diff --git a/Makefile.am b/Makefile.am
index 9dfa24b..2f130a8 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -18,3 +18,25 @@ libpinelog_la_SOURCES = pinelog.c
 libpinelog_la_CFLAGS = @PINELOG_CFLAGS@ $(WARN_CFLAGS)
 libpinelog_la_LDFLAGS = $(WARN_LDFLAGS)
 
+
+test_SRCFILES = test_pinelog.c $(libpinelog_la_SOURCES)
+test_CFLAGS = \
+	-DPINELOG_FATAL_STR='"F"' \
+	-DPINELOG_ERROR_STR='"E"' \
+	-DPINELOG_WARNING_STR='"W"' \
+	-DPINELOG_INFO_STR='"I"' \
+	-DPINELOG_DEBUG_STR='"D"' \
+	-DPINELOG_TRACE_STR='"T"' \
+	-DPINELOG_DEFAULT_LEVEL=PINELOG_LVL_TRACE \
+	-DPINELOG_DEFAULT_STREAM=stderr \
+	-DPINELOG_TEST
+
+LOG_DRIVER = env AM_TAP_AWK='$(AWK)' $(SHELL) $(top_srcdir)/tap-driver.sh
+TESTS = \
+	test_ts_lvl_tr
+
+check_PROGRAMS = $(TESTS)
+test_ts_lvl_tr_SOURCES = $(test_SRCFILES)
+test_ts_lvl_tr_CFLAGS = $(WARN_CFLAGS) $(test_CFLAGS) \
+	-DPINELOG_SHOW_DATE=1 -DPINELOG_SHOW_LEVEL=1 -DPINELOG_SHOW_BACKTRACE=1
+test_ts_lvl_tr_LDFLAGS = $(WARN_LDFLAGS)
diff --git a/configure.ac b/configure.ac
index 9c3ee02..133e102 100644
--- a/configure.ac
+++ b/configure.ac
@@ -10,6 +10,7 @@ AM_INIT_AUTOMAKE([-Wall foreign subdir-objects])
 AC_REQUIRE_AUX_FILE([tap-driver.sh])
 AC_PROG_CC
 AC_PROG_CC_STDC
+AC_PROG_AWK
 AM_PROG_AR
 LT_INIT
 PKG_PROG_PKG_CONFIG
diff --git a/pinelog.c b/pinelog.c
index ded5abc..1c793c5 100644
--- a/pinelog.c
+++ b/pinelog.c
@@ -43,28 +43,28 @@
 /**********************************************************************
  * Configure level strings
  *********************************************************************/
-#ifndef PINELOG_FATAL
-#define PINELOG_FATAL "FATAL"
+#ifndef PINELOG_FATAL_STR
+#define PINELOG_FATAL_STR "FATAL"
 #endif
 
-#ifndef PINELOG_ERROR
-#define PINELOG_ERROR "ERROR"
+#ifndef PINELOG_ERROR_STR
+#define PINELOG_ERROR_STR "ERROR"
 #endif
 
-#ifndef PINELOG_WARNING
-#define PINELOG_WARNING "WARNING"
+#ifndef PINELOG_WARNING_STR
+#define PINELOG_WARNING_STR "WARNING"
 #endif
 
-#ifndef PINELOG_INFO
-#define PINELOG_INFO "INFO"
+#ifndef PINELOG_INFO_STR
+#define PINELOG_INFO_STR "INFO"
 #endif
 
-#ifndef PINELOG_DEBUG
-#define PINELOG_DEBUG "DEBUG"
+#ifndef PINELOG_DEBUG_STR
+#define PINELOG_DEBUG_STR "DEBUG"
 #endif
 
-#ifndef PINELOG_TRACE
-#define PINELOG_TRACE "TRACE"
+#ifndef PINELOG_TRACE_STR
+#define PINELOG_TRACE_STR "TRACE"
 #endif
 
 /**********************************************************************
@@ -103,6 +103,13 @@ int pinelog_set_output_stream(FILE *stream)
     return 0;
 }
 
+#ifdef PINELOG_TEST
+FILE * pinelog_get_output_stream(void)
+{
+    return output_stream;
+}
+#endif
+
 int pinelog_set_output_file(const char *file)
 {
     FILE *stream;
@@ -151,41 +158,46 @@ void pinelog_log_message(int level, const char *file, int line, const char *fmt,
         level = PINELOG_LVL_TRACE;
     }
 
-    /* Validate and set output stream */
+    #if !HAVE_FUNC_ATTRIBUTE_CONSTRUCTOR
+    /*
+     * Validate and set output stream. Only necessary if the compiler doesn't
+     * support the constructor attribute
+     */
     if (output_stream == NULL) {
-        output_stream = stdout;
+        output_stream = PINELOG_DEFAULT_STREAM;
     }
+    #endif
 
     #if PINELOG_SHOW_DATE
     do {
         time_t t;
         struct tm *tmp;
-        char date_string[20];
+        char date_string[30];
         t = time(NULL);
         tmp = localtime(&t);
         strftime(date_string, sizeof(date_string), "%F %T ", tmp);
-        fputs(date_string, out_stream);
+        fputs(date_string, output_stream);
     } while (0);
     #endif
 
     #if PINELOG_SHOW_LEVEL
     do {
         static const char *level_strings[] = {
-            PINELOG_FATAL,
-            PINELOG_ERROR,
-            PINELOG_WARNING,
-            PINELOG_INFO,
-            PINELOG_DEBUG,
-            PINELOG_TRACE,
+            PINELOG_FATAL_STR,
+            PINELOG_ERROR_STR,
+            PINELOG_WARNING_STR,
+            PINELOG_INFO_STR,
+            PINELOG_DEBUG_STR,
+            PINELOG_TRACE_STR,
         };
 
-        fputs(level_strings[level], out_stream);
-        fputs(": ", out_stream);
-    } while (0)
+        fputs(level_strings[level], output_stream);
+        fputs(": ", output_stream);
+    } while (0);
     #endif
 
     #if PINELOG_SHOW_BACKTRACE
-        fprintf(out_stream, "%s:%d ", file, line);
+        fprintf(output_stream, "%s:%d ", file, line);
     #endif
 
     va_start(ap, fmt);
diff --git a/pinelog.h b/pinelog.h
index b7bc0ff..0020ad9 100644
--- a/pinelog.h
+++ b/pinelog.h
@@ -20,7 +20,9 @@
 
 #include "config.h"
 #include <stdio.h>
+#ifndef PINELOG_TEST
 #include <stdlib.h>
+#endif
 
 #ifdef __cplusplus
 extern "C" {
@@ -60,6 +62,15 @@ enum {
  */
 void pinelog_set_defaults(void);
 
+#ifdef PINELOG_TEST
+/**
+ * @brief Get the pointer to the output stream. Only used in test harness.
+ *
+ * @returns FILE pointer to output stream
+ */
+FILE * pinelog_get_output_stream(void);
+#endif
+
 /**
  * @brief Set the output stream. Must be a FILE pointer.
  *
@@ -113,11 +124,16 @@ __attribute__((format(printf, 4, 5)))
 
 void pinelog_log_message(int level, const char *file, int line, const char *fmt, ...);
 
+// Test harness will redefine pinelog_exit
+#ifndef PINELOG_TEST
+#define pinelog_exit exit
+#endif
+
 #define PINELOG_FATAL(fmt, ...) do { \
     if (PINELOG_LVL_FATAL <= pinelog_get_level()) { \
         pinelog_log_message(PINELOG_LVL_FATAL, __FILE__, __LINE__, fmt, ##__VA_ARGS__); \
     } \
-    exit(1); \
+    pinelog_exit(1); \
 } while (0)
 
 #define PINELOG_ERROR(fmt, ...) do { \
diff --git a/test_pinelog.c b/test_pinelog.c
new file mode 100644
index 0000000..dd568a8
--- /dev/null
+++ b/test_pinelog.c
@@ -0,0 +1,194 @@
+/*
+ * Pinelog lightweight logging library - test harness
+ *
+ * Copyright (C) 2021 Nirenjan Krishnan (nirenjan@nirenjan.org)
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include "pinelog.h"
+#include <stdio.h>
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+#include <limits.h>
+#include <time.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+/**********************************************************************
+ * Global variables
+ *********************************************************************/
+// Test ID (of current test case)
+static unsigned int test_id;
+
+// Observed output stream
+static FILE *observed_stream_w;
+static FILE *observed_stream_r;
+
+// Temporary pipe for observed data
+static char observed_fifo[NAME_MAX];
+
+// Buffer for expected output
+static char expected_output[1024];
+static size_t expected_len;
+
+static void test_case(const char *desc, bool test)
+{
+    test_id++;
+    if (test) {
+        printf("ok %u %s\n", test_id, desc);
+    } else {
+        printf("not ok %u %s\n", test_id, desc);
+    }
+}
+
+static void pinelog_exit(int status)
+{
+    fprintf(observed_stream_w, "EXIT(%d)\n", status);
+    expected_len += snprintf(&expected_output[expected_len],
+                             sizeof(expected_output) - expected_len,
+                             "EXIT(%d)\n", status);
+}
+
+static void dump_data(const char *type, size_t len, char *data)
+{
+    char *line;
+    printf("# %s (%lu bytes):\n", type, len);
+    line = strtok(data, "\n");
+    while (line != NULL) {
+        printf("#\t%s\n", line);
+        line = strtok(NULL, "\n");
+    }
+    printf("\n");
+}
+
+static void test_setup(int level, int filter, const char *file, int line, const char *fmt, ...)
+{
+    va_list ap;
+
+    expected_len = 0;
+    memset(expected_output, 0, sizeof(expected_output));
+
+    if (level <= filter) {
+        if (PINELOG_SHOW_DATE) {
+            time_t t;
+            struct tm *tmp;
+
+            t = time(NULL);
+            tmp = localtime(&t);
+            expected_len += strftime(&expected_output[expected_len],
+                                     sizeof(expected_output) - expected_len,
+                                     "%F %T ", tmp);
+        }
+
+        if (PINELOG_SHOW_LEVEL) {
+            const char * level_string[] = {
+                PINELOG_FATAL_STR,
+                PINELOG_ERROR_STR,
+                PINELOG_WARNING_STR,
+                PINELOG_INFO_STR,
+                PINELOG_DEBUG_STR,
+                PINELOG_TRACE_STR,
+            };
+            expected_len += snprintf(&expected_output[expected_len],
+                                     sizeof(expected_output) - expected_len,
+                                     "%s: ", level_string[level]);
+        }
+
+        if (PINELOG_SHOW_BACKTRACE) {
+            expected_len += snprintf(&expected_output[expected_len],
+                                     sizeof(expected_output) - expected_len,
+                                     "%s:%d ", file, line);
+        }
+
+        va_start(ap, fmt);
+        expected_len += vsnprintf(&expected_output[expected_len],
+                                  sizeof(expected_output) - expected_len,
+                                  fmt, ap);
+        va_end(ap);
+        expected_output[expected_len] = '\n';
+        expected_len++;
+    }
+}
+
+static void test_teardown(const char *desc)
+{
+    // Compare the output
+    static char observed[1024];
+    size_t observed_len;
+    int result;
+
+    observed_len = fread(observed, 1, sizeof(observed), observed_stream_r);
+
+    result = ((expected_len == observed_len) &&
+              (memcmp(expected_output, observed, expected_len) == 0));
+    test_case(desc, result);
+    if (!result) {
+        dump_data("expected", expected_len, expected_output);
+        dump_data("observed", observed_len, observed);
+    }
+}
+
+static void verify_defaults(void)
+{
+    test_case("Get default output stream",
+              pinelog_get_output_stream() == PINELOG_DEFAULT_STREAM);
+    test_case("Get default logging level",
+              pinelog_get_level() == PINELOG_DEFAULT_LEVEL);
+}
+
+#define PINELOG_WARNING PINELOG_WARN
+
+#define TEST_LOG(lvl, filter, fmt, ...) do { \
+    test_setup(PINELOG_LVL_ ## lvl, PINELOG_LVL_ ## filter, \
+               __FILE__, __LINE__, fmt, ##__VA_ARGS__); \
+    PINELOG_ ## lvl (fmt, ##__VA_ARGS__); \
+    test_teardown("Log " #lvl " filter " #filter); \
+} while(0)
+
+#define TEST(filter, fmt, ...) do { \
+    pinelog_set_level(PINELOG_LVL_ ## filter); \
+    TEST_LOG(TRACE, filter, fmt, ##__VA_ARGS__); \
+    TEST_LOG(DEBUG, filter, fmt, ##__VA_ARGS__); \
+    TEST_LOG(INFO, filter, fmt, ##__VA_ARGS__); \
+    TEST_LOG(WARNING, filter, fmt, ##__VA_ARGS__); \
+    TEST_LOG(ERROR, filter, fmt, ##__VA_ARGS__); \
+    TEST_LOG(FATAL, filter, fmt, ##__VA_ARGS__); \
+} while (0)
+
+int main(int argc, char **argv)
+{
+    int fifo_fd_r, fifo_fd_w;
+    snprintf(observed_fifo, sizeof(observed_fifo), "%s.fifo", argv[0]);
+    mkfifo(observed_fifo, 0777);
+
+    fifo_fd_r = open(observed_fifo, O_RDONLY | O_NONBLOCK);
+    fifo_fd_w = open(observed_fifo, O_WRONLY | O_NONBLOCK);
+    observed_stream_r = fdopen(fifo_fd_r, "r");
+    observed_stream_w = fdopen(fifo_fd_w, "w");
+
+    verify_defaults();
+
+    pinelog_set_output_stream(observed_stream_w);
+    TEST(TRACE, "testing %s... %d, %f, %u", "testing", -1, 0.0, 1);
+    TEST(DEBUG, "testing %s... %d, %f, %u", "testing", -1, 0.0, 1);
+    TEST(INFO, "testing %s... %d, %f, %u", "testing", -1, 0.0, 1);
+    TEST(WARNING, "testing %s... %d, %f, %u", "testing", -1, 0.0, 1);
+    TEST(ERROR, "testing %s... %d, %f, %u", "testing", -1, 0.0, 1);
+    TEST(FATAL, "testing %s... %d, %f, %u", "testing", -1, 0.0, 1);
+    TEST(NONE, "testing %s... %d, %f, %u", "testing", -1, 0.0, 1);
+
+    printf("1..%u\n", test_id);
+
+    fclose(observed_stream_w);
+    fclose(observed_stream_r);
+    close(fifo_fd_w);
+    close(fifo_fd_r);
+    unlink(observed_fifo);
+
+    return 0;
+}