The 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 ioctl requests. The argument d must be an open file descriptor.
The third argument to ioctl is traditionally named "char *argp". Most uses of ioctl in
.Fx 3.0 , however, require the third argument to be a
.Vt caddr_t or an
.Vt int .
An ioctl request has encoded in it whether the argument is an "in" argument or "out" argument, and the size of the argument argp in bytes. Macros and defines used in specifying an ioctl request are located in the file
.In sys/ioctl.h .
If an error has occurred, a value of -1 is returned and errno is set to indicate the error.
The ioctl system call will fail if: