Added documentation on logging functions

doc/index.rst

@@ -9,6 +9,7 @@ Criterion
     starter
     assert
     hooks
+    logging
     env
     output
     parameterized

doc/logging.rst

@@ -0,0 +1,61 @@
+Logging messages
+================
+
+Sometimes, it might be useful to print some output from within a test
+or fixture -- and while this can be done trivially with a ``printf``,
+it doesn't integrate well with the current output, nor does it work
+*at all* when the process is testing a redirected stdout.
+
+For these cases, Criterion exposes a logging facility:
+
+.. code-block:: c
+
+    #include <criterion/criterion.h>
+    #include <criterion/logging.h>
+
+    Test(suite_name, test_name) {
+        cr_log_info("This is an informational message. They are not displayed "
+            "by default.");
+        cr_log_warn("This is a warning. They indicate some possible malfunction "
+            "or misconfiguration in the test.");
+        cr_log_error("This is an error. They indicate serious problems and "
+            "are usually shown before the test is aborted.");
+    }
+
+``cr_log_info``, ``cr_log_warn`` and ``cr_log_error`` are all macros expanding
+to a call to the ``cr_log`` function. All of them take a mandatory format string,
+followed by optional parameters; for instance:
+
+.. code-block:: c
+
+    cr_log_info("%d + %d = %d", 1, 2, 3);
+
+If using C++, the output stream objects ``info``, ``warn`` and ``error`` are
+defined within the ``criterion::logging`` namespace, and can be used in
+conjunction with ``operator<<``:
+
+.. code-block:: c++
+
+    #include <criterion/criterion.h>
+    #include <criterion/logging.h>
+
+    using criterion::logging::info;
+    using criterion::logging::warn;
+    using criterion::logging::error;
+
+    Test(suite_name, test_name) {
+        info << "This is an informational message. "
+             << "They are not displayed by default."
+             << std::flush;
+        warn << "This is a warning. "
+             << "They indicate some possible malfunction "
+             << "or misconfiguration in the test."
+             << std::flush;
+        error << "This is an error. "
+             << "They indicate serious problems and "
+             << "are usually shown before the test is aborted."
+             << std::flush;
+    }
+
+Note that empty messages are ignored, and newlines in the log message splits
+the passed string into as many messages are there are lines.