1.\" $OpenBSD: read.2,v 1.27 2007/05/31 19:19:33 jmc Exp $ 2.\" $NetBSD: read.2,v 1.6 1995/02/27 12:35:47 cgd Exp $ 3.\" 4.\" Copyright (c) 1980, 1991, 1993 5.\" The Regents of the University of California. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. Neither the name of the University nor the names of its contributors 16.\" may be used to endorse or promote products derived from this software 17.\" without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 20.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 21.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 22.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 23.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 24.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 25.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 26.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 27.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 28.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 29.\" SUCH DAMAGE. 30.\" 31.\" @(#)read.2 8.4 (Berkeley) 2/26/94 32.\" 33.Dd $Mdocdate: May 31 2007 $ 34.Dt READ 2 35.Os 36.Sh NAME 37.Nm read , 38.Nm readv , 39.Nm pread , 40.Nm preadv 41.Nd read input 42.Sh SYNOPSIS 43.Fd #include <sys/types.h> 44.Fd #include <unistd.h> 45.Ft ssize_t 46.Fn read "int d" "void *buf" "size_t nbytes" 47.Ft ssize_t 48.Fn pread "int d" "void *buf" "size_t nbytes" "off_t offset" 49.Pp 50.Fd #include <sys/types.h> 51.Fd #include <sys/uio.h> 52.Fd #include <unistd.h> 53.Ft ssize_t 54.Fn readv "int d" "const struct iovec *iov" "int iovcnt" 55.Ft ssize_t 56.Fn preadv "int d" "const struct iovec *iov" "int iovcnt" "off_t offset" 57.Sh DESCRIPTION 58.Fn read 59attempts to read 60.Fa nbytes 61of data from the object referenced by the descriptor 62.Fa d 63into the buffer pointed to by 64.Fa buf . 65.Fn readv 66performs the same action, but scatters the input data 67into the 68.Fa iovcnt 69buffers specified by the members of the 70.Fa iov 71array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1]. 72.Fn pread 73and 74.Fn preadv 75perform the same functions, but read from the specified position in 76the file without modifying the file pointer. 77.Pp 78For 79.Fn readv 80and 81.Fn preadv , 82the 83.Fa iovec 84structure is defined as: 85.Bd -literal -offset indent 86struct iovec { 87 void *iov_base; 88 size_t iov_len; 89}; 90.Ed 91.Pp 92Each 93.Fa iovec 94entry specifies the base address and length of an area 95in memory where data should be placed. 96.Fn readv 97will always fill an area completely before proceeding 98to the next. 99.Pp 100On objects capable of seeking, the 101.Fn read 102starts at a position 103given by the pointer associated with 104.Fa d 105(see 106.Xr lseek 2 ) . 107Upon return from 108.Fn read , 109the pointer is incremented by the number of bytes actually read. 110.Pp 111Objects that are not capable of seeking always read from the current 112position. 113The value of the pointer associated with such an object is undefined. 114.Pp 115Upon successful completion, 116.Fn read , 117.Fn readv , 118.Fn pread , 119and 120.Fn preadv 121return the number of bytes actually read and placed in the buffer. 122The system guarantees to read the number of bytes requested if 123the descriptor references a normal file that has that many bytes left 124before the end-of-file, but in no other case. 125.Pp 126Note that 127.Fn readv 128and 129.Fn preadv 130will fail if the value of 131.Fa iovcnt 132exceeds the constant 133.Dv IOV_MAX . 134.Sh RETURN VALUES 135If successful, the 136number of bytes actually read is returned. 137Upon reading end-of-file, zero is returned. 138Otherwise, a \-1 is returned and the global variable 139.Va errno 140is set to indicate the error. 141.Sh ERRORS 142.Fn read , 143.Fn readv , 144.Fn pread , 145and 146.Fn preadv 147will succeed unless: 148.Bl -tag -width Er 149.It Bq Er EBADF 150.Fa d 151is not a valid file or socket descriptor open for reading. 152.It Bq Er EFAULT 153.Fa buf 154points outside the allocated address space. 155.It Bq Er EIO 156An I/O error occurred while reading from the file system, 157or the process is a member of a background process attempting to read 158from its controlling terminal, the process is ignoring or blocking 159the SIGTTIN signal or the process group is orphaned. 160.It Bq Er EINTR 161A read from a slow device 162(i.e. one that might block for an arbitrary amount of time) 163was interrupted by the delivery of a signal 164before any data arrived. 165.It Bq Er EINVAL 166The pointer associated with 167.Fa d 168was negative. 169.It Bq Er EAGAIN 170The file was marked for non-blocking I/O, 171and no data were ready to be read. 172.El 173.Pp 174In addition, 175.Fn read 176and 177.Fn pread 178may return the following error: 179.Bl -tag -width Er 180.It Bq Er EINVAL 181.Fa nbytes 182was larger than 183.Dv SSIZE_MAX . 184.El 185.Pp 186Also, 187.Fn readv 188and 189.Fn preadv 190may return one of the following errors: 191.Bl -tag -width Er 192.It Bq Er EINVAL 193.Fa iovcnt 194was less than or equal to 0, or greater than 195.Dv IOV_MAX . 196.It Bq Er EINVAL 197The sum of the 198.Fa iov_len 199values in the 200.Fa iov 201array overflowed an 202.Em ssize_t . 203.It Bq Er EFAULT 204Part of the 205.Fa iov 206points outside the process's allocated address space. 207.El 208.Sh SEE ALSO 209.Xr dup 2 , 210.Xr fcntl 2 , 211.Xr open 2 , 212.Xr pipe 2 , 213.Xr poll 2 , 214.Xr select 2 , 215.Xr socket 2 , 216.Xr socketpair 2 217.Sh STANDARDS 218The 219.Fn read 220function conforms to 221.St -p1003.1-90 . 222The 223.Fn readv 224and 225.Fn pread 226functions conform to 227.St -xpg4.2 . 228.Sh HISTORY 229The 230.Fn preadv 231function first appeared in 232.Ox 2.7 . 233The 234.Fn pread 235function appeared in 236.At V.4 . 237The 238.Fn readv 239function call appeared in 240.Bx 4.2 . 241The 242.Fn read 243function call appeared in 244.At v2 . 245.Sh CAVEATS 246Error checks should explicitly test for \-1. 247Code such as 248.Bd -literal 249 while ((nr = read(fd, buf, sizeof(buf))) > 0) 250.Ed 251.Pp 252is not maximally portable, as some platforms allow for 253.Va nbytes 254to range between 255.Dv SSIZE_MAX 256and 257.Dv SIZE_MAX 258\- 2, in which case the return value of an error-free 259.Fn read 260may appear as a negative number distinct from \-1. 261Proper loops should use 262.Bd -literal 263 while ((nr = read(fd, buf, sizeof(buf))) != -1 && nr != 0) 264.Ed 265