fix some gcc warnings

[Bruno Haible]

Hi,

When I compile a gnulib testdir with "gcc -Wall", I see the following warnings
(among others):

test-ffsl.c: In function 'main':
test-ffsl.c:44:12: warning: unused variable 'x' [-Wunused-variable]

test-posix_spawn3.c: In function 'parent_main':
test-posix_spawn3.c:78:11: warning: ignoring return value of 'freopen', 
declared with attribute warn_unused_result [-Wunused-result]

test-sethostname1.c: In function 'main':
test-sethostname1.c:34:17: warning: ignoring return value of 'sethostname', 
declared with attribute warn_unused_result [-Wunused-result]

Here's a proposed fix for them.


2016-10-16  Bruno Haible  <br...@clisp.org>

        Fix some "gcc -Wall" warnings.
        * tests/test-ffsl.c (main): Use variable x, not i.
        * tests/test-posix_spawn3.c (parent_main): Consider the return value of
        freopen.
        * tests/test-sethostname1.c (main): Explicitly ignore the return value
        of sethostname.

diff --git a/tests/test-ffsl.c b/tests/test-ffsl.c
index e903509..117bc4a 100644
--- a/tests/test-ffsl.c
+++ b/tests/test-ffsl.c
@@ -44,8 +44,8 @@ main (int argc, char *argv[])
   long int x;
   int i;
 
-  for (i = -128; i <= 128; i++)
-    ASSERT (ffsl (i) == naive (i));
+  for (x = -128; x <= 128; x++)
+    ASSERT (ffsl (x) == naive (x));
   for (i = 0; i < NBITS; i++)
     {
       ASSERT (ffsl (1UL << i) == naive (1UL << i));
diff --git a/tests/test-posix_spawn3.c b/tests/test-posix_spawn3.c
index 90745fd..f2a699c 100644
--- a/tests/test-posix_spawn3.c
+++ b/tests/test-posix_spawn3.c
@@ -75,7 +75,11 @@ parent_main (void)
     }
 
   /* Avoid reading from our stdin, as it could block.  */
-  freopen ("/dev/null", "rb", stdin);
+  if (freopen ("/dev/null", "rb", stdin) == NULL)
+    {
+      perror ("cannot redirect stdin");
+      return 1;
+    }
 
   /* Test whether posix_spawn_file_actions_addopen with this file name
      actually works, but spawning a child that reads from this file.  */
diff --git a/tests/test-sethostname1.c b/tests/test-sethostname1.c
index 5ed6a83..08441f3 100644
--- a/tests/test-sethostname1.c
+++ b/tests/test-sethostname1.c
@@ -30,8 +30,11 @@ main ()
   /* Some code that has a link-time dependency to the sethostname() function
      and that is likely not optimized away by compilers.  */
   if (do_dangerous_things)
-    /* Never executed.  */
-    sethostname ("oprah", 5);
+    {
+      /* Never executed.  */
+      int ret = sethostname ("oprah", 5);
+      (void) ret;
+    }
 
   return 0;
 }