[PATCH v3 3/9] argparse: allow user-override of help printing

[Bruce Richardson <bruce.richardson@intel.com>]

When the arguments passed to argparse include -h/--help then usage
information is automatically printed. Provide the capability for the
user to supply their own help function for this.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
---
 doc/guides/prog_guide/argparse_lib.rst | 16 ++++++++++++++++
 lib/argparse/rte_argparse.c            |  5 ++++-
 lib/argparse/rte_argparse.h            |  7 ++++++-
 3 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/doc/guides/prog_guide/argparse_lib.rst b/doc/guides/prog_guide/argparse_lib.rst
index 9f11714890..b309260d20 100644
--- a/doc/guides/prog_guide/argparse_lib.rst
+++ b/doc/guides/prog_guide/argparse_lib.rst
@@ -24,6 +24,8 @@ Features and Capabilities
   #. autosave: used for parsing known value types;
   #. callback: will invoke user callback to parse.
 
+- Supports automatic help and usage information.
+
 Usage Guide
 -----------
 
@@ -193,3 +195,17 @@ Then the user input could contain multiple ``--xyz`` arguments.
 
    The multiple times argument only support with optional argument
    and must be parsed by callback way.
+
+Help and Usage Information
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The argparse library supports automatic generation of help and usage information.
+When the input arguments include ``-h`` or ``--help``,
+it will print the usage information to standard output.
+If the default help output is not what is wanted,
+the user can provide a custom help printing function by setting the ``print_help`` field in the ``rte_argparse`` object.
+(If this field is set to NULL, the default help printing function will be used.)
+
+If the custom help printing function wants to use the text produced by the default help function,
+it can call the function ``rte_argparse_print_help()`` to get the help text printed to an output stream,
+for example: stdout or stderr.
diff --git a/lib/argparse/rte_argparse.c b/lib/argparse/rte_argparse.c
index d3b32c6357..e7b9bf573d 100644
--- a/lib/argparse/rte_argparse.c
+++ b/lib/argparse/rte_argparse.c
@@ -821,7 +821,10 @@ rte_argparse_parse(const struct rte_argparse *obj, int argc, char **argv)
 		goto error;
 
 	if (show_help) {
-		rte_argparse_print_help(stdout, obj);
+		if (obj->print_help != NULL)
+			obj->print_help(obj);
+		else
+			rte_argparse_print_help(stdout, obj);
 		exit(0);
 	}
 
diff --git a/lib/argparse/rte_argparse.h b/lib/argparse/rte_argparse.h
index baf278f6b9..63b49ba220 100644
--- a/lib/argparse/rte_argparse.h
+++ b/lib/argparse/rte_argparse.h
@@ -160,8 +160,13 @@ struct rte_argparse {
 	rte_arg_parser_t callback;
 	/** Opaque which used to invoke callback. */
 	void *opaque;
+	/**
+	 * Function pointer for printing usage when -h is passed.
+	 * If this is NULL, default printing function will be used.
+	 */
+	void (*print_help)(const struct rte_argparse *obj);
 	/** Reserved field used for future extension. */
-	void *reserved[16];
+	void *reserved[15];
 	/** Arguments configuration. Must ended with ARGPARSE_ARG_END(). */
 	struct rte_argparse_arg args[];
 };
-- 
2.48.1