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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 July 29, 2005 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 was interrupted before 173any data arrived by the delivery of a signal. 174.It Bq Er EINVAL 175The pointer associated with 176.Fa d 177was negative. 178.It Bq Er EAGAIN 179The file was marked for non-blocking I/O, 180and no data were ready to be read. 181.It Bq Er EISDIR 182The file descriptor is associated with a directory residing 183on a file system that does not allow regular read operations on 184directories (e.g.\& NFS). 185.It Bq Er EOPNOTSUPP 186The file descriptor is associated with a file system and file type that 187do not allow regular read operations on it. 188.It Bq Er EOVERFLOW 189The file descriptor is associated with a regular file, 190.Fa nbytes 191is greater than 0, 192.Fa offset 193is before the end-of-file, and 194.Fa offset 195is greater than or equal to the offset maximum established 196for this file system. 197.It Bq Er EINVAL 198The value 199.Fa nbytes 200is greater than 201.Dv INT_MAX . 202.El 203.Pp 204In addition, 205.Fn readv 206and 207.Fn preadv 208may return one of the following errors: 209.Bl -tag -width Er 210.It Bq Er EINVAL 211The 212.Fa iovcnt 213argument 214was less than or equal to 0, or greater than 215.Dv IOV_MAX . 216.It Bq Er EINVAL 217One of the 218.Fa iov_len 219values in the 220.Fa iov 221array was negative. 222.It Bq Er EINVAL 223The sum of the 224.Fa iov_len 225values in the 226.Fa iov 227array overflowed a 32-bit integer. 228.It Bq Er EFAULT 229Part of the 230.Fa iov 231array points outside the process's allocated address space. 232.El 233.Pp 234The 235.Fn pread 236and 237.Fn preadv 238system calls may also return the following errors: 239.Bl -tag -width Er 240.It Bq Er EINVAL 241The 242.Fa offset 243value was negative. 244.It Bq Er ESPIPE 245The file descriptor is associated with a pipe, socket, or FIFO. 246.El 247.Sh SEE ALSO 248.Xr dup 2 , 249.Xr fcntl 2 , 250.Xr getdirentries 2 , 251.Xr open 2 , 252.Xr pipe 2 , 253.Xr select 2 , 254.Xr socket 2 , 255.Xr socketpair 2 , 256.Xr fread 3 , 257.Xr readdir 3 258.Sh STANDARDS 259The 260.Fn read 261system call is expected to conform to 262.St -p1003.1-90 . 263The 264.Fn readv 265and 266.Fn pread 267system calls are expected to conform to 268.St -xpg4.2 . 269.Sh HISTORY 270The 271.Fn preadv 272system call appeared in 273.Fx 6.0 . 274The 275.Fn pread 276function appeared in 277.At V.4 . 278The 279.Fn readv 280system call appeared in 281.Bx 4.2 . 282The 283.Fn read 284function appeared in 285.At v6 . 286