  0 - ENOERROR */
 	"Operation not permitted",		/*  1 - EPERM */
 	"No such file or directory",		/*  2 - ENOENT */
 	"No such process",			/*  3 - ESRCH */

lib/libc/string/strerror.3

@@ -32,7 +32,7 @@
 .\"     @(#)strerror.3	8.1 (Berkeley) 6/9/93
 .\" $FreeBSD$
 .\"
-.Dd October 12, 2004
+.Dd April 5, 2011
 .Dt STRERROR 3
 .Os
 .Sh NAME
@@ -114,6 +114,9 @@ the range 0 <
 .Fa errnum
 <
 .Fa sys_nerr .
+The number 0 is also recognized, although applications that take advantage of
+this are likely to use unspecified values of
+.Va errno .
 .Pp
 If insufficient storage is provided in
 .Fa strerrbuf

lib/libc/string/strerror.c

@@ -87,7 +87,7 @@ strerror_r(int errnum, char *strerrbuf, size_t buflen)
 	catd = catopen("libc", NL_CAT_LOCALE);
 #endif
 
-	if (errnum < 1 || errnum >= sys_nerr) {
+	if (errnum < 0 || errnum >= sys_nerr) {
 		errstr(errnum,
 #if defined(NLS)
 			catgets(catd, 1, 0xffff, UPREFIX),

tools/regression/lib/libc/string/test-strerror.c

@@ -42,16 +42,11 @@ main(void)
 	char *sret;
 	int iret;
 
-	plan_tests(25);
+	plan_tests(27);
 
 	/*
 	 * strerror() failure tests.
 	 */
-	errno = 0;
-	sret = strerror(0);
-	ok1(strcmp(sret, "Unknown error: 0") == 0);
-	ok1(errno == EINVAL);
-
 	errno = 0;
 	sret = strerror(INT_MAX);
 	snprintf(buf, sizeof(buf), "Unknown error: %d", INT_MAX);
@@ -61,6 +56,11 @@ main(void)
 	/*
 	 * strerror() success tests.
 	 */
+	errno = 0;
+	sret = strerror(0);
+	ok1(strcmp(sret, "No error: 0") == 0);
+	ok1(errno == 0);
+
 	errno = 0;
 	sret = strerror(EPERM);
 	ok1(strcmp(sret, "Operation not permitted") == 0);
@@ -79,8 +79,8 @@ main(void)
 	 * strerror_r() failure tests.
 	 */
 	memset(buf, '*', sizeof(buf));
-	iret = strerror_r(0, buf, sizeof(buf));
+	iret = strerror_r(-1, buf, sizeof(buf));
-	ok1(strcmp(buf, "Unknown error: 0") == 0);
+	ok1(strcmp(buf, "Unknown error: -1") == 0);
 	ok1(iret == EINVAL);
 
 	memset(buf, '*', sizeof(buf));
@@ -116,6 +116,11 @@ main(void)
 	/*
 	 * strerror_r() success tests.
 	 */
+	memset(buf, '*', sizeof(buf));
+	iret = strerror_r(0, buf, sizeof(buf));
+	ok1(strcmp(buf, "No error: 0") == 0);
+	ok1(iret == 0);
+
 	memset(buf, '*', sizeof(buf));
 	iret = strerror_r(EDEADLK, buf, sizeof(buf));
 	ok1(strcmp(buf, "Resource deadlock avoided") == 0);