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Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)read.2 8.4 (Berkeley) 2/26/94 33.\" $FreeBSD$ 34.\" 35.Dd October 11, 2006 36.Dt READ 2 37.Os 38.Sh NAME 39.Nm read , 40.Nm readv , 41.Nm pread , 42.Nm preadv 43.Nd read input 44.Sh LIBRARY 45.Lb libc 46.Sh SYNOPSIS 47.In sys/types.h 48.In sys/uio.h 49.In unistd.h 50.Ft ssize_t 51.Fn read "int d" "void *buf" "size_t nbytes" 52.Ft ssize_t 53.Fn pread "int d" "void *buf" "size_t nbytes" "off_t offset" 54.Ft ssize_t 55.Fn readv "int d" "const struct iovec *iov" "int iovcnt" 56.Ft ssize_t 57.Fn preadv "int d" "const struct iovec *iov" "int iovcnt" "off_t offset" 58.Sh DESCRIPTION 59The 60.Fn read 61system call 62attempts to read 63.Fa nbytes 64of data from the object referenced by the descriptor 65.Fa d 66into the buffer pointed to by 67.Fa buf . 68The 69.Fn readv 70system call 71performs the same action, but scatters the input data 72into the 73.Fa iovcnt 74buffers specified by the members of the 75.Fa iov 76array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1]. 77The 78.Fn pread 79and 80.Fn preadv 81system calls 82perform the same functions, but read from the specified position in 83the file without modifying the file pointer. 84.Pp 85For 86.Fn readv 87and 88.Fn preadv , 89the 90.Fa iovec 91structure is defined as: 92.Pp 93.Bd -literal -offset indent -compact 94struct iovec { 95 void *iov_base; /* Base address. */ 96 size_t iov_len; /* Length. */ 97}; 98.Ed 99.Pp 100Each 101.Fa iovec 102entry specifies the base address and length of an area 103in memory where data should be placed. 104The 105.Fn readv 106system call 107will always fill an area completely before proceeding 108to the next. 109.Pp 110On objects capable of seeking, the 111.Fn read 112starts at a position 113given by the pointer associated with 114.Fa d 115(see 116.Xr lseek 2 ) . 117Upon return from 118.Fn read , 119the pointer is incremented by the number of bytes actually read. 120.Pp 121Objects that are not capable of seeking always read from the current 122position. 123The value of the pointer associated with such an 124object is undefined. 125.Pp 126Upon successful completion, 127.Fn read , 128.Fn readv , 129.Fn pread 130and 131.Fn preadv 132return the number of bytes actually read and placed in the buffer. 133The system guarantees to read the number of bytes requested if 134the descriptor references a normal file that has that many bytes left 135before the end-of-file, but in no other case. 136.Sh RETURN VALUES 137If successful, the 138number of bytes actually read is returned. 139Upon reading end-of-file, 140zero is returned. 141Otherwise, a -1 is returned and the global variable 142.Va errno 143is set to indicate the error. 144.Sh ERRORS 145The 146.Fn read , 147.Fn readv , 148.Fn pread 149and 150.Fn preadv 151system calls 152will succeed unless: 153.Bl -tag -width Er 154.It Bq Er EBADF 155The 156.Fa d 157argument 158is not a valid file or socket descriptor open for reading. 159.It Bq Er ECONNRESET 160The 161.Fa d 162argument refers to a socket, and the remote socket end is 163forcibly closed. 164.It Bq Er EFAULT 165The 166.Fa buf 167argument 168points outside the allocated address space. 169.It Bq Er EIO 170An I/O error occurred while reading from the file system. 171.It Bq Er EINTR 172A read from a slow device 173(i.e.\& one that might block for an arbitrary amount of time) 174was interrupted by the delivery of a signal 175before any data arrived. 176.It Bq Er EINVAL 177The pointer associated with 178.Fa d 179was negative. 180.It Bq Er EAGAIN 181The file was marked for non-blocking I/O, 182and no data were ready to be read. 183.It Bq Er EISDIR 184The file descriptor is associated with a directory residing 185on a file system that does not allow regular read operations on 186directories (e.g.\& NFS). 187.It Bq Er EOPNOTSUPP 188The file descriptor is associated with a file system and file type that 189do not allow regular read operations on it. 190.It Bq Er EOVERFLOW 191The file descriptor is associated with a regular file, 192.Fa nbytes 193is greater than 0, 194.Fa offset 195is before the end-of-file, and 196.Fa offset 197is greater than or equal to the offset maximum established 198for this file system. 199.It Bq Er EINVAL 200The value 201.Fa nbytes 202is greater than 203.Dv INT_MAX . 204.El 205.Pp 206In addition, 207.Fn readv 208and 209.Fn preadv 210may return one of the following errors: 211.Bl -tag -width Er 212.It Bq Er EINVAL 213The 214.Fa iovcnt 215argument 216was less than or equal to 0, or greater than 217.Dv IOV_MAX . 218.It Bq Er EINVAL 219One of the 220.Fa iov_len 221values in the 222.Fa iov 223array was negative. 224.It Bq Er EINVAL 225The sum of the 226.Fa iov_len 227values in the 228.Fa iov 229array overflowed a 32-bit integer. 230.It Bq Er EFAULT 231Part of the 232.Fa iov 233array points outside the process's allocated address space. 234.El 235.Pp 236The 237.Fn pread 238and 239.Fn preadv 240system calls may also return the following errors: 241.Bl -tag -width Er 242.It Bq Er EINVAL 243The 244.Fa offset 245value was negative. 246.It Bq Er ESPIPE 247The file descriptor is associated with a pipe, socket, or FIFO. 248.El 249.Sh SEE ALSO 250.Xr dup 2 , 251.Xr fcntl 2 , 252.Xr getdirentries 2 , 253.Xr open 2 , 254.Xr pipe 2 , 255.Xr select 2 , 256.Xr socket 2 , 257.Xr socketpair 2 , 258.Xr fread 3 , 259.Xr readdir 3 260.Sh STANDARDS 261The 262.Fn read 263system call is expected to conform to 264.St -p1003.1-90 . 265The 266.Fn readv 267and 268.Fn pread 269system calls are expected to conform to 270.St -xpg4.2 . 271.Sh HISTORY 272The 273.Fn preadv 274system call appeared in 275.Fx 6.0 . 276The 277.Fn pread 278function appeared in 279.At V.4 . 280The 281.Fn readv 282system call appeared in 283.Bx 4.2 . 284The 285.Fn read 286function appeared in 287.At v6 . 288