make read() return undef on errors as documented, and clarify docs

p4raw-id: //depot/perl@1328

1 files changed, 5 insertions, 0 deletions

diff --git a/pp_sys.c b/pp_sys.c
index 44e520be20..a7e8dd5b3b 100644
--- a/pp_sys.c
+++ b/pp_sys.c
@@ -1324,7 +1324,12 @@ PP(pp_sysread)
     }
     else
 #endif
+    {
 	length = PerlIO_read(IoIFP(io), buffer+offset, length);
+	/* fread() returns 0 on both error and EOF */
+	if (PerlIO_error(IoIFP(io)))
+	    length = -1;
+    }
     if (length < 0)
 	goto say_undef;
     SvCUR_set(bufsv, length+offset);