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NAMEioctl - control device
SYNOPSISIn sys/ioctl.h Ft int Fn ioctl int fd unsigned long request ...
DESCRIPTIONThe Fn ioctl system call manipulates the underlying device parameters of special files. In particular, many operating characteristics of character special files (e.g. terminals) may be controlled with Fn ioctl requests. The argument Fa fd must be an open file descriptor.
The third argument to Fn ioctl is traditionally named char *argp Most uses of Fn ioctl , however, require the third argument to be a Vt caddr_t or an Vt int .
An Fn ioctl Fa request has encoded in it whether the argument is an ``in'' argument or ``out'' argument, and the size of the argument Fa argp in bytes. Macros and defines used in specifying an ioctl Fa request are located in the file In sys/ioctl.h .
GENERIC IOCTLSSome generic ioctls are not implemented for all types of file descriptors. These include:
- FIONREAD int
- Get the number of bytes that are immediately available for reading.
- FIONWRITE int
- Get the number of bytes in the descriptor's send queue. These bytes are data which has been written to the descriptor but which are being held by the kernel for further processing. The nature of the required processing depends on the underlying device. For TCP sockets, these bytes have not yet been acknowledged by the other side of the connection.
- FIONSPACE int
- Get the free space in the descriptor's send queue. This value is the size of the send queue minus the number of bytes being held in the queue. Note: while this value represents the number of bytes that may be added to the queue, other resource limitations may cause a write not larger than the send queue's space to be blocked. One such limitation would be a lack of network buffers for a write to a network connection.
RETURN VALUESIf an error has occurred, a value of -1 is returned and errno is set to indicate the error.
ERRORSThe Fn ioctl system call will fail if:
- Bq Er EBADF
- The Fa fd argument is not a valid descriptor.
- Bq Er ENOTTY
- The Fa fd argument is not associated with a character special device.
- Bq Er ENOTTY
- The specified request does not apply to the kind of object that the descriptor Fa fd references.
- Bq Er EINVAL
- The Fa request or Fa argp argument is not valid.
- Bq Er EFAULT
- The Fa argp argument points outside the process's allocated address space.