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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 June 4, 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. 203Directories may only be read directly by root if the filesystem supports it and 204the 205 .Dv security.bsd.allow_read_dir 206sysctl MIB is set to a non-zero value. 207For most scenarios, the 208 .Xr readdir 3 209function should be used instead. 210 .It Bq Er EOPNOTSUPP 211The file descriptor is associated with a file system and file type that 212do not allow regular read operations on it. 213 .It Bq Er EOVERFLOW 214The file descriptor is associated with a regular file, 215 .Fa nbytes 216is greater than 0, 217 .Fa offset 218is before the end-of-file, and 219 .Fa offset 220is greater than or equal to the offset maximum established 221for this file system. 222 .It Bq Er EINVAL 223The value 224 .Fa nbytes 225is greater than 226 .Dv INT_MAX . 227 .El 228 .Pp 229In addition, 230 .Fn readv 231and 232 .Fn preadv 233may return one of the following errors: 234 .Bl -tag -width Er 235 .It Bq Er EINVAL 236The 237 .Fa iovcnt 238argument 239was less than or equal to 0, or greater than 240 .Dv IOV_MAX . 241 .It Bq Er EINVAL 242One of the 243 .Fa iov_len 244values in the 245 .Fa iov 246array was negative. 247 .It Bq Er EINVAL 248The sum of the 249 .Fa iov_len 250values in the 251 .Fa iov 252array overflowed a 32-bit integer. 253 .It Bq Er EFAULT 254Part of the 255 .Fa iov 256array points outside the process's allocated address space. 257 .El 258 .Pp 259The 260 .Fn pread 261and 262 .Fn preadv 263system calls may also return the following errors: 264 .Bl -tag -width Er 265 .It Bq Er EINVAL 266The 267 .Fa offset 268value was negative. 269 .It Bq Er ESPIPE 270The file descriptor is associated with a pipe, socket, or FIFO. 271 .El 272 .Sh SEE ALSO 273 .Xr dup 2 , 274 .Xr fcntl 2 , 275 .Xr getdirentries 2 , 276 .Xr open 2 , 277 .Xr pipe 2 , 278 .Xr select 2 , 279 .Xr socket 2 , 280 .Xr socketpair 2 , 281 .Xr fread 3 , 282 .Xr readdir 3 283 .Sh STANDARDS 284The 285 .Fn read 286system call is expected to conform to 287 .St -p1003.1-90 . 288The 289 .Fn readv 290and 291 .Fn pread 292system calls are expected to conform to 293 .St -xpg4.2 . 294 .Sh HISTORY 295The 296 .Fn preadv 297system call appeared in 298 .Fx 6.0 . 299The 300 .Fn pread 301function appeared in 302 .At V.4 . 303The 304 .Fn readv 305system call appeared in 306 .Bx 4.2 . 307The 308 .Fn read 309function appeared in 310 .At v1 . 311