#include <sys/uio.h>

       ssize_t readv(int fd, const struct iovec *iov, int iovcnt);

       ssize_t writev(int fd, const struct iovec *iov, int iovcnt);

       ssize_t preadv(int fd, const struct iovec *iov, int iovcnt,
                      off_t offset);

       ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt,
                       off_t offset);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       preadv(), pwritev(): _BSD_SOURCE

       The readv() system call reads iovcnt buffers from the  file  associated
       with the file descriptor fd into the buffers described by iov ("scatter

       The writev() system call writes iovcnt buffers of data described by iov
       to the file associated with the file descriptor fd ("gather output").

       The  pointer  iov  points  to  an array of iovec structures, defined in
       <sys/uio.h> as:

           struct iovec {
               void  *iov_base;    /* Starting address */
               size_t iov_len;     /* Number of bytes to transfer */

       The readv() system call works just like read(2)  except  that  multiple
       buffers are filled.

       The  writev() system call works just like write(2) except that multiple
       buffers are written out.

       Buffers are processed in array order.  This  means  that  readv()  com-
       pletely fills iov[0] before proceeding to iov[1], and so on.  (If there
       is insufficient data, then not all buffers pointed to  by  iov  may  be
       filled.)   Similarly, writev() writes out the entire contents of iov[0]
       before proceeding to iov[1], and so on.

       The data transfers performed by readv() and writev()  are  atomic:  the
       data  written  by  writev()  is  written  as a single block that is not
       intermingled with output  from  writes  in  other  processes  (but  see
       pipe(7) for an exception); analogously, readv() is guaranteed to read a
       contiguous block of data from the file, regardless of  read  operations
       performed  in  other  threads  or  processes that have file descriptors
       referring to the same open file description (see open(2)).
       The file offset is  not  changed  by  these  system  calls.   The  file
       referred to by fd must be capable of seeking.

       On  success,  readv()  and  preadv()  return  the number of bytes read;
       writev() and pwritev() return the number of bytes written.   On  error,
       -1 is returned, and errno is set appropriately.

       The  errors  are  as  given  for  read(2)  and  write(2).  Furthermore,
       preadv() and pwritev() can also fail for the same reasons as  lseek(2).
       Additionally, the following error is defined:

       EINVAL The  sum  of the iov_len values overflows an ssize_t value.  Or,
              the vector count iovcnt is less than zero or  greater  than  the
              permitted maximum.

       preadv()  and pwritev() first appeared in Linux 2.6.30; library support
       was added in glibc 2.10.

       readv(),  writev():  4.4BSD  (these  system  calls  first  appeared  in
       4.2BSD),  POSIX.1-2001.   Linux  libc5  used  size_t as the type of the
       iovcnt argument, and int as the return type.

       preadv(), pwritev(): nonstandard, but present also on the modern BSDs.

   Linux notes
       POSIX.1-2001 allows an implementation to place a limit on the number of
       items  that  can be passed in iov.  An implementation can advertise its
       limit by defining IOV_MAX in <limits.h> or at run time via  the  return
       value  from  sysconf(_SC_IOV_MAX).   On  Linux, the limit advertised by
       these mechanisms is 1024, which is the true kernel limit.  However, the
       glibc  wrapper  functions  do  some  extra work if they detect that the
       underlying kernel system call failed because this limit  was  exceeded.
       In  the case of readv() the wrapper function allocates a temporary buf-
       fer large enough for all of the items specified  by  iov,  passes  that
       buffer  in  a call to read(2), copies data from the buffer to the loca-
       tions specified by the iov_base fields of the elements of iov, and then
       frees the buffer.  The wrapper function for writev() performs the anal-
       ogous task using a temporary buffer and a call to write(2).

       It is not advisable to mix calls to readv() or writev(), which  operate
       on  file  descriptors,  with  the functions from the stdio library; the
       results will be undefined and probably not what you want.

       The following code sample demonstrates the use of writev():

           char *str0 = "hello ";
           char *str1 = "world\n";

       This page is part of release 3.54 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2010-11-17                          READV(2)
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