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

NAME         top

       readv,  writev,  preadv,  pwritev  - read or write data into multiple

SYNOPSIS         top

       #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:
           Glibc 2.19 and earlier:

DESCRIPTION         top

       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

       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

   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_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

RETURN VALUE         top

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

       Note that 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         top

       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

       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         top

       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

CONFORMING TO         top

       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

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

NOTES         top

       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

           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.

       It is not advisable to mix calls to readv() or writev(), which
       operate on file descriptors, with the functions from the stdio
       library; the results will be undefined and probably not what you

EXAMPLE         top

       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         top

       pread(2), read(2), write(2)

COLOPHON         top

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       latest version of this page, can be found at

Linux                            2016-03-15                         READV(2)