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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 December 1, 2017 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 unistd.h 44.Ft ssize_t 45.Fn read "int fd" "void *buf" "size_t nbytes" 46.Ft ssize_t 47.Fn pread "int fd" "void *buf" "size_t nbytes" "off_t offset" 48.In sys/uio.h 49.Ft ssize_t 50.Fn readv "int fd" "const struct iovec *iov" "int iovcnt" 51.Ft ssize_t 52.Fn preadv "int fd" "const struct iovec *iov" "int iovcnt" "off_t offset" 53.Sh DESCRIPTION 54The 55.Fn read 56system call 57attempts to read 58.Fa nbytes 59of data from the object referenced by the descriptor 60.Fa fd 61into the buffer pointed to by 62.Fa buf . 63The 64.Fn readv 65system call 66performs the same action, but scatters the input data 67into the 68.Fa iovcnt 69buffers specified by the members of the 70.Fa iov 71array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1]. 72The 73.Fn pread 74and 75.Fn preadv 76system calls 77perform the same functions, but read from the specified position in 78the file without modifying the file pointer. 79.Pp 80For 81.Fn readv 82and 83.Fn preadv , 84the 85.Fa iovec 86structure is defined as: 87.Pp 88.Bd -literal -offset indent -compact 89struct iovec { 90 void *iov_base; /* Base address. */ 91 size_t iov_len; /* Length. */ 92}; 93.Ed 94.Pp 95Each 96.Fa iovec 97entry specifies the base address and length of an area 98in memory where data should be placed. 99The 100.Fn readv 101system call 102will always fill an area completely before proceeding 103to the next. 104.Pp 105On objects capable of seeking, the 106.Fn read 107starts at a position 108given by the pointer associated with 109.Fa fd 110(see 111.Xr lseek 2 ) . 112Upon return from 113.Fn read , 114the pointer is incremented by the number of bytes actually read. 115.Pp 116Objects that are not capable of seeking always read from the current 117position. 118The value of the pointer associated with such an 119object is undefined. 120.Pp 121Upon successful completion, 122.Fn read , 123.Fn readv , 124.Fn pread 125and 126.Fn preadv 127return the number of bytes actually read and placed in the buffer. 128The system guarantees to read the number of bytes requested if 129the descriptor references a normal file that has that many bytes left 130before the end-of-file, but in no other case. 131.Sh RETURN VALUES 132If successful, the 133number of bytes actually read is returned. 134Upon reading end-of-file, 135zero is returned. 136Otherwise, a -1 is returned and the global variable 137.Va errno 138is set to indicate the error. 139.Sh ERRORS 140The 141.Fn read , 142.Fn readv , 143.Fn pread 144and 145.Fn preadv 146system calls 147will succeed unless: 148.Bl -tag -width Er 149.It Bq Er EBADF 150The 151.Fa fd 152argument 153is not a valid file or socket descriptor open for reading. 154.It Bq Er ECONNRESET 155The 156.Fa fd 157argument refers to a socket, and the remote socket end is 158forcibly closed. 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 EBUSY 167Failed to read from a file, e.g. /proc/<pid>/regs while <pid> is not stopped 168.It Bq Er EINTR 169A read from a slow device 170(i.e.\& one that might block for an arbitrary amount of time) 171was interrupted by the delivery of a signal 172before any data arrived. 173.It Bq Er EINVAL 174The pointer associated with 175.Fa fd 176was negative. 177.It Bq Er EAGAIN 178The file was marked for non-blocking I/O, 179and no data were ready to be read. 180.It Bq Er EISDIR 181The file descriptor is associated with a directory residing 182on a file system that does not allow regular read operations on 183directories (e.g.\& NFS). 184.It Bq Er EOPNOTSUPP 185The file descriptor is associated with a file system and file type that 186do not allow regular read operations on it. 187.It Bq Er EOVERFLOW 188The file descriptor is associated with a regular file, 189.Fa nbytes 190is greater than 0, 191.Fa offset 192is before the end-of-file, and 193.Fa offset 194is greater than or equal to the offset maximum established 195for this file system. 196.It Bq Er EINVAL 197The value 198.Fa nbytes 199is greater than 200.Dv INT_MAX . 201.El 202.Pp 203In addition, 204.Fn readv 205and 206.Fn preadv 207may return one of the following errors: 208.Bl -tag -width Er 209.It Bq Er EINVAL 210The 211.Fa iovcnt 212argument 213was less than or equal to 0, or greater than 214.Dv IOV_MAX . 215.It Bq Er EINVAL 216One of the 217.Fa iov_len 218values in the 219.Fa iov 220array was negative. 221.It Bq Er EINVAL 222The sum of the 223.Fa iov_len 224values in the 225.Fa iov 226array overflowed a 32-bit integer. 227.It Bq Er EFAULT 228Part of the 229.Fa iov 230array points outside the process's allocated address space. 231.El 232.Pp 233The 234.Fn pread 235and 236.Fn preadv 237system calls may also return the following errors: 238.Bl -tag -width Er 239.It Bq Er EINVAL 240The 241.Fa offset 242value was negative. 243.It Bq Er ESPIPE 244The file descriptor is associated with a pipe, socket, or FIFO. 245.El 246.Sh SEE ALSO 247.Xr dup 2 , 248.Xr fcntl 2 , 249.Xr getdirentries 2 , 250.Xr open 2 , 251.Xr pipe 2 , 252.Xr select 2 , 253.Xr socket 2 , 254.Xr socketpair 2 , 255.Xr fread 3 , 256.Xr readdir 3 257.Sh STANDARDS 258The 259.Fn read 260system call is expected to conform to 261.St -p1003.1-90 . 262The 263.Fn readv 264and 265.Fn pread 266system calls are expected to conform to 267.St -xpg4.2 . 268.Sh HISTORY 269The 270.Fn preadv 271system call appeared in 272.Fx 6.0 . 273The 274.Fn pread 275function appeared in 276.At V.4 . 277The 278.Fn readv 279system call appeared in 280.Bx 4.2 . 281The 282.Fn read 283function appeared in 284.At v1 . 285