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NAME

       readv,  writev,  preadv,  pwritev,  preadv2,  pwritev2  - 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);

       ssize_t preadv2(int fd, const struct iovec *iov, int iovcnt,
                       off_t offset, int flags);

       ssize_t pwritev2(int fd, const struct iovec *iov, int iovcnt,
                        off_t offset, int flags);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       preadv(), pwritev():
           Since glibc 2.19:
               _DEFAULT_SOURCE
           Glibc 2.19 and earlier:
               _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.

   preadv2() and pwritev2()
       These system calls are similar to preadv() and pwritev() calls, but add a fifth  argument,
       flags, which modifies the behavior on a per-call basis.

       Unlike  preadv() and pwritev(), if the offset argument is -1, then the current file offset
       is used and updated.

       The flags argument contains a bitwise OR of zero or more of the following flags:

       RWF_DSYNC (since Linux 4.7)
              Provide a  per-write  equivalent  of  the  O_DSYNC  open(2)  flag.   This  flag  is
              meaningful  only  for  pwritev2(),  and  its  effect applies only to the data range
              written by the system call.

       RWF_HIPRI (since Linux 4.6)
              High priority read/write.  Allows block-based filesystems to  use  polling  of  the
              device,   which   provides   lower  latency,  but  may  use  additional  resources.
              (Currently, this feature is usable only on  a  file  descriptor  opened  using  the
              O_DIRECT flag.)

       RWF_SYNC (since Linux 4.7)
              Provide a per-write equivalent of the O_SYNC open(2) flag.  This flag is meaningful
              only for pwritev2(), and its effect applies only to the data range written  by  the
              system call.

       RWF_NOWAIT (since Linux 4.14)
              Do  not  wait  for  data  which  is  not  immediately  available.   If this flag is
              specified, the preadv2() system call will return instantly if it would have to read
              data  from  the  backing storage or wait for a lock.  If some data was successfully
              read, it will return the number of bytes read.  If no  bytes  were  read,  it  will
              return  -1  and  set  errno to EAGAIN.  Currently, this flag is meaningful only for
              preadv2().

       RWF_APPEND (since Linux 4.16)
              Provide a per-write  equivalent  of  the  O_APPEND  open(2)  flag.   This  flag  is
              meaningful  only  for  pwritev2(),  and  its  effect applies only to the data range
              written by the system  call.   The  offset  argument  does  not  affect  the  write
              operation;  the  data  is  always appended to the end of the file.  However, if the
              offset argument is -1, the current file offset is updated.

RETURN VALUE

       On success, readv(), preadv() and preadv2() return the number  of  bytes  read;  writev(),
       pwritev() and pwritev2() return the number of bytes written.

       Note  that it 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(),  preadv2(),
       pwritev(),  and  pwritev2() can also fail for the same reasons as lseek(2).  Additionally,
       the following errors are 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.

       EOPNOTSUPP
              An unknown flag is specified in flags.

VERSIONS

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

       preadv2()  and pwritev2() first appeared in Linux 4.6.  Library support was added in glibc
       2.26.

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.

       preadv2(), pwritev2(): nonstandard Linux extension.

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.

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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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.