first time poking around in the sysfs attributes code and i'm a bit puzzled by something i found, i'm sure someone here (mulyadi? :-) will know the answer. the header file <linux/device.h> declares a number of show/store routines for various attribute types. for instance, here's for class: ssize_t (*show)(struct class *class, struct class_attribute *attr, char *buf); ssize_t (*store)(struct class *class, struct class_attribute *attr, const char *buf, size_t count); the above suggests that, since the show() and store() routines for class attributes take both a pointer to a class struct *AND* a pointer to an attribute itself, the same pair of routines can be used generally for a number of distinct class attributes, as long as the callback routine can distinguish among them. after all, the attributes being passed can be checked for their name field, and the show() and store() routines can then act accordingly. the same thing can be said for device attributes in that same header file: ssize_t (*show)(struct device *dev, struct device_attribute *attr, char *buf); ssize_t (*store)(struct device *dev, struct device_attribute *attr, const char *buf, size_t count); once again, the show and store routines are passed an attribute pointer so they can examine the name field of the attribute in question and act accordingly. but the same is *not* true for the bus class: ssize_t (*show)(struct bus_type *bus, char *buf); ssize_t (*store)(struct bus_type *bus, const char *buf, size_t count); note that these show and store routines *don't* incorporate an attribute pointer in the argument list, so you can't possibly define a pair of these routines that will properly process a number of attributes since those routines will have no idea how to distinguish between them, correct? here's an example from drivers/pci/pci.c: ===== start ===== static ssize_t pci_resource_alignment_show(struct bus_type *bus, char *buf) { return pci_get_resource_alignment_param(buf, PAGE_SIZE); } static ssize_t pci_resource_alignment_store(struct bus_type *bus, const char *buf, size_t count) { return pci_set_resource_alignment_param(buf, count); } BUS_ATTR(resource_alignment, 0644, pci_resource_alignment_show, pci_resource_alignment_store); static int __init pci_resource_alignment_sysfs_init(void) { return bus_create_file(&pci_bus_type, &bus_attr_resource_alignment); } ===== end ===== in the code above, a single attribute file, "resource_alignment", is created under /sys/bus/pci and, sure enough, it exists on my ubuntu system. but as you can see, the show() and store() routines associated with that file are absolutely hard-coded for that attribute only, and can't possible be used for any other attribute. is there a reason for this difference in design? if you look through <linux/device.h>, some of the attribute types (bus, driver) have show/store routines that don't take attribute pointers, while others (class, device) have show/store routines that *do*, allowing them to be more general. is this deliberate? and why? rday p.s. even when show()/store() routines take an attribute pointer and can switch among them, for the most part, the code doesn't seem to do that much, anyway. here's a perfect example from block/genhd.c, starting around line 922: static ssize_t disk_range_show(struct device *dev, struct device_attribute *attr, char *buf) { struct gendisk *disk = dev_to_disk(dev); return sprintf(buf, "%d\n", disk->minors); } static ssize_t disk_ext_range_show(struct device *dev, struct device_attribute *attr, char *buf) { struct gendisk *disk = dev_to_disk(dev); return sprintf(buf, "%d\n", disk_max_parts(disk)); } static ssize_t disk_removable_show(struct device *dev, struct device_attribute *attr, char *buf) { struct gendisk *disk = dev_to_disk(dev); return sprintf(buf, "%d\n", (disk->flags & GENHD_FL_REMOVABLE ? 1 : 0)); } ... etc etc ... note how those callback routines, given a pointer to the underlying attribute, ignore it completely, even when they're all related to an underlying "struct gendisk" and could have been written as a pair of large callbacks with a "switch" statement based on the attribute name. anyway, you get the idea. is there a reason for this difference? i may have more questions about this shortly as i read further. rday -- ======================================================================== Robert P. J. Day Ottawa, Ontario, CANADA http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday ========================================================================