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

NAME         top

       readv, writev - read or write data into multiple buffers

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);

DESCRIPTION         top

       The readv() function reads iovcnt buffers from the file associated with the
       file descriptor fd into the buffers described by iov ("scatter input").

       The writev() function 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() function works just like read(2) except that multiple buffers are
       filled.

       The writev() function 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
       open(2)).

RETURN VALUE         top

       On success, the readv() function returns the number of bytes read; the
       writev() function returns the number of bytes written.  On error, -1 is
       returned, and errno is set appropriately.

ERRORS         top

       The errors are as given for read(2) and write(2).  Additionally the following
       error is defined:

       EINVAL The sum of the iov_len values overflows an ssize_t value.  Or, the
              vector count iovcnt is less than zero or greater than the permitted
              maximum.

CONFORMING TO         top

       4.4BSD (the readv() and writev() functions first appeared in 4.2BSD),
       POSIX.1-2001.  Linux libc5 used size_t as the type of the iovcnt argument, and
       int as return type for these functions.

NOTES         top

Linux Notes

       POSIX.1-2001 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 Linux, the limit advertised by these mechanisms is
       1024, which is the true kernel limit.  However, the glibc wrapper functions do
       some extra work if they detect that the underlying kernel system call failed
       because this limit was exceeded.  In the case of readv() the wrapper function
       allocates a temporary buffer large enough for all of the items specified by
       iov, passes that buffer in a call to read(2), copies data from the buffer to
       the locations specified by the iov_base fields of the elements of iov, and
       then frees the buffer.  The wrapper function for writev() performs the
       analogous task using a temporary buffer and a call to write(2).

BUGS         top

       It is not advisable to mix calls to functions like 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 want.

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

       read(2), write(2)

COLOPHON         top

       This page is part of release 3.23 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can be found
       at http://www.kernel.org/doc/man-pages/.

Linux                                 2002-10-17                             READV(2)