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1.1 root 1: .\" Copyright (c) 1983 Regents of the University of California.
2: .\" All rights reserved. The Berkeley software License Agreement
3: .\" specifies the terms and conditions for redistribution.
4: .\"
5: .\" @(#)2.1.t 6.2 (Berkeley) 5/12/86
6: .\"
7: .sh "Generic operations
8: .PP
9: .PP
10: Many system abstractions support the
11: operations \fIread\fP, \fIwrite\fP and \fIioctl\fP. We describe
12: the basics of these common primitives here.
13: Similarly, the mechanisms whereby normally synchronous operations
14: may occur in a non-blocking or asynchronous fashion are
15: common to all system-defined abstractions and are described here.
16: .NH 3
17: Read and write
18: .PP
19: The \fIread\fP and \fIwrite\fP system calls can be applied
20: to communications channels, files, terminals and devices.
21: They have the form:
22: .DS
23: cc = read(fd, buf, nbytes);
24: result int cc; int fd; result caddr_t buf; int nbytes;
25:
26: cc = write(fd, buf, nbytes);
27: result int cc; int fd; caddr_t buf; int nbytes;
28: .DE
29: The \fIread\fP call transfers as much data as possible from the
30: object defined by \fIfd\fP to the buffer at address \fIbuf\fP of
31: size \fInbytes\fP. The number of bytes transferred is
32: returned in \fIcc\fP, which is \-1 if a return occurred before
33: any data was transferred because of an error or use of non-blocking
34: operations.
35: .PP
36: The \fIwrite\fP call transfers data from the buffer to the
37: object defined by \fIfd\fP. Depending on the type of \fIfd\fP,
38: it is possible that the \fIwrite\fP call will accept some portion
39: of the provided bytes; the user should resubmit the other bytes
40: in a later request in this case.
41: Error returns because of interrupted or otherwise incomplete operations
42: are possible.
43: .PP
44: Scattering of data on input or gathering of data for output
45: is also possible using an array of input/output vector descriptors.
46: The type for the descriptors is defined in \fI<sys/uio.h>\fP as:
47: .DS
48: ._f
49: struct iovec {
50: caddr_t iov_msg; /* base of a component */
51: int iov_len; /* length of a component */
52: };
53: .DE
54: The calls using an array of descriptors are:
55: .DS
56: cc = readv(fd, iov, iovlen);
57: result int cc; int fd; struct iovec *iov; int iovlen;
58:
59: cc = writev(fd, iov, iovlen);
60: result int cc; int fd; struct iovec *iov; int iovlen;
61: .DE
62: Here \fIiovlen\fP is the count of elements in the \fIiov\fP array.
63: .NH 3
64: Input/output control
65: .PP
66: Control operations on an object are performed by the \fIioctl\fP
67: operation:
68: .DS
69: ioctl(fd, request, buffer);
70: int fd, request; caddr_t buffer;
71: .DE
72: This operation causes the specified \fIrequest\fP to be performed
73: on the object \fIfd\fP. The \fIrequest\fP parameter specifies
74: whether the argument buffer is to be read, written, read and written,
75: or is not needed, and also the size of the buffer, as well as the
76: request.
77: Different descriptor types and subtypes within descriptor types
78: may use distinct \fIioctl\fP requests. For example,
79: operations on terminals control flushing of input and output
80: queues and setting of terminal parameters; operations on
81: disks cause formatting operations to occur; operations on tapes
82: control tape positioning.
83: .PP
84: The names for basic control operations are defined in \fI<sys/ioctl.h>\fP.
85: .NH 3
86: Non-blocking and asynchronous operations
87: .PP
88: A process that wishes to do non-blocking operations on one of
89: its descriptors sets the descriptor in non-blocking mode as
90: described in section 1.5.4. Thereafter the \fIread\fP call will
91: return a specific EWOULDBLOCK error indication if there is no data to be
92: \fIread\fP. The process may
93: \fIselect\fP the associated descriptor to determine when a read is
94: possible.
95: .PP
96: Output attempted when a descriptor can accept less than is requested
97: will either accept some of the provided data, returning a shorter than normal
98: length, or return an error indicating that the operation would block.
99: More output can be performed as soon as a \fIselect\fP call indicates
100: the object is writeable.
101: .PP
102: Operations other than data input or output
103: may be performed on a descriptor in a non-blocking fashion.
104: These operations will return with a characteristic error indicating
105: that they are in progress
106: if they cannot complete immediately. The descriptor
107: may then be \fIselect\fPed for \fIwrite\fP to find out
108: when the operation has been completed. When \fIselect\fP indicates
109: the descriptor is writeable, the operation has completed.
110: Depending on the nature of the descriptor and the operation,
111: additional activity may be started or the new state may be tested.
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