In user space, the ioctl system call has the following prototype:
int ioctl(int fd, unsigned long cmd, ...);The prototype stands out in the list of Unix system calls because of the dots, which usually mark the function as having a variable number ofarguments. In a real system, however, a system call cannot actually have a variable number of arguments. System calls must have a well-defined prototype, because user programs can access them only through hardware "gates".
So what are these hardware gates? The page numbers are 135 and 136.
Hardware "gates" are specific instructions that allow switching to the kernel's context, usually to let a program request something from the kernel. This might be an instruction like syscall, sysenter, or int 0x80, depending on your system.
I should note that these aren't usually called "hardware gates" in practice, but rather something like "system calls instructions."