IOConnectCallMethod

IOConnectCallMethod is a function from the IOKit framework that lets user-space clients invoke a method exposed by a kernel object, typically an IOUserClient implemented by a kernel extension. It enables communication between user space and a driver by supplying both scalar values and a structured input, and by receiving scalar and structured outputs.

The typical signature includes:

IOReturn IOConnectCallMethod(

io_connect_t connect,

uint32_t selector,

const uint64_t* inputScalar,

uint32_t inputScalarCount,

const void* inputStruct,

size_t inputStructSize,

uint64_t* outputScalar,

uint32_t* outputScalarCount,

void* outputStruct,

size_t* outputStructSize

);

The selector selects the external method index in the kernel object's external method table. inputScalar provides an array of 64-bit values; inputStruct and inputStructSize supply an optional structured input buffer. On return, outputScalar and outputStruct contain data produced by the method, with their counts and sizes updated accordingly.

IOConnectCallMethod is often contrasted with IOConnectCallStructMethod, which provides a more specialized path when the interaction uses only structured inputs and outputs. Both functions require a valid io_connect_t obtained from opening a connection to a registered IOUserClient, and the kernel side must expose the requested method through its external methods. The call returns an IOReturn code indicating success or the type of failure.