sd-daemon, SD_EMERG, SD_ALERT, SD_CRIT, SD_ERR, SD_WARNING, SD_NOTICE, SD_INFO, SD_DEBUG - APIs for new-style daemons
pkg-config --cflags --libs libsystemd
sd-daemon.h provides APIs for new-style daemons, as implemented by the systemd(1) service manager.
See sd_listen_fds(3), sd_notify(3), sd_booted(3), sd_is_fifo(3), sd_watchdog_enabled(3) for more information about the functions implemented. In addition to these functions, a couple of logging prefixes are defined as macros:
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 (or stdout-based logging) as implemented by systemd. If a systemd service definition file is configured with StandardError=journal or StandardError=kmsg (and similar with StandardOutput=), 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");
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.