1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 4. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)read.2 8.4 (Berkeley) 2/26/94 29.\" $FreeBSD$ 30.\" 31.Dd September 11, 2013 32.Dt READ 2 33.Os 34.Sh NAME 35.Nm read , 36.Nm readv , 37.Nm pread , 38.Nm preadv 39.Nd read input 40.Sh LIBRARY 41.Lb libc 42.Sh SYNOPSIS 43.In sys/types.h 44.In unistd.h 45.Ft ssize_t 46.Fn read "int fd" "void *buf" "size_t nbytes" 47.Ft ssize_t 48.Fn pread "int fd" "void *buf" "size_t nbytes" "off_t offset" 49.In sys/uio.h 50.Ft ssize_t 51.Fn readv "int fd" "const struct iovec *iov" "int iovcnt" 52.Ft ssize_t 53.Fn preadv "int fd" "const struct iovec *iov" "int iovcnt" "off_t offset" 54.Sh DESCRIPTION 55The 56.Fn read 57system call 58attempts to read 59.Fa nbytes 60of data from the object referenced by the descriptor 61.Fa fd 62into the buffer pointed to by 63.Fa buf . 64The 65.Fn readv 66system call 67performs the same action, but scatters the input data 68into the 69.Fa iovcnt 70buffers specified by the members of the 71.Fa iov 72array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1]. 73The 74.Fn pread 75and 76.Fn preadv 77system calls 78perform the same functions, but read from the specified position in 79the file without modifying the file pointer. 80.Pp 81For 82.Fn readv 83and 84.Fn preadv , 85the 86.Fa iovec 87structure is defined as: 88.Pp 89.Bd -literal -offset indent -compact 90struct iovec { 91 void *iov_base; /* Base address. */ 92 size_t iov_len; /* Length. */ 93}; 94.Ed 95.Pp 96Each 97.Fa iovec 98entry specifies the base address and length of an area 99in memory where data should be placed. 100The 101.Fn readv 102system call 103will always fill an area completely before proceeding 104to the next. 105.Pp 106On objects capable of seeking, the 107.Fn read 108starts at a position 109given by the pointer associated with 110.Fa fd 111(see 112.Xr lseek 2 ) . 113Upon return from 114.Fn read , 115the pointer is incremented by the number of bytes actually read. 116.Pp 117Objects that are not capable of seeking always read from the current 118position. 119The value of the pointer associated with such an 120object is undefined. 121.Pp 122Upon successful completion, 123.Fn read , 124.Fn readv , 125.Fn pread 126and 127.Fn preadv 128return the number of bytes actually read and placed in the buffer. 129The system guarantees to read the number of bytes requested if 130the descriptor references a normal file that has that many bytes left 131before the end-of-file, but in no other case. 132.Sh RETURN VALUES 133If successful, the 134number of bytes actually read is returned. 135Upon reading end-of-file, 136zero is returned. 137Otherwise, a -1 is returned and the global variable 138.Va errno 139is set to indicate the error. 140.Sh ERRORS 141The 142.Fn read , 143.Fn readv , 144.Fn pread 145and 146.Fn preadv 147system calls 148will succeed unless: 149.Bl -tag -width Er 150.It Bq Er EBADF 151The 152.Fa fd 153argument 154is not a valid file or socket descriptor open for reading. 155.It Bq Er ECONNRESET 156The 157.Fa fd 158argument refers to a socket, and the remote socket end is 159forcibly closed. 160.It Bq Er EFAULT 161The 162.Fa buf 163argument 164points outside the allocated address space. 165.It Bq Er EIO 166An I/O error occurred while reading from the file system. 167.It Bq Er EBUSY 168Failed to read from a file, e.g. /proc/<pid>/regs while <pid> is not stopped 169.It Bq Er EINTR 170A read from a slow device 171(i.e.\& one that might block for an arbitrary amount of time) 172was interrupted by the delivery of a signal 173before any data arrived. 174.It Bq Er EINVAL 175The pointer associated with 176.Fa fd 177was negative. 178.It Bq Er EAGAIN 179The file was marked for non-blocking I/O, 180and no data were ready to be read. 181.It Bq Er EISDIR 182The file descriptor is associated with a directory residing 183on a file system that does not allow regular read operations on 184directories (e.g.\& NFS). 185.It Bq Er EOPNOTSUPP 186The file descriptor is associated with a file system and file type that 187do not allow regular read operations on it. 188.It Bq Er EOVERFLOW 189The file descriptor is associated with a regular file, 190.Fa nbytes 191is greater than 0, 192.Fa offset 193is before the end-of-file, and 194.Fa offset 195is greater than or equal to the offset maximum established 196for this file system. 197.It Bq Er EINVAL 198The value 199.Fa nbytes 200is greater than 201.Dv INT_MAX . 202.El 203.Pp 204In addition, 205.Fn readv 206and 207.Fn preadv 208may return one of the following errors: 209.Bl -tag -width Er 210.It Bq Er EINVAL 211The 212.Fa iovcnt 213argument 214was less than or equal to 0, or greater than 215.Dv IOV_MAX . 216.It Bq Er EINVAL 217One of the 218.Fa iov_len 219values in the 220.Fa iov 221array was negative. 222.It Bq Er EINVAL 223The sum of the 224.Fa iov_len 225values in the 226.Fa iov 227array overflowed a 32-bit integer. 228.It Bq Er EFAULT 229Part of the 230.Fa iov 231array points outside the process's allocated address space. 232.El 233.Pp 234The 235.Fn pread 236and 237.Fn preadv 238system calls may also return the following errors: 239.Bl -tag -width Er 240.It Bq Er EINVAL 241The 242.Fa offset 243value was negative. 244.It Bq Er ESPIPE 245The file descriptor is associated with a pipe, socket, or FIFO. 246.El 247.Sh SEE ALSO 248.Xr dup 2 , 249.Xr fcntl 2 , 250.Xr getdirentries 2 , 251.Xr open 2 , 252.Xr pipe 2 , 253.Xr select 2 , 254.Xr socket 2 , 255.Xr socketpair 2 , 256.Xr fread 3 , 257.Xr readdir 3 258.Sh STANDARDS 259The 260.Fn read 261system call is expected to conform to 262.St -p1003.1-90 . 263The 264.Fn readv 265and 266.Fn pread 267system calls are expected to conform to 268.St -xpg4.2 . 269.Sh HISTORY 270The 271.Fn preadv 272system call appeared in 273.Fx 6.0 . 274The 275.Fn pread 276function appeared in 277.At V.4 . 278The 279.Fn readv 280system call appeared in 281.Bx 4.2 . 282The 283.Fn read 284function appeared in 285.At v6 . 286