diff --git a/src/plugins/metrics/metrics_parameters.ml b/src/plugins/metrics/metrics_parameters.ml
index ca410edbb5e529bfee912a5d8ed37aaa4ff31718..5d9bd963dbf2187682089cc8b9581ef220e9c366 100644
--- a/src/plugins/metrics/metrics_parameters.ml
+++ b/src/plugins/metrics/metrics_parameters.ml
@@ -49,9 +49,8 @@ module OutputFile =
       let option_name = "-metrics-output"
       let arg_name = "filename"
       let help = "print some metrics into the specified file; \
-                  the output format is recognized through the extension. \
-                  If the filename is only the extension (txt, html, json), \
-                  print to stdout but in the specified format."
+                  the output format is recognized through the extension: \
+                  .text/.txt for text, .html/.htm for HTML, or .json for JSON."
      end)
 
 module ValueCoverage =
diff --git a/tests/metrics/libc.c b/tests/metrics/libc.c
index 15e0328f762aa57887caa863f9459c63ed2a15c1..0c4aaf13fb44c75d6342950e46534288badae189 100644
--- a/tests/metrics/libc.c
+++ b/tests/metrics/libc.c
@@ -1,8 +1,9 @@
 /* run.config
    STDOPT: #"-metrics-no-libc -metrics-eva-cover"
    STDOPT: #"-metrics-libc -metrics-eva-cover"
-   STDOPT: #"-metrics-libc -metrics-output @PTEST_DIR@/oracle/libc.json"
-   LOG: @PTEST_DIR@/oracle/libc.json
+   STDOPT: #"-metrics-libc -metrics-output @PTEST_DIR@/result/libc.json"
+   LOG: libc.json
+   OPT:
 */
 #include <ctype.h>
 #include <stdio.h> // defines external variables
diff --git a/tests/metrics/oracle/libc.3.res.oracle b/tests/metrics/oracle/libc.3.res.oracle
new file mode 100644
index 0000000000000000000000000000000000000000..eeb5977dbc5fef0789c0e314ec2e68f5834bd8d2
--- /dev/null
+++ b/tests/metrics/oracle/libc.3.res.oracle
@@ -0,0 +1 @@
+[kernel] Parsing tests/metrics/libc.c (with preprocessing)