Linux Security Cookbook

9.34.1 Problem

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

9.34.2 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); }

9.34.3 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.

9.34.4 See Also

syslog(3).