An example of how to extend printf() using a thread local arena to display custom data types.

A_Printf_Extension_Example

 ...
 

demo.c

/*
*/

	#include "printf_ext.h"

//********************************************************************************************************
// Public functions
//********************************************************************************************************

int main(int argc, const char* argv[])
{
	(void)argc;(void)argv;

	complex float ca = 23.471 + 41.231*I;
	complex float cb = -18.19 + 9.473*I;

	printf_ext("\n\n\
This GIST demonstrates an easy way of extending printf\n\
Instead of calling printf() directly, call a wrapper function called printf_ext().\n\
printf_ext() uses vprintf(), but also resets a thread local string arena.\n\
This string arena can then be used by functions which describe whatever types you like.\n\
In this example, we can use prext_cplex_pol() and prext_cplex_rec() to display complex values in polar or rectangular form\n\
\n\
Some values in rectangular form:\n\
%s * %s = %s rectangular\n\
\n\
Some values in polar form:\n\
%s * %s = %s polar\n\
\n\
One of the benefits of extending printf in this way, is that %%s also takes a width parameter and offers left & right alignment\n\
so you can align your text. eg;\n\
%25s * %-25s = %s \n\
%25s * %-25s = %s \n\
\n"
, prext_cplex_rec(ca), prext_cplex_rec(cb), prext_cplex_rec(ca * cb)
, prext_cplex_pol(ca), prext_cplex_pol(cb), prext_cplex_pol(ca * cb)
, prext_cplex_rec(ca), prext_cplex_rec(cb), prext_cplex_rec(ca * cb)
, prext_cplex_pol(ca), prext_cplex_pol(cb), prext_cplex_pol(ca * cb)
);

	return 0;
}

Makefile

#----------------------------------------------------------------------------
#

# Target file name (without extension).
TARGET = demo

# List C source files here. (C dependencies are automatically generated.)
# To exclude certain files in a folder remove the $(wildcard) and 
# list them seperated by spaces, ie src/main.c src/util.c 
SRC = $(wildcard *.c)

# List any extra directories to look for include files here.
#     Each directory must be seperated by a space.
#     Use forward slashes for directory separators.
#     For a directory that has spaces, enclose it in quotes.
EXTRAINCDIRS = . 

# Object and list files directory
#     To put .o and .lst files alongside .c files use a dot (.), do NOT make
#     this an empty or blank macro!
#     If source files are in sub directories, matching subdirectories must exist under this folder for the .o files
#	  This is a pain, if you can fix this, please do and share.
OBJLSTDIR = .

# Compiler flag to set the C Standard level.
#     c89   = "ANSI" C
#     gnu89 = c89 plus GCC extensions
#     c99   = ISO C99 standard (not yet fully implemented)
#     gnu99 = c99 plus GCC extensions
CSTANDARD = -std=gnu99

# Place -D or -U options here for C sources
CDEFS =


#---------------- Compiler Options C ----------------
#  -g 			 debug information
#  -f...:        tuning, see GCC manual and avr-libc documentation
#  -Wall...:     warning level
CFLAGS += $(CDEFS)
CFLAGS += -Wall
CFLAGS += -Wno-unused-function
CFLAGS += -Wno-unused-but-set-variable
CFLAGS += $(CSTANDARD)
CFLAGS += $(patsubst %,-I%,$(EXTRAINCDIRS))
CFLAGS += -fsanitize=address
CFLAGS += -Wextra
CFLAGS += -fsanitize=undefined

# List any extra directories to look for libraries here.
#     Each directory must be seperated by a space.
#     Use forward slashes for directory separators.
#     For a directory that has spaces, enclose it in quotes.
EXTRALIBDIRS = .
EXTRALIBS = -lm

#---------------- Linker Options ----------------

LDFLAGS = $(patsubst %,-L%,$(EXTRALIBDIRS))
LDFLAGS += $(EXTRALIBS)

#============================================================================

# Define programs and commands.
SHELL = sh
CC = gcc
REMOVE = rm -f
REMOVEDIR = rm -rf
COPY = cp

# Define Messages
# English
MSG_ERRORS_NONE = Errors: none
MSG_BEGIN = -------- begin --------
MSG_END = --------  end  --------
MSG_LINKING = Linking:
MSG_COMPILING = Compiling C:
MSG_CLEANING = Cleaning project:

# Define all object files.
OBJ = $(SRC:%.c=$(OBJLSTDIR)/%.o)

# Compiler flags to generate dependency files.
GENDEPFLAGS = -MMD -MP -MF .dep/$(@F).d

# Combine all necessary flags and optional flags.
# Add target processor to flags.
ALL_CFLAGS = -I. $(CFLAGS) $(GENDEPFLAGS)

# Default target.
all: begin gccversion build end


build: tgt

tgt: $(TARGET)

# Eye candy.
# the following magic strings to be generated by the compile job.
begin:
	@echo
	@echo $(MSG_BEGIN)

end:
	@echo $(MSG_END)
	@echo

# Display compiler version information.
gccversion : 
	@$(CC) --version


# Link: create output file from object files.
.SECONDARY : $(TARGET)
.PRECIOUS : $(OBJ)
$(TARGET): $(OBJ)
	@echo
	@echo $(MSG_LINKING) $@
	$(CC) $(ALL_CFLAGS) $^ --output $@ $(LDFLAGS)

# Compile: create object files from C source files.
$(OBJLSTDIR)/%.o : %.c
	@echo
	@echo $(MSG_COMPILING) $<
	$(CC) -c $(ALL_CFLAGS) $< -o $@ 

# Target: clean project.
clean: begin clean_list end

clean_list :
	@echo
	@echo $(MSG_CLEANING)
	$(REMOVE) $(SRC:%.c=$(OBJLSTDIR)/%.o)
	$(REMOVE) $(SRC:%.c=$(OBJLSTDIR)/%.lst)
	$(REMOVEDIR) .dep

# Create object files directory
$(shell mkdir $(OBJLSTDIR) 2>/dev/null)

# Include the dependency files.
-include $(shell mkdir .dep 2>/dev/null) $(wildcard .dep/*)

# Listing of phony targets.
.PHONY : all begin end  gccversion build tgt clean clean_list 

printf_ext.c

/*
  Extendable printf wrapper using a thread local string arena for the extensions.
*/

	#include <assert.h>
	#include <stdarg.h>
	#include <complex.h>
	#include <stdio.h>

	#include "printf_ext.h"

//********************************************************************************************************
// Configurable defines
//********************************************************************************************************

	#define PRINTF_EXT_ARENA_SIZE 1024

//********************************************************************************************************
// Local defines
//********************************************************************************************************

	#define TERMINATOR_SIZE	1

	struct arena_struct
	{
		char body[PRINTF_EXT_ARENA_SIZE];
		char* ptr;
		int remaining_size;
	};

//********************************************************************************************************
// Private variables
//********************************************************************************************************

	static __thread struct arena_struct arena = {.ptr = NULL};

//********************************************************************************************************
// Private prototypes
//********************************************************************************************************

	static void arena_reset(struct arena_struct* arena);
	static void arena_prepare(struct arena_struct* arena);
	static char* arena_advance(struct arena_struct* arena, int char_count);

//********************************************************************************************************
// Public functions
//********************************************************************************************************

int printf_ext(const char* fmt, ...)
{
	va_list va;
	va_start(va, fmt);
	const int retval = vprintf_ext(fmt, va);
	va_end(va);
	return retval;
}

int vprintf_ext(const char* fmt, va_list va)
{
	const int retval = vprintf(fmt, va);
	arena_reset(&arena);
	return retval;
}

char* prext_cplex_pol(complex double c)
{
	arena_prepare(&arena);

	int char_count = snprintf(arena.ptr, arena.remaining_size, "(%.3f @%.3f)", cabs(c), carg(c));

	return arena_advance(&arena, char_count);
}

char* prext_cplex_rec(complex double c)
{
	arena_prepare(&arena);

	int char_count =  snprintf(arena.ptr, arena.remaining_size, "(%.3f, %.3f)", creal(c), cimag(c));

	return arena_advance(&arena, char_count);
}

//********************************************************************************************************
// Private functions
//********************************************************************************************************

static void arena_reset(struct arena_struct* arena)
{
	arena->ptr = NULL;
}

static void arena_prepare(struct arena_struct* arena)
{
	if(arena->ptr == NULL)
	{
		arena->ptr = arena->body;
		arena->remaining_size = PRINTF_EXT_ARENA_SIZE;
	};
}

static char* arena_advance(struct arena_struct* arena, int char_count)
{
	char* retval = arena->ptr;
	int size = char_count + TERMINATOR_SIZE;
	assert(size <= arena->remaining_size);
	arena->ptr += size;
	arena->remaining_size -= size;
	return retval;
}

printf_ext.h

	#include <stdarg.h>
	#include <complex.h>

//********************************************************************************************************
// Public prototypes
//********************************************************************************************************

	int printf_ext(const char* fmt, ...)	__attribute__((format(printf, 1, 2)));
	int vprintf_ext(const char* fmt, va_list va);

	char* prext_cplex_pol(complex double c);
	char* prext_cplex_rec(complex double c);