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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() 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; 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 (but see BUGS).  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 to indicate the error.

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 de‐
       tected 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 ver‐
       sion 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.

BUGS
       Linux 5.9 and 5.10 have a bug where preadv2() with the RWF_NOWAIT flag may return 0 even when not at end of file.

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)

Linux                                                                          2021-08-27                                                                       READV(2)