![[BACK]](/cvsweb/icons/back.gif){kind=icon}
Return to vread.2v CVS log ![[TXT]](/cvsweb/icons/text.gif){kind=icon}
![[DIR]](/cvsweb/icons/dir.gif){kind=icon}
Up to [Research Unix] / researchv10dc / man / manb|
Return to vread.2v CVS log | Up to [Research Unix] / researchv10dc / man / manb |
1.1 root 1: .TH VREAD 2V deprecated
2: .UC 4
3: .SH NAME
4: vread \- read virtually
5: .SH SYNOPSIS
6: .nf
7: .B vread(fildes, buffer, nbytes)
8: .B char *buffer;
9: .fi
10: .SH DESCRIPTION
11: .B N.B.:
12: This call is likely to be replaced by more general virtual memory facilities
13: in the near future.
14: .PP
15: A file descriptor is a word returned from a successful
16: .I open,
17: .I creat,
18: .I dup
19: or
20: .I pipe
21: call.
22: .I Buffer
23: is the location of
24: .I nbytes
25: contiguous bytes into which the input will be placed.
26: It is not guaranteed that all
27: .I nbytes
28: will be read (see
29: .IR read (2)).
30: In particular, if the returned value is 0, then end-of-file has been reached.
31: .PP
32: Unlike
33: .IR read (2),
34: .I vread
35: does not necessarily or immediately fetch the data requested from
36: .I fildes,
37: but merely insures that the data will be fetched from the file descriptor
38: .I "sometime before"
39: the first reference to the data, at the system's discretion.
40: Thus
41: .I vread
42: allows the system, among other possibilities,
43: to choose to read data on demand,
44: with whatever granularity is allowed by the memory management hardware,
45: or to just read it in immediately as with
46: .I read.
47: A companion
48: .IR vwrite (2)
49: call may be used with
50: .I vread
51: to provide an efficient mechanism for updating large files.
52: The behavior of
53: .I vread
54: if other processes are writing to
55: .I fildes
56: is not defined.
57: .PP
58: Both the address of
59: .I buffer
60: and the current offset in
61: .I fildes
62: (as told by
63: .IR tell (2))
64: must be aligned to a multiple of VALSIZ (defined in
65: .B <valign.h> ).
66: The library routine
67: .IR valloc (3)
68: allocates properly aligned blocks from the free list.
69: .PP
70: Note for non-virtual systems: the
71: .I vread
72: system call can be simulated (exactly, if less efficiently) by
73: .I read.
74: If the unit on which a
75: .I vread
76: is done is not capable of supporting efficient demand initialization
77: (e.g. a terminal or a pipe), then the system may choose to treat a call to
78: .I vread
79: as if it were a call to
80: .I read
81: at its discretion.
82: .SH SEE ALSO
83: read(2), write(2), vwrite (2), valloc(3)
84: .SH DIAGNOSTICS
85: A 0 is returned at end-of-file. If the read was otherwise unsuccessful,
86: a -1 is returned. Physical I/O errors, non-aligned or bad buffer addresses,
87: preposterous
88: .I nbytes,
89: file descriptor not that of an input file, and file offset not properly
90: aligned can all generate errors.
91: .SH BUGS
92: You can't
93: .I close
94: a file descriptor which you have
95: .I vread
96: from while there are still pages in the file which haven't been fetched by the
97: system into your address space. In no case can a file descriptor which had
98: such pages at the point of a
99: .I vfork
100: be closed during the
101: .I vfork.
102: .PP
103: The system refuses to truncate a file to which any process has a pending
104: .I vread.
105: .PP
106: There is no primitive inverting
107: .I vread
108: to release the binding
109: .I vread
110: sets up so that the file may be closed.
111: This can be only be done, clumsily, by reading another (plain) file onto the
112: buffer area, or pulling the break back with
113: .IR break (2)
114: to completely release the pages.
115: .PP
116: This call is peculiar to this version of UNIX.
117: It will be superseded by more general
118: virtual memory facilities in future versions of the system.
This archive runs on limited infrastructure. Preserving old code on modern bandwidth. Automated agents are requested to crawl responsibly.