See sd_listen_fds(3), sd_notify(3), sd_booted(3), sd_is_fifo(3) for more information about the functions implemented. In addition to these functions a couple of logging prefixes are defined as macros:

#define SD_EMERG   "<0>"  /* system is unusable */
#define SD_ALERT   "<1>"  /* action must be taken immediately */
#define SD_CRIT    "<2>"  /* critical conditions */
#define SD_ERR     "<3>"  /* error conditions */
#define SD_WARNING "<4>"  /* warning conditions */
#define SD_NOTICE  "<5>"  /* normal but significant condition */
#define SD_INFO    "<6>"  /* informational */
#define SD_DEBUG   "<7>"  /* debug-level messages */

These prefixes are intended to be used in conjunction with STDERR-based logging as implemented by systemd. If a systemd service definition file is configured with StandardError=syslog or StandardError=kmsg these prefixes can be used to encode a log level in lines printed. This is similar to the kernel printk()-style logging. See klogctl(2) for more information.

The log levels are identical to syslog(3)'s log level system. To use these prefixes simply prefix every line with one of these strings. A line that is not prefixed will be logged at the default log level SD_INFO.

Example 1. Hello World

A daemon may log with the log level NOTICE by issuing this call:

fprintf(stderr, SD_NOTICE "Hello World!\n");