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ioctl – control device
- int ioctl(int fildes , int request, /* arg */ …);
ioctl() performs a variety of control functions on devices and STREAMS. For non-STREAMS files, the functions performed by this call are device-specific control functions. request and an optional third argument with varying type are passed to the file designated by fildes and are interpreted by the device driver.
fildes is an open file descriptor that refers to a device. request selects the control function to be performed and depends on the device being addressed. arg represents a third argument that has additional information that is needed by this specific device to perform the requested function. The data type of arg depends upon the particular control request, but it is either an int or a pointer to a device-specific data structure.
In addition to device-specific and STREAMS functions, generic functions are provided by more than one device driver, for example, the general terminal interface.
Refer to each specific Unison server for a list of supported IOCTL requests.
Upon successful completion, the value returned depends upon the device control function, but must be a non-negative integer. Otherwise, a value of -1 is returned and errno is set to indicate the error.
This function is a member of Unison’s IOLIB family of functions. IOLIB is implemented as a message passing and generalized interface layer. Each Unison I/O server is responsible for its own error reporting.
For an exact list of error codes returned by a particular server, refer to that server’s documentation in the Unison Programmer’s Guide for each specific platform.
Servers may implement these errors codes in response to this function.
ioctl() fails for any type of file if one or more of the following are true:
- fildes is not a valid open file descriptor.
- fildes is not associated with a device driver that accepts control functions.
ioctl() also fails if the device driver detects an error. In this case, the error is passed through ioctl() without change to the caller. A particular driver might not have all of the following error cases. Under the following conditions, requests to device drivers may fail and set errno to:
- request requires a data transfer to or from a buffer pointed to by but arg points to an illegal address.
- request or arg is not valid for this device.
- Some physical I/O error has occurred.
- fildes is on a remote machine and the link to that machine is no longer active.
- The request and arg are valid for this device driver, but the service requested can not be performed on this particular subdevice.