Add usage and integration instructions to README

Makefile.am

@@ -15,7 +15,7 @@ noinst_LTLIBRARIES = libpinelog.la
 
 # pinelog logging library
 libpinelog_la_SOURCES = pinelog.c
-libpinelog_la_CFLAGS = @PINELOG_CFLAGS@ $(WARN_CFLAGS)
+libpinelog_la_CFLAGS = @PINELOG_CFLAGS@ $(WARN_CFLAGS) -I $(top_builddir)
 libpinelog_la_LDFLAGS = $(WARN_LDFLAGS)
 
 
@@ -29,7 +29,7 @@ test_CFLAGS = \
 	-DPINELOG_TRACE_STR='"T"' \
 	-DPINELOG_DEFAULT_LEVEL=PINELOG_LVL_TRACE \
 	-DPINELOG_DEFAULT_STREAM=stderr \
-	-DPINELOG_TEST
+	-DPINELOG_TEST -I $(top_builddir)
 
 LOG_DRIVER = env AM_TAP_AWK='$(AWK)' $(SHELL) $(top_srcdir)/tap-driver.sh
 TESTS = \

README.md

@@ -4,3 +4,131 @@ Pinelog - a lightweight logging API
 Pinelog is a lightweight logging API for C programs that's designed to be
 included in your program source code. Parameters for Pinelog are configured at
 build time by means of preprocessor flags.
+
+# Usage
+## Logging macros
+
+Pinelog uses `printf` style formatting, using the following list of macros. The
+macro indicates the level at which the message is logged.
+
+* `PINELOG_FATAL`
+* `PINELOG_ERROR`
+* `PINELOG_WARN`
+* `PINELOG_INFO`
+* `PINELOG_DEBUG`
+* `PINELOG_TRACE`
+
+**Note:** `PINELOG_FATAL` is used when the program encounters a fatal condition
+and needs to abort. This will log the fatal message and terminate the program
+with an exit code of 1.
+
+### Example
+
+```C
+PINELOG_INFO("configuration file %s not found, using defaults", config_file);
+```
+
+## Logging levels
+
+The default logging level is `ERROR`, and this can be controlled by the
+preprocessor flag `PINELOG_DEFAULT_LEVEL`.
+
+The program can control the level at which messages can be logged at runtime,
+by using the `pinelog_set_level` function. This function takes in the level
+definition, which is one of the following, in increasing order of priority.
+
+* `PINELOG_LVL_TRACE`
+* `PINELOG_LVL_DEBUG`
+* `PINELOG_LVL_INFO`
+* `PINELOG_LVL_WARNING`
+* `PINELOG_LVL_ERROR`
+* `PINELOG_LVL_FATAL`
+* `PINELOG_LVL_NONE`
+
+Setting the level to a given priority suppresses all log messages of lower
+priority, i.e., if the level is set to `PINELOG_LVL_ERROR`, messages at
+`WARNING` level and below will be suppressed, but `ERROR` and `FATAL` messages
+will be logged.
+
+**Note:** `PINELOG_LVL_NONE` suppresses all log messages, but `PINELOG_FATAL`
+will still terminate the program, even though nothing is logged.
+
+### Example
+
+```C
+pinelog_set_level(PINELOG_LVL_WARNING);
+```
+
+```
+-DPINELOG_DEFAULT_LEVEL=PINELOG_LVL_WARNING
+```
+
+## Output redirection
+
+Pinelog defaults to writing the log messages to standard output, and this can
+be controlled by the preprocessor flag `PINELOG_DEFAULT_STREAM`.
+
+However, the application can redirect log messages at runtime to a different
+`FILE *` stream, or to a file by using one of the following two methods:
+
+```C
+FILE *out = fopen("/run/app.fifo", "w");
+pinelog_set_output_stream(out);
+pinelog_set_output_file("/var/log/app.log");
+```
+
+```
+-DPINELOG_DEFAULT_STREAM=stderr
+```
+
+## Logging format
+
+Pinelog uses an opinionated logging format that is fixed as follows. Fields
+within `[]` are optional and controlled by build time flags.
+
+    [2021-07-14 11:08:04 ][ERROR: ][./test_pinelog.c:108 ]formatted message.
+
+The program can be controlled by the following preprocessor flags, all of which
+default to `0` (disabled). Set the flag to `1` to enable it.
+
+* `PINELOG_SHOW_DATE` - Display the ISO 8601 date and time when the message is
+  logged.
+* `PINELOG_SHOW_LEVEL` - Display the level at which the message is logged.
+* `PINELOG_SHOW_BACKTRACE` - Display the file and line where the message is
+  logged.
+
+Set these flags by using the `-D` compiler argument, .e.g.
+`-DPINELOG_SHOW_LEVEL=1 -DPINELOG_SHOW_DATE=1`
+
+### Level strings
+
+The application can control the level strings displayed by means of preprocessor
+flags, if the application wishes to display the log messages in a language other
+than English. This can be achieved by means of the following preprocessor
+definitions.
+
+* `PINELOG_FATAL_STR`
+* `PINELOG_ERROR_STR`
+* `PINELOG_WARNING_STR`
+* `PINELOG_INFO_STR`
+* `PINELOG_DEBUG_STR`
+* `PINELOG_TRACE_STR`
+
+### Example
+
+```
+-DPINELOG_ERROR_STR=\"E\" -DPINELOG_FATAL_STR=\"F\"
+```
+
+# Integrating Pinelog
+
+Pinelog is intended to be integrated into your application source tree, either
+by means of including the sources directly, or by including the repository as
+a Git submodule or subtree.
+
+The default build of Pinelog uses an autotools generated `config.h` file, which
+includes checks for the following GCC attributes. If you don't care about these,
+then either create a dummy config.h which includes the macros
+`HAVE_FUNC_ATTRIBUTE_CONSTRUCTOR` and `HAVE_FUNC_ATTRIBUTE_FORMAT`, or use the
+`AX_GCC_FUNC_ATTRIBUTE` macro to check for the `constructor` and `format`
+attributes in your application's `configure.ac` file.