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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: src/lib/libc/sys/read.2,v 1.9.2.6 2001/12/14 18:34:01 ru Exp $ 30.\" 31.Dd June 8, 2020 32.Dt READ 2 33.Os 34.Sh NAME 35.Nm read , 36.Nm readv , 37.Nm pread , 38.Nm preadv , 39.Nm extpread , 40.Nm extpreadv 41.Nd read input 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.In sys/types.h 46.In sys/uio.h 47.In unistd.h 48.Ft ssize_t 49.Fn read "int d" "void *buf" "size_t nbytes" 50.Ft ssize_t 51.Fn readv "int d" "const struct iovec *iov" "int iovcnt" 52.Ft ssize_t 53.Fn pread "int d" "void *buf" "size_t nbytes" "off_t offset" 54.Ft ssize_t 55.Fn preadv "int d" "const struct iovec *iov" "int iovcnt" "off_t offset" 56.Ft ssize_t 57.Fn extpread "int d" "void *buf" "size_t nbytes" "int flags" "off_t offset" 58.Ft ssize_t 59.Fn extpreadv "int d" "const struct iovec *iov" "int iovcnt" "int flags" "off_t offset" 60.Sh DESCRIPTION 61.Fn Read 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 . 68.Fn Readv 69performs the same action, but scatters the input data 70into the 71.Fa iovcnt 72buffers specified by the members of the 73.Fa iov 74array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1]. 75The 76.Fn pread , 77.Fn preadv , 78.Fn extpread , 79and 80.Fn extpreadv 81calls perform the same function, but read from the specified position in 82the file without modifying the file pointer. 83.Pp 84For 85.Fn readv 86and 87.Fn preadv , 88the 89.Fa iovec 90structure is defined as: 91.Pp 92.Bd -literal -offset indent -compact 93struct iovec { 94 char *iov_base; /* Base address. */ 95 size_t iov_len; /* Length. */ 96}; 97.Ed 98.Pp 99Each 100.Fa iovec 101entry specifies the base address and length of an area 102in memory where data should be placed. 103.Fn Readv 104and 105.Fn preadv 106will always fill an area completely before proceeding 107to the next. 108.Pp 109The 110.Fn extpread 111and 112.Fn extpreadv 113allow a 114.Ar flags 115argument 116to also be passed in, controlling blocking/non-blocking and other features 117on a call-by-call basis, ignoring the related default for the descriptor. 118Allowed flags are: 119.Bl -tag -width ".Dv O_FNONBLOCKING" 120.It Dv O_FBLOCKING 121Force the system call to operate in a blocking fashion. 122.It Dv O_FNONBLOCKING 123Force the system call to operate in a non-blocking fashion. 124.It Dv O_FAPPEND 125Force append mode for the operation. 126.It Dv O_FOFFSET 127Force offset mode for the operation. 128.El 129.Pp 130On objects capable of seeking, the 131.Fn read 132starts at a position 133given by the pointer associated with 134.Fa d 135(see 136.Xr lseek 2 ) . 137Upon return from 138.Fn read , 139the pointer is incremented by the number of bytes actually read. 140.Pp 141Objects that are not capable of seeking always read from the current 142position. The value of the pointer associated with such an 143object is undefined. 144.Pp 145Upon successful completion, 146.Fn read , 147.Fn readv , 148.Fn pread , 149and 150.Fn preadv 151return the number of bytes actually read and placed in the buffer. 152The system guarantees to read the number of bytes requested if 153the descriptor references a normal file that has that many bytes left 154before the end-of-file, but in no other case. 155.Sh RETURN VALUES 156If successful, the 157number of bytes actually read is returned. 158Upon reading end-of-file, 159zero is returned. 160Otherwise, a -1 is returned and the global variable 161.Va errno 162is set to indicate the error. 163.Sh ERRORS 164.Fn read , 165.Fn readv , 166.Fn pread , 167.Fn preadv , 168.Fn extpread 169and 170.Fn extpreadv 171will succeed unless: 172.Bl -tag -width Er 173.It Bq Er EBADF 174.Fa D 175is not a valid file or socket descriptor open for reading. 176.It Bq Er EFAULT 177.Fa Buf 178points outside the allocated address space. 179.It Bq Er EIO 180An I/O error occurred while reading from the file system. 181.It Bq Er EINTR 182A read from a slow device was interrupted before 183any data arrived by the delivery of a signal. 184.It Bq Er EINVAL 185The pointer associated with 186.Fa d 187was negative. 188.It Bq Er EISDIR 189The file descriptor 190.Fa d 191refers to a directory and the implementation does not allow the directory 192to be read using 193.Fn read 194or 195.Fn pread . 196The 197.Fn readdir 198function should be used instead. 199.It Bq Er EAGAIN 200The file was marked for non-blocking I/O, 201and no data were ready to be read. 202.It Bq Er ENOBUFS 203A message was not delivered because it would have overflowed the buffer. 204.El 205.Pp 206In addition, 207.Fn readv , 208.Fn preadv 209and 210.Fn extpreadv 211may return one of the following errors: 212.Bl -tag -width Er 213.It Bq Er EINVAL 214.Fa Iovcnt 215was less than or equal to 0, or greater than 16. 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 231points outside the process's allocated address space. 232.El 233.Pp 234The 235.Fn pread , 236.Fn preadv , 237.Fn extpread 238and 239.Fn extpreadv 240calls may also return the following errors: 241.Bl -tag -width Er 242.It Bq Er EINVAL 243The specified file offset is invalid. 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 open 2 , 251.Xr pipe 2 , 252.Xr select 2 , 253.Xr socket 2 , 254.Xr socketpair 2 , 255.Xr readdir 3 256.Sh STANDARDS 257The 258.Fn read 259function call is expected to conform to 260.St -p1003.1-90 . 261The 262.Fn readv 263and 264.Fn pread 265functions are expected to conform to 266.St -xpg4.2 . 267.Pp 268The 269.Fn extpread 270and 271.Fn extpreadv 272functions are 273.Dx 274specific extensions. 275.Sh HISTORY 276The 277.Fn preadv 278function call was added in 279.Dx 1.5 . 280The 281.Fn pread 282function call 283appeared in 284.At V.4 . 285The 286.Fn readv 287function call 288appeared in 289.Bx 4.2 . 290A 291.Fn read 292function call appeared in 293.At v6 . 294