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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: src/lib/libc/sys/read.2,v 1.9.2.6 2001/12/14 18:34:01 ru Exp $ 34.\" $DragonFly: src/lib/libc/sys/read.2,v 1.2 2003/06/17 04:26:47 dillon Exp $ 35.\" 36.Dd February 26, 1994 37.Dt READ 2 38.Os 39.Sh NAME 40.Nm read , 41.Nm readv , 42.Nm pread 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 readv "int d" "const struct iovec *iov" "int iovcnt" 54.Ft ssize_t 55.Fn pread "int d" "void *buf" "size_t nbytes" "off_t offset" 56.Sh DESCRIPTION 57.Fn Read 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 . 64.Fn Readv 65performs the same action, but scatters the input data 66into the 67.Fa iovcnt 68buffers specified by the members of the 69.Fa iov 70array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1]. 71.Fn Pread 72performs the same function, but reads from the specified position in 73the file without modifying the file pointer. 74.Pp 75For 76.Fn readv , 77the 78.Fa iovec 79structure is defined as: 80.Pp 81.Bd -literal -offset indent -compact 82struct iovec { 83 char *iov_base; /* Base address. */ 84 size_t iov_len; /* Length. */ 85}; 86.Ed 87.Pp 88Each 89.Fa iovec 90entry specifies the base address and length of an area 91in memory where data should be placed. 92.Fn Readv 93will always fill an area completely before proceeding 94to the next. 95.Pp 96On objects capable of seeking, the 97.Fn read 98starts at a position 99given by the pointer associated with 100.Fa d 101(see 102.Xr lseek 2 ) . 103Upon return from 104.Fn read , 105the pointer is incremented by the number of bytes actually read. 106.Pp 107Objects that are not capable of seeking always read from the current 108position. The value of the pointer associated with such an 109object is undefined. 110.Pp 111Upon successful completion, 112.Fn read , 113.Fn readv , 114and 115.Fn pread 116return the number of bytes actually read and placed in the buffer. 117The system guarantees to read the number of bytes requested if 118the descriptor references a normal file that has that many bytes left 119before the end-of-file, but in no other case. 120.Sh RETURN VALUES 121If successful, the 122number of bytes actually read is returned. 123Upon reading end-of-file, 124zero is returned. 125Otherwise, a -1 is returned and the global variable 126.Va errno 127is set to indicate the error. 128.Sh ERRORS 129.Fn Read , 130.Fn readv , 131and 132.Fn pread 133will succeed unless: 134.Bl -tag -width Er 135.It Bq Er EBADF 136.Fa D 137is not a valid file or socket descriptor open for reading. 138.It Bq Er EFAULT 139.Fa Buf 140points outside the allocated address space. 141.It Bq Er EIO 142An I/O error occurred while reading from the file system. 143.It Bq Er EINTR 144A read from a slow device was interrupted before 145any data arrived by the delivery of a signal. 146.It Bq Er EINVAL 147The pointer associated with 148.Fa d 149was negative. 150.It Bq Er EAGAIN 151The file was marked for non-blocking I/O, 152and no data were ready to be read. 153.El 154.Pp 155In addition, 156.Fn readv 157may return one of the following errors: 158.Bl -tag -width Er 159.It Bq Er EINVAL 160.Fa Iovcnt 161was less than or equal to 0, or greater than 16. 162.It Bq Er EINVAL 163One of the 164.Fa iov_len 165values in the 166.Fa iov 167array was negative. 168.It Bq Er EINVAL 169The sum of the 170.Fa iov_len 171values in the 172.Fa iov 173array overflowed a 32-bit integer. 174.It Bq Er EFAULT 175Part of the 176.Fa iov 177points outside the process's allocated address space. 178.El 179.Pp 180The 181.Fn pread 182call may also return the following errors: 183.Bl -tag -width Er 184.It Bq Er EINVAL 185The specified file offset is invalid. 186.It Bq Er ESPIPE 187The file descriptor is associated with a pipe, socket, or FIFO. 188.El 189.Sh SEE ALSO 190.Xr dup 2 , 191.Xr fcntl 2 , 192.Xr open 2 , 193.Xr pipe 2 , 194.Xr select 2 , 195.Xr socket 2 , 196.Xr socketpair 2 197.Sh STANDARDS 198The 199.Fn read 200function call is expected to conform to 201.St -p1003.1-90 . 202The 203.Fn readv 204and 205.Fn pread 206functions are expected to conform to 207.St -xpg4.2 . 208.Sh HISTORY 209The 210.Fn pread 211function call 212appeared in 213.At V.4 . 214The 215.Fn readv 216function call 217appeared in 218.Bx 4.2 . 219A 220.Fn read 221function call appeared in 222.At v6 . 223