SD_DEVICE_REF(3)                 sd_device_ref                SD_DEVICE_REF(3)

NAME
       sd_device_ref, sd_device_unref, sd_device_unrefp - Create or destroy
       references to a device object

SYNOPSIS
       #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[1]. 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.

RETURN VALUE
       sd_device_ref() always returns the argument, and sd_device_unref()
       always returns NULL.

HISTORY
       sd_device_ref(), sd_device_unref(), and sd_device_unrefp() were added
       in version 251.

NOTES
        1. Clean-up Variable Attribute
           https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html

systemd 255                                                   SD_DEVICE_REF(3)