Re: [Bug-gnulib] invalid use of errno after ferror

[Bruno Haible]

Paul Eggert wrote:
> Isn't this also true for lib/mountlist.c?

You are right; I had overlooked that the third argument of fread() is 1.
If Jim doesn't object, I'll add a comment about this:

--- mountlist.c 9 Sep 2003 22:47:38 -0000       1.42
+++ mountlist.c 17 Sep 2003 10:19:40 -0000
@@ -612,6 +612,7 @@
 
     if (ferror (fp))
       {
+       /* The last fread() call must have failed.  */
        int saved_errno = errno;
        fclose (fp);
        errno = saved_errno;

--------------------------------------------------------------------------

> 2003-09-15  Paul Eggert  <address@hidden>
>
>       * lib/getndelim2.c (getndelim2): Don't trash errno when a read
>       fails, so that the caller gets the proper errno.

Thanks for this patch. Following it, I'd like to change the description
of the getndelim2 and related functions to read:

   Return the number of bytes read and stored at *LINEPTR + OFFSET (not
   including the NUL terminator), or -1 on error or EOF.  Error can be
   distinguished from EOF through ferror (stream); in the case of error
   'errno' will be set upon return.

However this description does not hold on non-POSIX systems that don't
set errno in their stdio routines. To fix this, I'd propose this patch:

--- getndelim2.c        15 Sep 2003 22:34:18 -0000      1.4
+++ getndelim2.c        17 Sep 2003 10:25:47 -0000
@@ -95,12 +95,16 @@
            }
        }
 
+      /* Clear errno, so that on non-POSIX systems the caller doesn't see a
+        wrong value of errno when we return -1.  */
+      errno = 0;
+
       c = getc (stream);
       if (c == EOF)
        {
          /* Return partial line, if any.  */
          if (read_pos == *lineptr)
-           return -1;
+           return -1; /* if error occurred, errno is set here */
          else
            break;
        }

--------------------------------------------------------------------------

>       Don't assume ferror sets errno.  Bug reported by Bruno Haible.
>       * src/cksum.c (cksum): Don't assume ferror sets errno.

But here you could actually still provide the errno value by calling
fwriteerror (stdout) instead of ferror (stdout). That's why I added the
fwriteerror module to gnulib.

Bruno