how to add __FUNCTION__ into syslog - c

Question

I found the following code can add log into syslog. But if i also want to add __FUNCTION__ info to the syslog, how to do it?
openlog(basename(argv[0]), LOG_CONS | LOG_PID | LOG_NDELAY, LOG_DAEMON);
int logger(int priority, const char *format, ...) {
int result = 0;
va_list args;
va_start(args, format);
vsyslog(priority, format, args);
vfprintf(stderr, format, args);
va_end(args);
return result;
}

Answer 1

First, modify logger to accept the function name:
int logger_internal(const char* func, int priority, const char* format, ...);
But call it through the macro:
#define logger(priority, format, ...) \
logger_internal(__FUNCTION__, priority, format, __VA_ARGS__)
Modifying the body of logger_internal to take the extra parameter into account is a bit annoying, but not difficult. Probably the most straightforward way would be to prepend the function to the format argument; so something like (untested):
int logger_internal(const char* func, int priority, const char* format, ...) {
va_list args;
size_t func_l = strlen(func);
size_t format_l = strlen(format);
char* fmt = malloc(func_l + format_l + 2);
memcpy(fmt, func, func_l);
fmt[func_l] = ' ';
memcpy(&(fmt[func_l + 1]), format, format_l);
fmt[func_l + 1 + format_l] = '\0';
va_start(args, format);
vsyslog(priority, fmt, args);
va_end(args);
free(fmt);
}
But obviously that part would depend on what you wanted your syslog entry to look like.