man2/readv.2

   1 .\" Copyright (C) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
   2 .\" and (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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  23 .\" License.
  24 .\" Modified Sat Jul 24 18:34:44 1993 by Rik Faith (faith@cs.unc.edu)
  25 .\" Merged readv.[23], 2002-10-17, aeb
  26 .\" 2007-04-30 mtk, A fairly major rewrite to fix errors and
  27 .\"     add more details.
  28 .\"
  29 .TH READV 2  2002-10-17 "Linux" "Linux Programmer's Manual"
  30 .SH NAME
  31 readv, writev \- read or write data into multiple buffers
  32 .SH SYNOPSIS
  33 .nf
  34 .B #include <sys/uio.h>
  35 .sp
  36 .BI "ssize_t readv(int " fd ", const struct iovec *" iov ", int " iovcnt );
  37 .sp
  38 .BI "ssize_t writev(int " fd ", const struct iovec *" iov ", int " iovcnt );
  39 .fi
  40 .SH DESCRIPTION
  41 The
  42 .BR readv ()
  43 function reads
  44 .I iovcnt
  45 buffers from the file associated with the file descriptor
  46 .I fd
  47 into the buffers described by
  48 .IR iov
  49 ("scatter input").
  50 .PP
  51 The
  52 .BR writev ()
  53 function writes
  54 .I iovcnt
  55 buffers of data described by
  56 .I iov
  57 to the file associated with the file descriptor
  58 .IR fd
  59 ("gather output").
  60 .PP
  61 The pointer
  62 .I iov
  63 points to an array of
  64 .I iovec
  65 structures,
  66 defined in
  67 .I <sys/uio.h>
  68 as:
  69 .PP
  70 .br
  71 .in +0.25in
  72 .nf
  73 struct iovec {
  74     void  *iov_base;    /* Starting address */
  75     size_t iov_len;     /* Number of bytes to transfer */
  76 };
  77 .fi
  78 .in 0.25in
  79 .PP
  80 The
  81 .BR readv ()
  82 function works just like
  83 .BR read (2)
  84 except that multiple buffers are filled.
  85 .PP
  86 The
  87 .BR writev ()
  88 function works just like
  89 .BR write (2)
  90 except that multiple buffers are written out.
  91 .PP
  92 Buffers are processed in array order.
  93 This means that
  94 .BR readv ()
  95 completely fills
  96 .IR iov [0]
  97 before proceeding to
  98 .IR iov [1],
  99 and so on.
 100 (If there is insufficient data, then not all buffers pointed to by
 101 .I iov
 102 may be filled.)
 103 Similarly,
 104 .BR writev ()
 105 writes out the entire contents of
 106 .IR iov [0]
 107 before proceeding to
 108 .IR iov [1],
 109 and so on.
 110 .PP
 111 The data transfers performed by
 112 .BR readv ()
 113 and
 114 .BR writev ()
 115 are atomic: the data written by
 116 .BR writev ()
 117 is written as a single block that is not intermingled with output
 118 from writes in other processes (but see
 119 .BR pipe (7)
 120 for an exception);
 121 analogously,
 122 .BR readv ()
 123 is guaranteed to read a contiguous block of data from the file,
 124 regardless of read operations performed in other threads or processes
 125 that have file descriptors referring to the same open file description
 126 (see
 127 .BR open (2)).
 128 .SH "RETURN VALUE"
 129 On success, the
 130 .BR readv ()
 131 function returns the number of bytes read; the
 132 .BR writev ()
 133 function returns the number of bytes written.
 134 On error, \-1 is returned, and \fIerrno\fP is set appropriately.
 135 .SH ERRORS
 136 The errors are as given for
 137 .BR read (2)
 138 and
 139 .BR write (2).
 140 Additionally the following error is defined:
 141 .TP
 142 .B EINVAL
 143 The sum of the
 144 .I iov_len
 145 values overflows an
 146 .I ssize_t
 147 value.
 148 Or, the vector count \fIiovcnt\fP is less than zero or greater than the
 149 permitted maximum.
 150 .SH "CONFORMING TO"
 151 4.4BSD (the
 152 .BR readv ()
 153 and
 154 .BR writev ()
 155 functions first appeared in 4.2BSD), POSIX.1-2001.
 156 Linux libc5 used \fIsize_t\fP as the type of the \fIiovcnt\fP parameter,
 157 and \fIint\fP as return type for these functions.
 158 .\" The readv/writev system calls were buggy before Linux 1.3.40.
 159 .\" (Says release.libc.)
 160 .SH NOTES
 161 .SS Linux Notes
 162 POSIX.1-2001 allows an implementation to place a limit on
 163 the number of items that can be passed in
 164 .IR iov .
 165 An implementation can advertise its limit by defining
 166 .B IOV_MAX
 167 in
 168 .IR <limits.h>
 169 or at run time via the return value from
 170 .IR sysconf(_SC_IOV_MAX) .
 171 On Linux, the limit advertised by these mechanisms is 1024,
 172 which is the true kernel limit.
 173 However, the glibc wrapper functions do some extra work if
 174 they detect that the underlying kernel system call failed because this
 175 limit was exceeded.
 176 In the case of
 177 .BR readv ()
 178 the wrapper function allocates a temporary buffer large enough
 179 for all of the items specified by
 180 .IR iov ,
 181 passes that buffer in a call to
 182 .BR read (2),
 183 copies data from the buffer to the locations specified by the
 184 .I iov_base
 185 fields of the elements of
 186 .IR iov ,
 187 and then frees the buffer.
 188 The wrapper function for
 189 .BR writev ()
 190 performs the analogous task using a temporary buffer and a call to
 191 .BR write (2).
 192 .SH BUGS
 193 It is not advisable to mix calls to functions like
 194 .BR readv ()
 195 or
 196 .BR writev (),
 197 which operate on file descriptors, with the functions from the stdio
 198 library; the results will be undefined and probably not what you want.
 199 .SH EXAMPLE
 200 The following code sample demonstrates the use of
 201 .BR writev ():
 202
 203 .in +0.5i
 204 .nf
 205 char *str0 = "hello ";
 206 char *str1 = "world\\n";
 207 struct iovec iov[2];
 208 ssize_t nwritten;
 209
 210 iov[0].iov_base = str0;
 211 iov[0].iov_len = strlen(str0);
 212 iov[1].iov_base = str1;
 213 iov[1].iov_len = strlen(str1);
 214
 215 nwritten = writev(STDOUT_FILENO, iov, 2);
 216 .fi
 217 .in
 218 .SH "SEE ALSO"
 219 .BR read (2),
 220 .BR write (2)