read.2

Unix操作系统minix 2.0源码

.\" Copyright (c) 1980 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.\"	@(#)read.2	6.6 (Berkeley) 5/23/86
.\"
.TH READ 2 "May 23, 1986"
.UC 4
.SH NAME
read \- read input
.SH SYNOPSIS
.nf
.ft B
#include <sys/types.h>
#include <unistd.h>

ssize_t read(int \fId\fP, void *\fIbuf\fP, size_t \fInbytes\fP)
.fi
.SH DESCRIPTION
.B Read
attempts to read
.I nbytes
of data from the object referenced by the descriptor
.I d
into the buffer pointed to by
.IR buf .
.PP
On objects capable of seeking, the
.B read
starts at a position
given by the pointer associated with
.IR d 
(see
.BR lseek (2)).
Upon return from
.BR read ,
the pointer is incremented by the number of bytes actually read.
.PP
Objects that are not capable of seeking always read from the current
position.  The value of the pointer associated with such an
object is undefined.
.PP
Upon successful completion,
.B read
return the number of bytes actually read and placed in the buffer.
The system guarantees to read the number of bytes requested if
the descriptor references a normal file that has that many bytes left
before the end-of-file, but in no other case.
.PP
If the returned value is 0, then
end-of-file has been reached.
.SH "RETURN VALUE
If successful, the
number of bytes actually read is returned.
Otherwise, a \-1 is returned and the global variable
.B errno
is set to indicate the error.
.SH "ERRORS
.B Read
will fail if one or more of the following are true:
.TP 15
[EBADF]
\fID\fP is not a valid descriptor open for reading.
.TP 15
[EFAULT]
\fIBuf\fP points outside the allocated address space.
.TP 15
[EIO]
An I/O error occurred while reading from the file system.
.TP 15
[EINTR]
A read from a slow device was interrupted before
any data arrived by the delivery of a signal.
.TP 15
[EAGAIN]
The file was marked for non-blocking I/O,
and no data were ready to be read.
.SH "SEE ALSO"
.BR dup (2),
.BR fcntl (2),
.BR open (2),
.BR pipe (2),
.BR write (2).