pwritev

SYNOPSIS
       #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

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

       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)).

   preadv() and pwritev()

RETURN VALUE
       On  success,  readv()  and  preadv()  return  the number of bytes read;
       writev() and pwritev() return the number of bytes written.

       Note that is not an error for a successful call to transfer fewer bytes
       than requested (see read(2) and write(2)).

       On error, -1 is returned, and errno is set appropriately.

ERRORS
       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.

       EINVAL The  vector  count  iovcnt is less than zero or greater than the
              permitted maximum.

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

CONFORMING TO
       readv(),  writev():  POSIX.1-2001,  POSIX.1-2008,  4.4BSD (these system
       calls first appeared in 4.2BSD).

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

NOTES
       POSIX.1 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 modern Linux systems, the limit is
       1024.  Back in Linux 2.0 days, this limit was 16.

   C library/kernel differences
       The raw preadv() and pwritev() system calls have call  signatures  that
       differ  slightly  from  that of the corresponding GNU C library wrapper
       functions shown in  the  SYNOPSIS.   The  final  argument,  offset,  is
       unpacked  by  the  wrapper  functions  into two arguments in the system
       calls:

           unsigned long pos_l, unsigned long pos

       These arguments contain, respectively, the low order and high order  32
       bits of offset.

   Historical C library/kernel differences
       To  deal  with  the  fact  that IOV_MAX was so low on early versions of
       Linux, the glibc wrapper functions for readv() and  writev()  did  some
       extra  work  if  they  detected  that the underlying kernel system call
       failed because this limit was exceeded.  In the case  of  readv(),  the
       arbitrarily  selected  kernel  version).   And  since glibc 2.20 (which
       requires a minimum Linux kernel version of 2.6.32), the  glibc  wrapper
       functions always just directly invoke the system calls.

       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.

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

           char *str0 = "hello ";
           char *str1 = "world\n";
           struct iovec iov[2];
           ssize_t nwritten;

           iov[0].iov_base = str0;
           iov[0].iov_len = strlen(str0);
           iov[1].iov_base = str1;
           iov[1].iov_len = strlen(str1);

           nwritten = writev(STDOUT_FILENO, iov, 2);

SEE ALSO
       pread(2), read(2), write(2)

COLOPHON
       This  page  is  part of release 4.04 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-07-23                          READV(2)
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