systemd sd_device_ref 3 sd_device_ref sd_device_unref sd_device_unrefp Create or destroy references to a device object #include <systemd/sd-device.h> sd_device* sd_device_ref sd_device *device sd_device* sd_device_unref sd_device *device void sd_device_unrefp sd_device **device sd_device_ref() increases the internal reference counter of device by one. sd_device_unref() decreases the internal reference counter of device by one. Once the reference count has dropped to zero, device is destroyed and cannot be used anymore, so further calls to sd_device_ref() or sd_device_unref() are illegal. sd_device_unrefp() is similar to sd_device_unref() but takes a pointer to a pointer to an sd_device object. This call is useful in conjunction with GCC's and LLVM's Clean-up Variable Attribute. Note that this function is defined as an inline function. Use a declaration like the following, in order to allocate a device object that is freed automatically as the code block is left: { __attribute__((cleanup(sd_device_unrefp))) sd_device *device = NULL; int r; … r = sd_device_new_from_syspath(&device, "…"); if (r < 0) { errno = -r; fprintf(stderr, "Failed to allocate device: %m\n"); } … } sd_device_ref() and sd_device_unref() execute no operation if the argument is NULL. sd_device_unrefp() will first dereference its argument, which must not be NULL, and will execute no operation if that is NULL. sd_device_ref() always returns the argument, and sd_device_unref() always returns NULL.