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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 July 6, 2019 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.Pp 132In accordance with 133.St -p1003.1-2004 , 134both 135.Xr read 2 136and 137.Xr write 2 138syscalls are atomic with respect to each other in the effects on file 139content, when they operate on regular files. 140If two threads each call one of the 141.Xr read 2 142or 143.Xr write 2 , 144syscalls, each call will see either all of the changes of the other call, 145or none of them. 146The 147.Fx 148kernel implements this guarantee by locking the file ranges affected by 149the calls. 150.Sh RETURN VALUES 151If successful, the 152number of bytes actually read is returned. 153Upon reading end-of-file, 154zero is returned. 155Otherwise, a -1 is returned and the global variable 156.Va errno 157is set to indicate the error. 158.Sh ERRORS 159The 160.Fn read , 161.Fn readv , 162.Fn pread 163and 164.Fn preadv 165system calls 166will succeed unless: 167.Bl -tag -width Er 168.It Bq Er EBADF 169The 170.Fa fd 171argument 172is not a valid file or socket descriptor open for reading. 173.It Bq Er ECONNRESET 174The 175.Fa fd 176argument refers to a socket, and the remote socket end is 177forcibly closed. 178.It Bq Er EFAULT 179The 180.Fa buf 181argument 182points outside the allocated address space. 183.It Bq Er EIO 184An I/O error occurred while reading from the file system. 185.It Bq Er EBUSY 186Failed to read from a file, e.g. /proc/<pid>/regs while <pid> is not stopped 187.It Bq Er EINTR 188A read from a slow device 189(i.e.\& one that might block for an arbitrary amount of time) 190was interrupted by the delivery of a signal 191before any data arrived. 192.It Bq Er EINVAL 193The pointer associated with 194.Fa fd 195was negative. 196.It Bq Er EAGAIN 197The file was marked for non-blocking I/O, 198and no data were ready to be read. 199.It Bq Er EISDIR 200The file descriptor is associated with a directory residing 201on a file system that does not allow regular read operations on 202directories (e.g.\& NFS). 203.It Bq Er EOPNOTSUPP 204The file descriptor is associated with a file system and file type that 205do not allow regular read operations on it. 206.It Bq Er EOVERFLOW 207The file descriptor is associated with a regular file, 208.Fa nbytes 209is greater than 0, 210.Fa offset 211is before the end-of-file, and 212.Fa offset 213is greater than or equal to the offset maximum established 214for this file system. 215.It Bq Er EINVAL 216The value 217.Fa nbytes 218is greater than 219.Dv INT_MAX . 220.El 221.Pp 222In addition, 223.Fn readv 224and 225.Fn preadv 226may return one of the following errors: 227.Bl -tag -width Er 228.It Bq Er EINVAL 229The 230.Fa iovcnt 231argument 232was less than or equal to 0, or greater than 233.Dv IOV_MAX . 234.It Bq Er EINVAL 235One of the 236.Fa iov_len 237values in the 238.Fa iov 239array was negative. 240.It Bq Er EINVAL 241The sum of the 242.Fa iov_len 243values in the 244.Fa iov 245array overflowed a 32-bit integer. 246.It Bq Er EFAULT 247Part of the 248.Fa iov 249array points outside the process's allocated address space. 250.El 251.Pp 252The 253.Fn pread 254and 255.Fn preadv 256system calls may also return the following errors: 257.Bl -tag -width Er 258.It Bq Er EINVAL 259The 260.Fa offset 261value was negative. 262.It Bq Er ESPIPE 263The file descriptor is associated with a pipe, socket, or FIFO. 264.El 265.Sh SEE ALSO 266.Xr dup 2 , 267.Xr fcntl 2 , 268.Xr getdirentries 2 , 269.Xr open 2 , 270.Xr pipe 2 , 271.Xr select 2 , 272.Xr socket 2 , 273.Xr socketpair 2 , 274.Xr fread 3 , 275.Xr readdir 3 276.Sh STANDARDS 277The 278.Fn read 279system call is expected to conform to 280.St -p1003.1-90 . 281The 282.Fn readv 283and 284.Fn pread 285system calls are expected to conform to 286.St -xpg4.2 . 287.Sh HISTORY 288The 289.Fn preadv 290system call appeared in 291.Fx 6.0 . 292The 293.Fn pread 294function appeared in 295.At V.4 . 296The 297.Fn readv 298system call appeared in 299.Bx 4.2 . 300The 301.Fn read 302function appeared in 303.At v1 . 304