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man:pwritev2

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.
     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.
     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 4.16 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 2018-04-30 READV(2)

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