Q: When using I/O Kit, how can I figure out what these huge error codes
mean? A: I/O Kit uses the same error encoding scheme used by the Mach
kernel. I/O Kit errors are 32 bits long and their layout is shown in Listing 1
(from mach/error.h).
hi lo
| system(6) | subsystem(12) | code(14) |
| | Listing 1. error number layout |
The hi 6 bits are known as the system value, the next
12 bits are known as the subsystem value, and the low 14 bits make up the error
code. The first place to look for information
about error codes is IOKit/IOReturn.h. Here you'll find a list of
all the generic I/O Kit errors that aren't specific to a particular family. Now
for an example... By using the information in IOReturn.h,
we can determine that an I/O Kit error of -536870206, which in hex is 0xE00002C2,
will map to... |
system | = 0x38 |
which is equal to sys_iokit error. | | subSystem |
= 0x0 | so we know it's a sub_iokit_common
error. | | code |
= 0x2C2 | which means this is a kIOReturnBadArgument
error. |
There are convenient macros for extracting these
three values from an I/O Kit error code which are shown in Listing 2 (from mach/error.h).
#define err_get_system(err) (((err)>>26)&0x3f)
#define err_get_sub(err) (((err)>>14)&0xfff)
#define err_get_code(err) ((err)&0x3fff)
|
| Listing 2. macros to extract I/O Kit error information. |
It's also possible to get error codes back from I/O Kit with a system value
of sys_mach_ipc (0x4, defined in mach/error.h).The most
common of these is 268435459 or hex 0x10000003 which is the error MACH_SEND_INVALID_DEST
defined in the Kernel.framework header mach/message.h. To understand
why you can get a Mach error code, it helps to understand that I/O Kit uses Mach
messages to communicate between user space and the kernel. The io_service_t
type passed to calls such as IOServiceCreatePlugInInterfaceForService
is defined as mach_port_t in the IOKit.framework header IOTypes.h.
So, a MACH_SEND_INVALID_DEST error in this context means that a io_service_t
is invalid. This can mean either that the io_service_t is uninitialized
or that it was previously freed. Remember also that each I/O Kit family
defines its own family-specific error codes in a different header file. For example,
USB error codes are located in IOKit/usb/USB.h inside Kernel.framework,
while most FireWire error codes are located in IOKit/firewire/IOFireWireFamilyCommon.h,
also inside Kernel.framework.
[Feb
06 2002]
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