Linux Security Cookbook

9.34. Writing Log Entries via C

Problem

You want to add information to the system log from a C program.

Solution

Use the system library functions openlog , syslog, and closelog (see The syslog API):

               syslog-demo.c:
#define _GNU_SOURCE     /* for basename( ) in <string.h> */
#include <syslog.h>
#include <string.h>
int count = 0;
char *host = "some-machine";
int main(int argc, char *argv[]) {
        openlog(basename(argv[0]), LOG_PID, LOG_LOCAL3);
        syslog(LOG_WARNING, "%d connection attempts from %s", count, host);
        syslog(LOG_AUTHPRIV|LOG_ERR, "intruder alert!");
        syslog(LOG_ERR, "can't open configuration file: %m");
        closelog( );
        return(0);
}

Discussion

Like Perl scripts [Recipe 9.33], C programs can pass the %m format specifier to syslog to include system error messages, corresponding to strerror(errno). Be sure to use %m only when a system error has occurred, to avoid misleading messages.

See Also

syslog(3).