calling openlog before every syslog C++

Question

I have created my own C library which my co-workers intend to use. In this wrapper, I intend to use syslog and depending on an input parameter, I wish to switch between LOCAL0 and LOCAL1.

The easiest way I found is to do openlog() with LOCAL0 or LOCAL1, depending on an input param, and then do syslog() and `closelog().

I have all 3 in the same wrapper API (something simliar to below):

void syslog_wrap_api(int flag, const char *msg)
{
setlogmask(LOG_UPTO (LOG_INFO));
if(flag == 0)
    openlog("myapplog",LOG_NDELAY,LOG_LOCAL0);
else
    openlog("myapplog",LOG_NDELAY,LOG_LOCAL1);
syslog(LOG_INFO,"%s",msg);
closelog()
}

My question is, can this API cause problems under stress (performance issues)?

Answer 1

This is, in my opinion, the wrong way to go about it, but the documentation is confusing.

Whether you have problems under stress is going to be be determined by the implementation of openlog in your C library. It might well be problematic. More to the point, in a multithreaded environment, one thread may issue the syslog after another thread issues the openlog, meaning the wrong priority is used.

The key to how to do this right is in the man page:

The facility argument establishes a default to be used if none is specified in subsequent calls to syslog(). Values for option and facility are given below ... The priority argument is formed by OR-ing the facility and the level values (explained below).

The clue as to how to exploit this is here:

The use of openlog() is optional; it will automatically be called by syslog() if necessary, in which case ident will default to NULL.

If you don't use openlog, there must be some way of expressing the facility. The answer is that you can specify the facility as zero to openlog (or not use it at all), and OR in the facility to the priority argument of syslog.

So, something like:

syslog(LOG_INFO | (flag?LOG_LOCAL1:LOG_LOCAL0),"%s",msg);