log_impl.c

#include <stdarg.h>
#include <stdio.h>
#include <inttypes.h>

#define LOG_ERR "<1>"
#define LOG_WARN "<2>"
#define LOG_INFO "<3>"
#define LOG_DEBUG "<4>"

#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
#  define MULTICHAR_CONSTANT(a,b,c,d) ((int32_t)((a) | (b) << 8 | (c) << 16 | (d) << 24))
#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
#  define MULTICHAR_CONSTANT(d,c,b,a) ((int32_t)((a) | (b) << 8 | (c) << 16 | (d) << 24))
#elif __BYTE_ORDER__ == __ORDER_PDP_ENDIAN__
#  error A PDP? Seriously?
#endif

#define log_debug(...)   _log(LOG_DEBUG __VA_ARGS__)
#define log_info(...)    _log(LOG_INFO  __VA_ARGS__)
#define log_warning(...) _log(LOG_WARN  __VA_ARGS__)
#define log_error(...)   _log(LOG_ERR   __VA_ARGS__)

enum {
    LOG_LEVEL_NONE = MULTICHAR_CONSTANT('<', '0', '>', 0),
    LOG_LEVEL_ERR = MULTICHAR_CONSTANT('<', '1', '>', 0),
    LOG_LEVEL_WARN = MULTICHAR_CONSTANT('<', '2', '>', 0),
    LOG_LEVEL_INFO = MULTICHAR_CONSTANT('<', '3', '>', 0),
    LOG_LEVEL_DEBUG = MULTICHAR_CONSTANT('<', '4', '>', 0),
};

int _log(const char *str, ...)
{
    unsigned int level = *((uint32_t *)str);

    /* ignore 4th char it may or may not be a NUL byte */
#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
    level &= 0xffffff;
#else
    level >>= 8;
#endif

    printf("%#x: %s\n", level, str + 3);

    return 0;
}

int main()
{
    log_error("this is a message");
    log_warning("this is a message");
    log_info("this is a message");
    log_debug("this is a message");

    log_debug("");

    return 0;
}