267 lines
6.4 KiB
Groff
267 lines
6.4 KiB
Groff
.\" Copyright (c) 1980, 1991, 1993
|
|
.\" The Regents of the University of California. All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\" 3. All advertising materials mentioning features or use of this software
|
|
.\" must display the following acknowledgement:
|
|
.\" This product includes software developed by the University of
|
|
.\" California, Berkeley and its contributors.
|
|
.\" 4. Neither the name of the University nor the names of its contributors
|
|
.\" may be used to endorse or promote products derived from this software
|
|
.\" without specific prior written permission.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
|
|
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
|
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
|
|
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
|
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
|
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
|
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
|
.\" SUCH DAMAGE.
|
|
.\"
|
|
.\" @(#)read.2 8.4 (Berkeley) 2/26/94
|
|
.\" $FreeBSD$
|
|
.\"
|
|
.Dd February 26, 1994
|
|
.Dt READ 2
|
|
.Os BSD 4
|
|
.Sh NAME
|
|
.Nm read ,
|
|
.Nm readv ,
|
|
.Nm pread
|
|
.Nd read input
|
|
.Sh SYNOPSIS
|
|
.Fd #include <sys/types.h>
|
|
.Fd #include <sys/uio.h>
|
|
.Fd #include <unistd.h>
|
|
.Ft ssize_t
|
|
.Fn read "int d" "void *buf" "size_t nbytes"
|
|
.Ft ssize_t
|
|
.Fn readv "int d" "const struct iovec *iov" "int iovcnt"
|
|
.Ft ssize_t
|
|
.Fn pread "int d" "void *buf" "size_t nbytes" "off_t offset"
|
|
.Sh DESCRIPTION
|
|
.Fn Read
|
|
attempts to read
|
|
.Fa nbytes
|
|
of data from the object referenced by the descriptor
|
|
.Fa d
|
|
into the buffer pointed to by
|
|
.Fa buf .
|
|
.Fn Readv
|
|
performs the same action, but scatters the input data
|
|
into the
|
|
.Fa iovcnt
|
|
buffers specified by the members of the
|
|
.Fa iov
|
|
array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1].
|
|
.Fn Pread
|
|
performs the same function, but reads from the specified position in
|
|
the file without modifying the file pointer.
|
|
.Pp
|
|
For
|
|
.Fn readv ,
|
|
the
|
|
.Fa iovec
|
|
structure is defined as:
|
|
.Pp
|
|
.Bd -literal -offset indent -compact
|
|
struct iovec {
|
|
char *iov_base; /* Base address. */
|
|
size_t iov_len; /* Length. */
|
|
};
|
|
.Ed
|
|
.Pp
|
|
Each
|
|
.Fa iovec
|
|
entry specifies the base address and length of an area
|
|
in memory where data should be placed.
|
|
.Fn Readv
|
|
will always fill an area completely before proceeding
|
|
to the next.
|
|
.Pp
|
|
On objects capable of seeking, the
|
|
.Fn read
|
|
starts at a position
|
|
given by the pointer associated with
|
|
.Fa d
|
|
(see
|
|
.Xr lseek 2 ) .
|
|
Upon return from
|
|
.Fn 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,
|
|
.Fn read ,
|
|
.Fn readv ,
|
|
and
|
|
.Fn pread
|
|
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
|
|
.Sh IMPLEMENTATION NOTES
|
|
.Pp
|
|
In the non-threaded library
|
|
.Fn read
|
|
is implemented as the
|
|
.Va read
|
|
syscall.
|
|
.Pp
|
|
In the threaded library, the
|
|
.Va read
|
|
syscall is assembled to
|
|
.Fn _thread_sys_read
|
|
and
|
|
.Fn read
|
|
is implemented as a function which locks
|
|
.Va d
|
|
for read, then calls
|
|
.Fn _thread_sys_read .
|
|
If the call to
|
|
.Fn _thread_sys_read
|
|
would block, a context switch is performed. Before returning,
|
|
.Fn read
|
|
unlocks
|
|
.Va d .
|
|
.Pp
|
|
In the non-threaded library
|
|
.Fn readv
|
|
is implemented as the
|
|
.Va readv
|
|
syscall.
|
|
.Pp
|
|
In the threaded library, the
|
|
.Va readv
|
|
syscall is assembled to
|
|
.Fn _thread_sys_readv
|
|
and
|
|
.Fn readv
|
|
is implemented as a function which locks
|
|
.Va d
|
|
for read, then calls
|
|
.Fn _thread_sys_readv .
|
|
If the call to
|
|
.Fn _thread_sys_readv
|
|
would block, a context switch is performed. Before returning,
|
|
.Fn readv
|
|
unlocks
|
|
.Va d .
|
|
.Sh RETURN VALUES
|
|
If successful, the
|
|
number of bytes actually read is returned. Upon reading end-of-file,
|
|
zero is returned.
|
|
Otherwise, a -1 is returned and the global variable
|
|
.Va errno
|
|
is set to indicate the error.
|
|
.Sh ERRORS
|
|
.Fn Read ,
|
|
.Fn readv ,
|
|
and
|
|
.Fn pread
|
|
will succeed unless:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EBADF
|
|
.Fa D
|
|
is not a valid file or socket descriptor open for reading.
|
|
.It Bq Er EFAULT
|
|
.Fa Buf
|
|
points outside the allocated address space.
|
|
.It Bq Er EIO
|
|
An I/O error occurred while reading from the file system.
|
|
.It Bq Er EINTR
|
|
A read from a slow device was interrupted before
|
|
any data arrived by the delivery of a signal.
|
|
.It Bq Er EINVAL
|
|
The pointer associated with
|
|
.Fa d
|
|
was negative.
|
|
.It Bq Er EAGAIN
|
|
The file was marked for non-blocking I/O,
|
|
and no data were ready to be read.
|
|
.El
|
|
.Pp
|
|
In addition,
|
|
.Fn readv
|
|
may return one of the following errors:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EINVAL
|
|
.Fa Iovcnt
|
|
was less than or equal to 0, or greater than 16.
|
|
.It Bq Er EINVAL
|
|
One of the
|
|
.Fa iov_len
|
|
values in the
|
|
.Fa iov
|
|
array was negative.
|
|
.It Bq Er EINVAL
|
|
The sum of the
|
|
.Fa iov_len
|
|
values in the
|
|
.Fa iov
|
|
array overflowed a 32-bit integer.
|
|
.It Bq Er EFAULT
|
|
Part of the
|
|
.Fa iov
|
|
points outside the process's allocated address space.
|
|
.El
|
|
.Pp
|
|
The
|
|
.Fn pread
|
|
call may also return the following errors:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EINVAL
|
|
The specified file offset is invalid.
|
|
.It Bq Er ESPIPE
|
|
The file descriptor is associated with a pipe, socket, or FIFO.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr dup 2 ,
|
|
.Xr fcntl 2 ,
|
|
.Xr open 2 ,
|
|
.Xr pipe 2 ,
|
|
.Xr select 2 ,
|
|
.Xr socket 2 ,
|
|
.Xr socketpair 2
|
|
.Sh STANDARDS
|
|
The
|
|
.Fn read
|
|
function call is expected to conform to
|
|
.St -p1003.1-90 .
|
|
The
|
|
.Fn readv
|
|
and
|
|
.Fn pread
|
|
functions are expected to conform to
|
|
.St -xpg4.2 .
|
|
.Sh HISTORY
|
|
The
|
|
.Fn pread
|
|
function call
|
|
appeared in
|
|
.At V.4 .
|
|
The
|
|
.Fn readv
|
|
function call
|
|
appeared in
|
|
.Bx 4.2 .
|
|
A
|
|
.Fn read
|
|
function call appeared in
|
|
.At v6 .
|