bsd/man/man2/write.2

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  32 .\"     @(#)write.2     8.5 (Berkeley) 4/2/94
  33 .\" $FreeBSD: src/lib/libc/sys/write.2,v 1.12.2.7 2001/12/14 18:34:02 ru Exp $
  34 .\"
  35 .Dd April 2, 1994
  36 .Dt WRITE 2
  37 .Os
  38 .Sh NAME
  39 .Nm write ,
  40 .Nm writev ,
  41 .Nm pwrite
  42 .Nd write output
  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 write "int d" "const void *buf" "size_t nbytes"
  51 .Ft ssize_t
  52 .Fn writev "int d" "const struct iovec *iov" "int iovcnt"
  53 .Ft ssize_t
  54 .Fn pwrite "int d" "const void *buf" "size_t nbytes" "off_t offset"
  55 .Sh DESCRIPTION
  56 .Fn Write
  57 attempts to write
  58 .Fa nbytes
  59 of data to the object referenced by the descriptor
  60 .Fa d
  61 from the buffer pointed to by
  62 .Fa buf .
  63 .Fn Writev
  64 performs the same action, but gathers the output data
  65 from 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 Pwrite
  71 performs the same function, but writes to the specified position in
  72 the file without modifying the file pointer.
  73 .Pp
  74 For
  75 .Fn writev ,
  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 from which data should be written.
  91 .Fn Writev
  92 will always write a complete area before proceeding
  93 to the next.
  94 .Pp
  95 On objects capable of seeking, the
  96 .Fn write
  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 write ,
 104 the pointer is incremented by the number of bytes which were written.
 105 .Pp
 106 Objects that are not capable of seeking always write from the current
 107 position.  The value of the pointer associated with such an object
 108 is undefined.
 109 .Pp
 110 If the real user is not the super-user, then
 111 .Fn write
 112 clears the set-user-id bit on a file.
 113 This prevents penetration of system security
 114 by a user who
 115 .Dq captures
 116 a writable set-user-id file
 117 owned by the super-user.
 118 .Pp
 119 When using non-blocking I/O on objects such as sockets that are subject
 120 to flow control,
 121 .Fn write
 122 and
 123 .Fn writev
 124 may write fewer bytes than requested;
 125 the return value must be noted,
 126 and the remainder of the operation should be retried when possible.
 127 .Sh RETURN VALUES
 128 Upon successful completion the number of bytes which were written
 129 is returned.  Otherwise a -1 is returned and the global variable
 130 .Va errno
 131 is set to indicate the error.
 132 .Sh ERRORS
 133 .Fn Write ,
 134 .Fn writev ,
 135 and
 136 .Fn pwrite
 137 will fail and the file pointer will remain unchanged if:
 138 .Bl -tag -width Er
 139 .It Bq Er EBADF
 140 .Fa D
 141 is not a valid descriptor open for writing.
 142 .It Bq Er EPIPE
 143 An attempt is made to write to a pipe that is not open
 144 for reading by any process.
 145 .It Bq Er EPIPE
 146 An attempt is made to write to a socket of type
 147 .Dv SOCK_STREAM
 148 that is not connected to a peer socket.
 149 .It Bq Er EFBIG
 150 An attempt was made to write a file that exceeds the process's
 151 file size limit or the maximum file size.
 152 .It Bq Er EFAULT
 153 Part of
 154 .Fa iov
 155 or data to be written to the file
 156 points outside the process's allocated address space.
 157 .It Bq Er EINVAL
 158 The pointer associated with
 159 .Fa d
 160 was negative.
 161 .It Bq Er ENOSPC
 162 There is no free space remaining on the file system
 163 containing the file.
 164 .It Bq Er EDQUOT
 165 The user's quota of disk blocks on the file system
 166 containing the file has been exhausted.
 167 .It Bq Er EIO
 168 An I/O error occurred while reading from or writing to the file system.
 169 .It Bq Er EINTR
 170 A signal interrupted the write before it could be completed.
 171 .It Bq Er EAGAIN
 172 The file was marked for non-blocking I/O,
 173 and no data could be written immediately.
 174 .El
 175 .Pp
 176 In addition,
 177 .Fn writev
 178 may return one of the following errors:
 179 .Bl -tag -width Er
 180 .It Bq Er EDESTADDRREQ
 181 The destination is no longer available when writing to a
 182 .Ux
 183 domain datagram socket on which
 184 .Xr connect 2
 185 had been used to set a destination address.
 186 .It Bq Er EINVAL
 187 .Fa Iovcnt
 188 was less than or equal to 0, or greater than
 189 .Dv UIO_MAXIOV .
 190 .It Bq Er EINVAL
 191 One of the
 192 .Fa iov_len
 193 values in the
 194 .Fa iov
 195 array was negative.
 196 .It Bq Er EINVAL
 197 The sum of the
 198 .Fa iov_len
 199 values in the
 200 .Fa iov
 201 array overflowed a 32-bit integer.
 202 .It Bq Er ENOBUFS
 203 The mbuf pool has been completely exhausted when writing to a socket.
 204 .El
 205 .Pp
 206 The
 207 .Fn pwrite
 208 call may also return the following errors:
 209 .Bl -tag -width Er
 210 .It Bq Er EINVAL
 211 The specified file offset is invalid.
 212 .It Bq Er ESPIPE
 213 The file descriptor is associated with a pipe, socket, or FIFO.
 214 .El
 215 .Sh SEE ALSO
 216 .Xr fcntl 2 ,
 217 .Xr lseek 2 ,
 218 .Xr open 2 ,
 219 .Xr pipe 2 ,
 220 .Xr select 2
 221 .Sh STANDARDS
 222 The
 223 .Fn write
 224 function call is expected to conform to
 225 .St -p1003.1-90 .
 226 The
 227 .Fn writev
 228 and
 229 .Fn pwrite
 230 functions are expected to conform to
 231 .St -xpg4.2 .
 232 .Sh HISTORY
 233 The
 234 .Fn pwrite
 235 function call
 236 appeared in
 237 .At V.4 .
 238 The
 239 .Fn writev
 240 function call
 241 appeared in
 242 .Bx 4.2 .
 243 A
 244 .Fn write
 245 function call appeared in
 246 .At v6 .