minix/lib/libc/sys/read.2
Ben Gras 2fe8fb192f Full switch to clang/ELF. Drop ack. Simplify.
There is important information about booting non-ack images in
docs/UPDATING. ack/aout-format images can't be built any more, and
booting clang/ELF-format ones is a little different. Updating to the
new boot monitor is recommended.

Changes in this commit:

	. drop boot monitor -> allowing dropping ack support
	. facility to copy ELF boot files to /boot so that old boot monitor
	  can still boot fairly easily, see UPDATING
	. no more ack-format libraries -> single-case libraries
	. some cleanup of OBJECT_FMT, COMPILER_TYPE, etc cases
	. drop several ack toolchain commands, but not all support
	  commands (e.g. aal is gone but acksize is not yet).
	. a few libc files moved to netbsd libc dir
	. new /bin/date as minix date used code in libc/
	. test compile fix
	. harmonize includes
	. /usr/lib is no longer special: without ack, /usr/lib plays no
	  kind of special bootstrapping role any more and bootstrapping
	  is done exclusively through packages, so releases depend even
	  less on the state of the machine making them now.
	. rename nbsd_lib* to lib*
	. reduce mtree
2012-02-14 14:52:02 +01:00

263 lines
6.3 KiB
Groff

.\" $NetBSD: read.2,v 1.33 2010/04/05 07:53:47 wiz Exp $
.\"
.\" Copyright (c) 1980, 1991, 1993
.\" The Regents of the University of California. All rights reserved.
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.\" modification, are permitted provided that the following conditions
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.\" 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. 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
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.\" 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)
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.\" SUCH DAMAGE.
.\"
.\" @(#)read.2 8.4 (Berkeley) 2/26/94
.\"
.Dd April 3, 2010
.Dt READ 2
.Os
.Sh NAME
.Nm read ,
.Nm readv ,
.Nm pread ,
.Nm preadv
.Nd read input
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft ssize_t
.Fn read "int d" "void *buf" "size_t nbytes"
.Ft ssize_t
.Fn pread "int d" "void *buf" "size_t nbytes" "off_t offset"
.In sys/uio.h
.Ft ssize_t
.Fn readv "int d" "const struct iovec *iov" "int iovcnt"
.Ft ssize_t
.Fn preadv "int d" "const struct iovec *iov" "int iovcnt" "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
and
.Fn preadv
perform the same functions, but read from the specified position in
the file without modifying the file pointer.
.Pp
For
.Fn readv
and
.Fn preadv ,
the
.Fa iovec
structure is defined as:
.Pp
.Bd -literal -offset indent -compact
struct iovec {
void *iov_base;
size_t iov_len;
};
.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 file pointer associated with
.Fa d
(see
.Xr lseek 2 ) .
Upon return from
.Fn read ,
the file 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 file pointer associated with such an object is undefined.
.Pp
Upon successful completion,
.Fn read ,
.Fn readv ,
.Fn pread ,
and
.Fn preadv
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.
.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 ,
.Fn pread ,
and
.Fn preadv
will succeed unless:
.Bl -tag -width Er
.It Bq Er EAGAIN
The file was marked for non-blocking I/O,
and no data were ready to be read.
.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 EINTR
A read from a slow device
(i.e. one that might block for an arbitrary amount of time)
was interrupted by the delivery of a signal
before any data arrived.
See
.Xr sigaction 2
for more information on the interaction between signals and system
calls.
.It Bq Er EINVAL
The file pointer associated with
.Fa d
was negative; or
the total length of the I/O is more than can be expressed by the ssize_t
return value.
.It Bq Er EIO
An I/O error occurred while reading from the file system.
.El
.Pp
In addition,
.Fn readv
and
.Fn preadv
may return one of the following errors:
.Bl -tag -width Er
.It Bq Er EFAULT
Part of the
.Fa iov
points outside the process's allocated address space.
.It Bq Er EINVAL
.Fa iovcnt
was less than or equal to 0, or greater than
.Dv {IOV_MAX} ;
or one of the
.Fa iov_len
values in the
.Fa iov
array was negative; or
the sum of the
.Fa iov_len
values in the
.Fa iov
array overflowed a 32-bit integer.
.El
.Pp
The
.Fn pread
and
.Fn preadv
calls 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 poll 2 ,
.Xr select 2 ,
.Xr sigaction 2 ,
.Xr socket 2 ,
.Xr socketpair 2
.Sh STANDARDS
The
.Fn read
function conforms to
.St -p1003.1-90 .
The
.Fn readv
and
.Fn pread
functions conform to
.St -xpg4.2 .
.Sh HISTORY
The
.Fn preadv
function call
appeared in
.Nx 1.4 .
The
.Fn pread
function call
appeared in
.At V.4 .
The
.Fn readv
function call
appeared in
.Bx 4.2 .
The
.Fn read
function call appeared in
.At v2 .
.Sh CAVEATS
Error checks should explicitly test for \-1.
Code such as
.Bd -literal
while ((nr = read(fd, buf, sizeof(buf))) \*[Gt] 0)
.Ed
.Pp
is not maximally portable, as some platforms allow for
.Va nbytes
to range between
.Dv SSIZE_MAX
and
.Dv SIZE_MAX
\- 2, in which case the return value of an error-free
.Fn read
may appear as a negative number distinct from \-1.
Proper loops should use
.Bd -literal
while ((nr = read(fd, buf, sizeof(buf))) != -1 \*[Am]\*[Am] nr != 0)
.Ed