bionic (2) pwritev.2.gz

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NAME

       readv, writev, preadv, pwritev, preadv2, pwritev2 - 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);

       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:
               _DEFAULT_SOURCE
           Glibc 2.19 and earlier:
               _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.

   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_DSYNC (since Linux 4.7)
              Provide  a  per-write  equivalent  of  the O_DSYNC open(2) flag.  This flag is meaningful only for
              pwritev2(), and its effect applies only to the data range written by the system call.

       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 flag.)

       RWF_SYNC (since Linux 4.7)
              Provide a per-write equivalent of the O_SYNC open(2) flag.   This  flag  is  meaningful  only  for
              pwritev2(), and its effect applies only to the data range written by the system call.

       RWF_NOWAIT (since Linux 4.14)
              Do not wait for data which is not immediately available.  If this flag is specified, the preadv2()
              system call will return instantly if it would have to read data from the backing storage  or  wait
              for  a  lock.  If some data was successfully read, it will return the number of bytes read.  If no
              bytes were read, it will return -1 and set errno to EAGAIN.  Currently, this  flag  is  meaningful
              only for preadv2().

RETURN VALUE

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

       Note that it 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(), preadv2(), pwritev(), and
       pwritev2() can also fail for the same reasons  as  lseek(2).   Additionally,  the  following  errors  are
       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.

       EINVAL An unknown flag is specified in flags.

VERSIONS

       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.  Library support was added in glibc 2.26.

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.

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

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.

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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