bsd/man/man2/read.2

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  32 .\"     @(#)read.2      8.4 (Berkeley) 2/26/94
  33 .\" $FreeBSD: src/lib/libc/sys/read.2,v 1.9.2.6 2001/12/14 18:34:01 ru Exp $
  34 .\"
  35 .Dd February 26, 1994
  36 .Dt READ 2
  37 .Os
  38 .Sh NAME
  39 .Nm pread ,
  40 .Nm read ,
  41 .Nm readv
  42 .Nd read input
  43 .Sh LIBRARY
  44 .Lb libc
  45 .Sh SYNOPSIS
  46 .In sys/types.h
  47 .In sys/uio.h
  48 .In unistd.h
  49 .Ft ssize_t
  50 .Fo pread
  51 .Fa "int d"
  52 .Fa "void *buf"
  53 .Fa "size_t nbyte"
  54 .Fa "off_t offset"
  55 .Fc
  56 .Ft ssize_t
  57 .Fo read
  58 .Fa "int fildes"
  59 .Fa "void *buf"
  60 .Fa "size_t nbyte"
  61 .Fc
  62 .Ft ssize_t
  63 .Fo readv
  64 .Fa "int d"
  65 .Fa "const struct iovec *iov"
  66 .Fa "int iovcnt"
  67 .Fc
  68 .Sh DESCRIPTION
  69 .Fn Read
  70 attempts to read
  71 .Fa nbyte
  72 bytes of data from the object referenced by the descriptor
  73 .Fa fildes
  74 into the buffer pointed to by
  75 .Fa buf .
  76 .Fn Readv
  77 performs the same action,
  78 but scatters the input data into the
  79 .Fa iovcnt
  80 buffers specified by the members of the
  81 .Fa iov
  82 array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1].
  83 .Fn Pread
  84 performs the same function,
  85 but reads from the specified position in the file
  86 without modifying the file pointer.
  87 .Pp
  88 For
  89 .Fn readv ,
  90 the
  91 .Fa iovec
  92 structure is defined as:
  93 .Pp
  94 .Bd -literal -offset indent -compact
  95 struct iovec {
  96         char   *iov_base;  /* Base address. */
  97         size_t iov_len;    /* Length. */
  98 };
  99 .Ed
 100 .Pp
 101 Each
 102 .Fa iovec
 103 entry specifies the base address and length of an area
 104 in memory where data should be placed.
 105 .Fn Readv
 106 will always fill an area completely before proceeding
 107 to the next.
 108 .Pp
 109 On objects capable of seeking, the
 110 .Fn read
 111 starts at a position
 112 given by the pointer associated with
 113 .Fa fildes
 114 (see
 115 .Xr lseek 2 ) .
 116 Upon return from
 117 .Fn read ,
 118 the pointer is incremented by the number of bytes actually read.
 119 .Pp
 120 Objects that are not capable of seeking always read from the current
 121 position.  The value of the pointer associated with such an
 122 object is undefined.
 123 .Pp
 124 Upon successful completion,
 125 .Fn read ,
 126 .Fn readv ,
 127 and
 128 .Fn pread
 129 return the number of bytes actually read and placed in the buffer.
 130 The system guarantees to read the number of bytes requested if
 131 the descriptor references a normal file that has that many bytes left
 132 before the end-of-file, but in no other case.
 133 .Sh RETURN VALUES
 134 If successful, the
 135 number of bytes actually read is returned.
 136 Upon reading end-of-file,
 137 zero is returned.
 138 Otherwise, a -1 is returned and the global variable
 139 .Va errno
 140 is set to indicate the error.
 141 .Sh ERRORS
 142 The
 143 .Fn pread ,
 144 .Fn read ,
 145 and
 146 .Fn readv
 147 calls
 148 will succeed unless:
 149 .Bl -tag -width Er
 150 .\" ===========
 151 .It Bq Er EAGAIN
 152 The file was marked for non-blocking I/O,
 153 and no data were ready to be read.
 154 .\" ===========
 155 .It Bq Er EBADF
 156 .Fa fildes
 157 is not a valid file or socket descriptor open for reading.
 158 .\" ===========
 159 .It Bq Er EFAULT
 160 .Fa Buf
 161 points outside the allocated address space.
 162 .\" ===========
 163 .It Bq Er EINTR
 164 A read from a slow device was interrupted before
 165 any data arrived by the delivery of a signal.
 166 .It Bq Er EINVAL
 167 The pointer associated with
 168 .Fa fildes
 169 was negative.
 170 .\" ===========
 171 .It Bq Er EIO
 172 An I/O error occurred while reading from the file system.
 173 .\" ===========
 174 .\" .It Bq Er EIO
 175 .\" The process is a member of a background process
 176 .\" attempting to read from its controlling terminal.
 177 .\" ===========
 178 .\" .It Bq Er EIO
 179 .\" The process is ignoring or blocking the SIGTTIN signal.
 180 .\" ===========
 181 .It Bq Er EIO
 182 The process group is orphaned.
 183 .\" ===========
 184 .It Bq Er EIO
 185 The file is a regular file,
 186 .Fa nbyte
 187 is greater than 0,
 188 the starting position is before the end-of-file,
 189 and the starting position is greater than or equal
 190 to the offset maximum established
 191 for the open file descriptor associated with
 192 .Fa fildes .
 193 .\" ===========
 194 .It Bq Er EISDIR
 195 An attempt is made to read a directory.
 196 .\" ===========
 197 .It Bq Er ENOBUFS
 198 An attempt to allocate a memory buffer fails.
 199 .\" ===========
 200 .It Bq Er ENOMEM
 201 Insufficient memory is available.
 202 .\" ===========
 203 .It Bq Er ENXIO
 204 An action is requested of a device that does not exist..
 205 .\" ===========
 206 .It Bq Er ENXIO
 207 A requested action cannot be performed by the device.
 208 .El
 209 .Pp
 210 The
 211 .Fn pread
 212 call may also return the following errors:
 213 .Bl -tag -width Er
 214 .\" ===========
 215 .It Bq Er EINVAL
 216 The specified file offset is invalid.
 217 .\" ===========
 218 .It Bq Er ESPIPE
 219 The file descriptor is associated with a pipe, socket, or FIFO.
 220 .El
 221 .Pp
 222 The
 223 .Fn read
 224 call may also return the following errors:
 225 .Bl -tag -width Er
 226 .\" ===========
 227 .It Bq Er ECONNRESET
 228 The connection is closed by the peer
 229 during a read attempt on a socket.
 230 .\" ===========
 231 .It Bq Er ENOTCONN
 232 A read is attempted on an unconnected socket.
 233 .\" ===========
 234 .It Bq Er ETIMEDOUT
 235 A transmission timeout occurs
 236 during a read attempt on a socket.
 237 .El
 238 .Pp
 239 The
 240 .Fn readv
 241 call may also return one of the following errors:
 242 .Bl -tag -width Er
 243 .\" ===========
 244 .It Bq Er EFAULT
 245 Part of the
 246 .Fa iov
 247 points outside the process's allocated address space.
 248 .\" ===========
 249 .It Bq Er EINVAL
 250 .Fa Iovcnt
 251 was less than or equal to 0, or greater than 16.
 252 .\" ===========
 253 .It Bq Er EINVAL
 254 One of the
 255 .Fa iov_len
 256 values in the
 257 .Fa iov
 258 array was negative.
 259 .\" ===========
 260 .It Bq Er EINVAL
 261 The sum of the
 262 .Fa iov_len
 263 values in the
 264 .Fa iov
 265 array overflowed a 32-bit integer.
 266 .El
 267 .Sh LEGACY SYNOPSIS
 268 .Fd #include <sys/types.h>
 269 .Fd #include <sys/uio.h>
 270 .Fd #include <unistd.h>
 271 .Pp
 272 The include files
 273 .In sys/types.h
 274 and
 275 .In sys/uio.h
 276 are necessary for all functions.
 277 .Sh SEE ALSO
 278 .Xr dup 2 ,
 279 .Xr fcntl 2 ,
 280 .Xr open 2 ,
 281 .Xr pipe 2 ,
 282 .Xr select 2 ,
 283 .Xr socket 2 ,
 284 .Xr socketpair 2 ,
 285 .Xr compat 5
 286 .Sh STANDARDS
 287 The
 288 .Fn read
 289 function call is expected to conform to
 290 .St -p1003.1-90 .
 291 The
 292 .Fn readv
 293 and
 294 .Fn pread
 295 functions are expected to conform to
 296 .St -xpg4.2 .
 297 .Sh HISTORY
 298 The
 299 .Fn pread
 300 function call
 301 appeared in
 302 .At V.4 .
 303 The
 304 .Fn readv
 305 function call
 306 appeared in
 307 .Bx 4.2 .
 308 A
 309 .Fn read
 310 function call appeared in
 311 .At v6 .