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NAME

       readv, writev, preadv, pwritev - 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);

   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()  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)).

   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.

       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

       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.

       EINVAL 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():  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.

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 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 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

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