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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 January 22, 2012 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 d" "void *buf" "size_t nbytes" 47.Ft ssize_t 48.Fn pread "int d" "void *buf" "size_t nbytes" "off_t offset" 49.In sys/uio.h 50.Ft ssize_t 51.Fn readv "int d" "const struct iovec *iov" "int iovcnt" 52.Ft ssize_t 53.Fn preadv "int d" "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 d 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 d 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 d 153argument 154is not a valid file or socket descriptor open for reading. 155.It Bq Er ECONNRESET 156The 157.Fa d 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 EINTR 168A read from a slow device 169(i.e.\& one that might block for an arbitrary amount of time) 170was interrupted by the delivery of a signal 171before any data arrived. 172.It Bq Er EINVAL 173The pointer associated with 174.Fa d 175was negative. 176.It Bq Er EAGAIN 177The file was marked for non-blocking I/O, 178and no data were ready to be read. 179.It Bq Er EISDIR 180The file descriptor is associated with a directory residing 181on a file system that does not allow regular read operations on 182directories (e.g.\& NFS). 183.It Bq Er EOPNOTSUPP 184The file descriptor is associated with a file system and file type that 185do not allow regular read operations on it. 186.It Bq Er EOVERFLOW 187The file descriptor is associated with a regular file, 188.Fa nbytes 189is greater than 0, 190.Fa offset 191is before the end-of-file, and 192.Fa offset 193is greater than or equal to the offset maximum established 194for this file system. 195.It Bq Er EINVAL 196The value 197.Fa nbytes 198is greater than 199.Dv INT_MAX . 200.El 201.Pp 202In addition, 203.Fn readv 204and 205.Fn preadv 206may return one of the following errors: 207.Bl -tag -width Er 208.It Bq Er EINVAL 209The 210.Fa iovcnt 211argument 212was less than or equal to 0, or greater than 213.Dv IOV_MAX . 214.It Bq Er EINVAL 215One of the 216.Fa iov_len 217values in the 218.Fa iov 219array was negative. 220.It Bq Er EINVAL 221The sum of the 222.Fa iov_len 223values in the 224.Fa iov 225array overflowed a 32-bit integer. 226.It Bq Er EFAULT 227Part of the 228.Fa iov 229array points outside the process's allocated address space. 230.El 231.Pp 232The 233.Fn pread 234and 235.Fn preadv 236system calls may also return the following errors: 237.Bl -tag -width Er 238.It Bq Er EINVAL 239The 240.Fa offset 241value was negative. 242.It Bq Er ESPIPE 243The file descriptor is associated with a pipe, socket, or FIFO. 244.El 245.Sh SEE ALSO 246.Xr dup 2 , 247.Xr fcntl 2 , 248.Xr getdirentries 2 , 249.Xr open 2 , 250.Xr pipe 2 , 251.Xr select 2 , 252.Xr socket 2 , 253.Xr socketpair 2 , 254.Xr fread 3 , 255.Xr readdir 3 256.Sh STANDARDS 257The 258.Fn read 259system call is expected to conform to 260.St -p1003.1-90 . 261The 262.Fn readv 263and 264.Fn pread 265system calls are expected to conform to 266.St -xpg4.2 . 267.Sh HISTORY 268The 269.Fn preadv 270system call appeared in 271.Fx 6.0 . 272The 273.Fn pread 274function appeared in 275.At V.4 . 276The 277.Fn readv 278system call appeared in 279.Bx 4.2 . 280The 281.Fn read 282function appeared in 283.At v6 . 284