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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 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() completely 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()
       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  performs  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.

       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 wrapper function allocated a temporary
       buffer large enough for all of the items specified by iov, passed that  buffer  in  a  call  to  read(2),
       copied data from the buffer to the locations specified by the iov_base fields of the elements of iov, and
       then freed the buffer.  The wrapper function for writev() performed the analogous task using a  temporary
       buffer and a call to write(2).

       The  need  for  this  extra  effort  in  the  glibc wrapper functions went away with Linux 2.2 and later.
       However, glibc continued to provide this behavior until version 2.10.  Starting with glibc  version  2.9,
       the  wrapper  functions  provide  this  behavior only if the library detects that the system is running a
       Linux kernel older than version 2.6.18 (an 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

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