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           #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
           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
           buffer  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 ";

    Linux 2010-11-17 READV(2)


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