Many system abstractions support the operations read, write and ioctl. We describe the basics of these common primitives here. Similarly, the mechanisms whereby normally synchronous operations may occur in a non-blocking or asynchronous fashion are common to all system-defined abstractions and are described here. Read and write

The read and write system calls can be applied to communications channels, files, terminals and devices. They have the form: cc = read(fd, buf, nbytes); result int cc; int fd; result caddr_t buf; int nbytes; cc = write(fd, buf, nbytes); result int cc; int fd; caddr_t buf; int nbytes; The read call transfers as much data as possible from the object defined by fd to the buffer at address buf of size nbytes. The number of bytes transferred is returned in cc, which is -1 if a return occurred before any data was transferred because of an error or use of non-blocking operations.

The write call transfers data from the buffer to the object defined by fd. Depending on the type of fd, it is possible that the write call will accept some portion of the provided bytes; the user should resubmit the other bytes in a later request in this case. Error returns because of interrupted or otherwise incomplete operations are possible.

Scattering of data on input or gathering of data for output is also possible using an array of input/output vector descriptors. The type for the descriptors is defined in <sys/uio.h> as: ._f struct iovec { caddr_t iov_msg; /* base of a component */ int iov_len; /* length of a component */ }; The calls using an array of descriptors are: cc = readv(fd, iov, iovlen); result int cc; int fd; struct iovec *iov; int iovlen; cc = writev(fd, iov, iovlen); result int cc; int fd; struct iovec *iov; int iovlen; Here iovlen is the count of elements in the iov array. Input/output control

Control operations on an object are performed by the ioctl operation: ioctl(fd, request, buffer); int fd, request; caddr_t buffer; This operation causes the specified request to be performed on the object fd. The request parameter specifies whether the argument buffer is to be read, written, read and written, or is not needed, and also the size of the buffer, as well as the request. Different descriptor types and subtypes within descriptor types may use distinct ioctl requests. For example, operations on terminals control flushing of input and output queues and setting of terminal parameters; operations on disks cause formatting operations to occur; operations on tapes control tape positioning.

The names for basic control operations are defined in <sys/ioctl.h>. Non-blocking and asynchronous operations

A process that wishes to do non-blocking operations on one of its descriptors sets the descriptor in non-blocking mode as described in section 1.5.4. Thereafter the read call will return a specific EWOULDBLOCK error indication if there is no data to be read. The process may select the associated descriptor to determine when a read is possible.

Output attempted when a descriptor can accept less than is requested will either accept some of the provided data, returning a shorter than normal length, or return an error indicating that the operation would block. More output can be performed as soon as a select call indicates the object is writeable.

Operations other than data input or output may be performed on a descriptor in a non-blocking fashion. These operations will return with a characteristic error indicating that they are in progress if they cannot complete immediately. The descriptor may then be selected for write to find out when the operation has been completed. When select indicates the descriptor is writeable, the operation has completed. Depending on the nature of the descriptor and the operation, additional activity may be started or the new state may be tested.