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SD_DEVICE_REF(3) sd_device_ref 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[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)
fprintf(stderr, "Failed to allocate device: %s\n", strerror(-r));
... }

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.


Clean-up Variable Attribute
systemd 251