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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 16, 2004 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 EFAULT 160The 161.Fa buf 162argument 163points outside the allocated address space. 164.It Bq Er EIO 165An I/O error occurred while reading from the file system. 166.It Bq Er EINTR 167A read from a slow device was interrupted before 168any data arrived by the delivery of a signal. 169.It Bq Er EINVAL 170The pointer associated with 171.Fa d 172was negative. 173.It Bq Er EAGAIN 174The file was marked for non-blocking I/O, 175and no data were ready to be read. 176.It Bq Er EISDIR 177The file descriptor is associated with a directory residing 178on a file system that does not allow regular read operations on 179directories (e.g.\& NFS). 180.It Bq Er EOPNOTSUPP 181The file descriptor is associated with a file system and file type that 182do not allow regular read operations on it. 183.It Bq Er EOVERFLOW 184The file descriptor is associated with a regular file, 185.Fa nbytes 186is greater than 0, 187.Fa offset 188is before the end-of-file, and 189.Fa offset 190is greater than or equal to the offset maximum established 191for this file system. 192.It Bq Er EINVAL 193The value 194.Fa nbytes 195is greater than 196.Dv INT_MAX . 197.El 198.Pp 199In addition, 200.Fn readv 201and 202.Fn preadv 203may return one of the following errors: 204.Bl -tag -width Er 205.It Bq Er EINVAL 206The 207.Fa iovcnt 208argument 209was less than or equal to 0, or greater than 210.Dv IOV_MAX . 211.It Bq Er EINVAL 212One of the 213.Fa iov_len 214values in the 215.Fa iov 216array was negative. 217.It Bq Er EINVAL 218The sum of the 219.Fa iov_len 220values in the 221.Fa iov 222array overflowed a 32-bit integer. 223.It Bq Er EFAULT 224Part of the 225.Fa iov 226array points outside the process's allocated address space. 227.El 228.Pp 229The 230.Fn pread 231and 232.Fn preadv 233system calls may also return the following errors: 234.Bl -tag -width Er 235.It Bq Er EINVAL 236The 237.Fa offset 238value was negative. 239.It Bq Er ESPIPE 240The file descriptor is associated with a pipe, socket, or FIFO. 241.El 242.Sh SEE ALSO 243.Xr dup 2 , 244.Xr fcntl 2 , 245.Xr getdirentries 2 , 246.Xr open 2 , 247.Xr pipe 2 , 248.Xr select 2 , 249.Xr socket 2 , 250.Xr socketpair 2 , 251.Xr fread 3 , 252.Xr readdir 3 253.Sh STANDARDS 254The 255.Fn read 256system call is expected to conform to 257.St -p1003.1-90 . 258The 259.Fn readv 260and 261.Fn pread 262system calls are expected to conform to 263.St -xpg4.2 . 264.Sh HISTORY 265The 266.Fn preadv 267system call appeared in 268.Fx 6.0 . 269The 270.Fn pread 271function appeared in 272.At V.4 . 273The 274.Fn readv 275system call appeared in 276.Bx 4.2 . 277The 278.Fn read 279function appeared in 280.At v6 . 281