READV(2)                   Linux Programmer's Manual                  READV(2)
       readv, writev - read or write data into multiple buffers

       #include <sys/uio.h>

       ssize_t readv(int fd, const struct iovec *vector, int count);

       ssize_t writev(int fd, const struct iovec *vector, int count);

       The  readv()  function reads count blocks from the file associated with
       the file descriptor fd into the multiple buffers described by vector.

       The writev() function writes at most count blocks described  by  vector
       to the file associated with the file descriptor fd.

       The pointer vector points to a struct iovec defined in <sys/uio.h> as

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

       Buffers are processed in the order specified.

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

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

       On  success, the readv() function returns the number of bytes read; the
       writev() function returns 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).  Additionally the
       following error is defined:

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

       4.4BSD (the readv() and writev() functions first appeared  in  4.2BSD),
       POSIX.1-2001.  Linux libc5 used size_t as the type of the count parame-
       ter, and int as return type for these functions.

       POSIX.1-2001 allows an implementation to place a limit on the number of
       items  that  can  be passed in vector.  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  vector,  passes
       that  buffer  in  a  call to read(), copies data from the buffer to the
       locations specified by the iov_base fields of the elements  of  vector,
       and  then frees the buffer.  The wrapper function for writev() performs
       the analogous task using a temporary buffer and a call to write().

       It is not advisable to mix calls to functions like 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.

       read(2), write(2)

                                  2002-10-17                          READV(2)