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 read ,
  40 .Nm readv ,
  41 .Nm pread
  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 .Fn read "int d" "void *buf" "size_t nbytes"
  51 .Ft ssize_t
  52 .Fn readv "int d" "const struct iovec *iov" "int iovcnt"
  53 .Ft ssize_t
  54 .Fn pread "int d" "void *buf" "size_t nbytes" "off_t offset"
  55 .Sh DESCRIPTION
  56 .Fn Read
  57 attempts to read
  58 .Fa nbytes
  59 of data from the object referenced by the descriptor
  60 .Fa d
  61 into the buffer pointed to by
  62 .Fa buf .
  63 .Fn Readv
  64 performs the same action, but scatters the input data
  65 into the
  66 .Fa iovcnt
  67 buffers specified by the members of the
  68 .Fa iov
  69 array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1].
  70 .Fn Pread
  71 performs the same function, but reads from the specified position in
  72 the file without modifying the file pointer.
  73 .Pp
  74 For
  75 .Fn readv ,
  76 the
  77 .Fa iovec
  78 structure is defined as:
  79 .Pp
  80 .Bd -literal -offset indent -compact
  81 struct iovec {
  82         char   *iov_base;  /* Base address. */
  83         size_t iov_len;    /* Length. */
  84 };
  85 .Ed
  86 .Pp
  87 Each
  88 .Fa iovec
  89 entry specifies the base address and length of an area
  90 in memory where data should be placed.
  91 .Fn Readv
  92 will always fill an area completely before proceeding
  93 to the next.
  94 .Pp
  95 On objects capable of seeking, the
  96 .Fn read
  97 starts at a position
  98 given by the pointer associated with
  99 .Fa d
 100 (see
 101 .Xr lseek 2 ) .
 102 Upon return from
 103 .Fn read ,
 104 the pointer is incremented by the number of bytes actually read.
 105 .Pp
 106 Objects that are not capable of seeking always read from the current
 107 position.  The value of the pointer associated with such an
 108 object is undefined.
 109 .Pp
 110 Upon successful completion,
 111 .Fn read ,
 112 .Fn readv ,
 113 and
 114 .Fn pread
 115 return the number of bytes actually read and placed in the buffer.
 116 The system guarantees to read the number of bytes requested if
 117 the descriptor references a normal file that has that many bytes left
 118 before the end-of-file, but in no other case.
 119 .Sh RETURN VALUES
 120 If successful, the
 121 number of bytes actually read is returned.
 122 Upon reading end-of-file,
 123 zero is returned.
 124 Otherwise, a -1 is returned and the global variable
 125 .Va errno
 126 is set to indicate the error.
 127 .Sh ERRORS
 128 .Fn Read ,
 129 .Fn readv ,
 130 and
 131 .Fn pread
 132 will succeed unless:
 133 .Bl -tag -width Er
 134 .It Bq Er EBADF
 135 .Fa D
 136 is not a valid file or socket descriptor open for reading.
 137 .It Bq Er EFAULT
 138 .Fa Buf
 139 points outside the allocated address space.
 140 .It Bq Er EIO
 141 An I/O error occurred while reading from the file system.
 142 .It Bq Er EINTR
 143 A read from a slow device was interrupted before
 144 any data arrived by the delivery of a signal.
 145 .It Bq Er EINVAL
 146 The pointer associated with
 147 .Fa d
 148 was negative.
 149 .It Bq Er EAGAIN
 150 The file was marked for non-blocking I/O,
 151 and no data were ready to be read.
 152 .El
 153 .Pp
 154 In addition,
 155 .Fn readv
 156 may return one of the following errors:
 157 .Bl -tag -width Er
 158 .It Bq Er EINVAL
 159 .Fa Iovcnt
 160 was less than or equal to 0, or greater than 16.
 161 .It Bq Er EINVAL
 162 One of the
 163 .Fa iov_len
 164 values in the
 165 .Fa iov
 166 array was negative.
 167 .It Bq Er EINVAL
 168 The sum of the
 169 .Fa iov_len
 170 values in the
 171 .Fa iov
 172 array overflowed a 32-bit integer.
 173 .It Bq Er EFAULT
 174 Part of the
 175 .Fa iov
 176 points outside the process's allocated address space.
 177 .El
 178 .Pp
 179 The
 180 .Fn pread
 181 call may also return the following errors:
 182 .Bl -tag -width Er
 183 .It Bq Er EINVAL
 184 The specified file offset is invalid.
 185 .It Bq Er ESPIPE
 186 The file descriptor is associated with a pipe, socket, or FIFO.
 187 .El
 188 .Sh SEE ALSO
 189 .Xr dup 2 ,
 190 .Xr fcntl 2 ,
 191 .Xr open 2 ,
 192 .Xr pipe 2 ,
 193 .Xr select 2 ,
 194 .Xr socket 2 ,
 195 .Xr socketpair 2
 196 .Sh STANDARDS
 197 The
 198 .Fn read
 199 function call is expected to conform to
 200 .St -p1003.1-90 .
 201 The
 202 .Fn readv
 203 and
 204 .Fn pread
 205 functions are expected to conform to
 206 .St -xpg4.2 .
 207 .Sh HISTORY
 208 The
 209 .Fn pread
 210 function call
 211 appeared in
 212 .At V.4 .
 213 The
 214 .Fn readv
 215 function call
 216 appeared in
 217 .Bx 4.2 .
 218 A
 219 .Fn read
 220 function call appeared in
 221 .At v6 .