readv(2)



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

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()  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()
       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 follow-
       ing 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.

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.

       EINVAL 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
       This  page  is  part of release 4.13 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                          READV(2)

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