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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.   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.
       Linux libc5 used size_t as the type of the iovcnt argument, and int as the return type.

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

NOTES

   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

       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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       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.