Reimplementation of printf() using printf() in C - c

Question

I'm making a embedded linux project and I want to do a simple debug messages library where I can disable my debug messages (using pre-compilation directives) when my code is in production phase and substitute for some type of log in a database (in the future). I'm having trouble with reimplementation of printf() because of va_list. This is my code until now:
My source file:
#include "debug_msgs.h"
#define ENABLE_DEBUG_MSGS 1U
#if ENABLE_DEBUG_MSGS
int print(const char *fmt, ...) {
int n = -1;
va_list ap;
va_start(ap, fmt);
n = printf(fmt, ap);
va_end(ap);
return n;
}
#else
int print(const char *fmt, ...) {
return 0;
}
#endif
My header file:
#ifndef DEBUG_MSGS_H
#define DEBUG_MSGS_H
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdarg.h>
#include <stdint.h>
#include <unistd.h>
int print(const char *fmt, ...);
#endif
My problem is:
My implementation compiles (I know this doesn't mean nothing), but when I do a simple test like:
int num1 = 10;
int num2 = 100;
int num3 = 1000;
float floating = 3.14;
char str[] = {"Please work, please"};
int number=-1;
number = print("\nTesting %d %d %d %f %s\n", num1, num2, num3, floating, str);
printf("\nnumber = %d\n",number); //Just to check the return
I get this output:
Testing -1095005212 100 -1095005212 -0.000002 H*�
number = 52
Complementary informations:
This is my platform info:
Linux mdm9607 3.18.44 #1 PREEMPT Tue Sep 13 19:45:33 UTC 2022 armv7l GNU/Linux
My compiler info:
arm-oe-linux-gnueabi-gcc
My $CFLAGS:
-O2 -fexpensive-optimizations -frename-registers -fomit-frame-pointer -MMD -MP
My $LDFLAGS:
-Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed
LDFLAGS += -lumdp -lumdpcommon
LDFLAGS += -lpthread -lxml2 -lrt
LDFLAGS += -lm

Answer 1

Instead of printf, you should be using vprintf or vfprintf with a va_list argument:
#include <stdarg.h>
#include <stdio.h>
int print(const char *fmt, ...) {
int n;
va_list ap;
va_start(ap, fmt);
n = vprintf(fmt, ap);
va_end(ap);
return n;
}
The erroneous code compiles because printf, as a vararg function, can accept any argument types after the format string. Yet you can avoid type mismatches for printf arguments by enabling more compiler warnings: gcc -Wall -Wextra -Werror would at least complain about using printf with a non constant format string and would perform compile time type checking if the format string is a string literal.