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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 March 30, 2020 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 EINTEGRITY 186Corrupted data was detected while reading from the file system. 187.It Bq Er EBUSY 188Failed to read from a file, e.g. /proc/<pid>/regs while <pid> is not stopped 189.It Bq Er EINTR 190A read from a slow device 191(i.e.\& one that might block for an arbitrary amount of time) 192was interrupted by the delivery of a signal 193before any data arrived. 194.It Bq Er EINVAL 195The pointer associated with 196.Fa fd 197was negative. 198.It Bq Er EAGAIN 199The file was marked for non-blocking I/O, 200and no data were ready to be read. 201.It Bq Er EISDIR 202The file descriptor is associated with a directory residing 203on a file system that does not allow regular read operations on 204directories (e.g.\& NFS). 205.It Bq Er EOPNOTSUPP 206The file descriptor is associated with a file system and file type that 207do not allow regular read operations on it. 208.It Bq Er EOVERFLOW 209The file descriptor is associated with a regular file, 210.Fa nbytes 211is greater than 0, 212.Fa offset 213is before the end-of-file, and 214.Fa offset 215is greater than or equal to the offset maximum established 216for this file system. 217.It Bq Er EINVAL 218The value 219.Fa nbytes 220is greater than 221.Dv INT_MAX . 222.El 223.Pp 224In addition, 225.Fn readv 226and 227.Fn preadv 228may return one of the following errors: 229.Bl -tag -width Er 230.It Bq Er EINVAL 231The 232.Fa iovcnt 233argument 234was less than or equal to 0, or greater than 235.Dv IOV_MAX . 236.It Bq Er EINVAL 237One of the 238.Fa iov_len 239values in the 240.Fa iov 241array was negative. 242.It Bq Er EINVAL 243The sum of the 244.Fa iov_len 245values in the 246.Fa iov 247array overflowed a 32-bit integer. 248.It Bq Er EFAULT 249Part of the 250.Fa iov 251array points outside the process's allocated address space. 252.El 253.Pp 254The 255.Fn pread 256and 257.Fn preadv 258system calls may also return the following errors: 259.Bl -tag -width Er 260.It Bq Er EINVAL 261The 262.Fa offset 263value was negative. 264.It Bq Er ESPIPE 265The file descriptor is associated with a pipe, socket, or FIFO. 266.El 267.Sh SEE ALSO 268.Xr dup 2 , 269.Xr fcntl 2 , 270.Xr getdirentries 2 , 271.Xr open 2 , 272.Xr pipe 2 , 273.Xr select 2 , 274.Xr socket 2 , 275.Xr socketpair 2 , 276.Xr fread 3 , 277.Xr readdir 3 278.Sh STANDARDS 279The 280.Fn read 281system call is expected to conform to 282.St -p1003.1-90 . 283The 284.Fn readv 285and 286.Fn pread 287system calls are expected to conform to 288.St -xpg4.2 . 289.Sh HISTORY 290The 291.Fn preadv 292system call appeared in 293.Fx 6.0 . 294The 295.Fn pread 296function appeared in 297.At V.4 . 298The 299.Fn readv 300system call appeared in 301.Bx 4.2 . 302The 303.Fn read 304function appeared in 305.At v1 . 306