READV(2)                  (2020-12-21)                   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

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          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); analo-
          gously, 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 speci-
          fies 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.

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          RWF_HIPRI (since Linux 4.6)
               High priority read/write.  Allows block-based filesys-
               tems 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 avail-
               able.  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 num-
          ber 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 trans-
          fer 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).  Further-
          more, 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.

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          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 signa-
          tures 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 ver-
          sions 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

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          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 func-
          tions 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 pro-
          vide this behavior only if the library detects that the sys-
          tem 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.

     EXAMPLES
          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 5.10 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/.

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