Many system abstractions support the operations .Fn read , .Fn write , and .Fn 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. .Sh 3 "Read and write

The .Fn read and .Fn write system calls can be applied to communications channels, files, terminals and devices. They have the form: .Fd read 3 "read input cc = read(fd, buf, nbytes); result int cc; int fd; result caddr_t buf; int nbytes; .Fd write 3 "write output cc = write(fd, buf, nbytes); result int cc; int fd; caddr_t buf; int nbytes; The .Fn 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. A return value of 0 is used to indicate an end-of-file condition.

The .Fn write call transfers data from the buffer to the object defined by fd. Depending on the type of fd, it is possible that the .Fn 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, in which case no data have been transferred.

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:

The iov_base field should be treated as if its type were ``void *'' as POSIX and other versions of the structure may use that type. Thus, pointer arithmetic should not use this value without a cast. The calls using an array of iovec structures are: .Fd readv 3 "read gathered input cc = readv(fd, iov, iovlen); result int cc; int fd; struct iovec *iov; int iovlen; .Fd writev 3 "write scattered output 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. .Sh 3 "Input/output control

Control operations on an object are performed by the .Fn ioctl operation: .Fd ioctl 3 "control device 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 .Fn 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>. .Sh 3 "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 .Xr 1.5.4 . Thereafter the .Fn read call will return a specific EWOULDBLOCK error indication if there is no data to be .Fn read . The process may .Fn 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 .Fn select call indicates the object is writable.

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 .Fn select 'ed for .Fn write to find out when the operation has been completed. When .Fn select indicates the descriptor is writable, 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.