READV(2)                            Linux Programmer's Manual                            READV(2)



NAME
       readv, writev, preadv, pwritev - read or write data into multiple buffers

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  asso‐
       ciated 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()  completely  fills  iov[0]
       before proceeding to iov[1], and so on.  (If there is insufficient data, then not all buf‐
       fers 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()
       The preadv() system call combines the functionality of readv() and pread(2).  It  performs
       the  same  task  as  readv(), but adds a fourth argument, offset, which specifies the file
       offset at which the input operation is to be performed.

       The pwritev() system call combines the functionality of writev() and pwrite(2).   It  per‐
       forms  the  same task as writev(), but adds a fourth argument, offset, which specifies the
       file offset at which the output operation is to be performed.

       The file offset is not changed by these system calls.  The file referred to by fd must  be
       capable of seeking.

RETURN VALUE
       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  appropri‐
       ately.

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.   Or,  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(): 4.4BSD (these system calls first appeared in 4.2BSD), POSIX.1-2001.

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

NOTES
   C library/kernel ABI differences
       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 <lim‐
       its.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  sys‐
       tem  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 locations
       specified by the iov_base fields of the elements of iov, and then frees the  buffer.   The
       wrapper  function  for writev() performs the analogous task using a temporary buffer and a
       call to write(2).

BUGS
       It is not advisable to mix calls to readv() or writev(), which operate  on  file  descrip‐
       tors,  with the functions from the stdio library; the results will be undefined and proba‐
       bly 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 3.74 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                                       2014-08-19                                   READV(2)