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

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

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);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       preadv(), pwritev(): _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.

RETURN VALUE
       On  success,  readv()  and  preadv()  return  the number of bytes read;
       writev() and pwritev() return the number of bytes written.   On  error,
       -1 is returned, and errno is set appropriately.

ERRORS
       The  errors  are  as  given  for  read(2)  and  write(2).  Furthermore,
       preadv() and pwritev() can also fail for the same reasons as  lseek(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.

VERSIONS
       preadv()  and pwritev() first appeared in Linux 2.6.30; library support
       was added in glibc 2.10.

CONFORMING TO
       readv(),  writev():  4.4BSD  (these  system  calls  first  appeared  in
       4.2BSD), POSIX.1-2001.

       preadv(), pwritev(): nonstandard, but present also on the modern BSDs.

NOTES
   C library/kernel ABI differences
       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 buf-
       fer  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  loca-
       tions specified by the iov_base fields of the elements of iov, and then
       frees the buffer.  The wrapper function for writev() performs the anal-
       ogous task using a temporary buffer and a call to write(2).

BUGS
       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 want.

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 3.74 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
       http://www.kernel.org/doc/man-pages/.

Linux                             2014-08-19                          READV(2)

Czas wygenerowania: 0.01163 sek.


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