readv(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

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

NAME         top

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

       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         top

       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().

       RWF_APPEND (since Linux 4.16)
              Provide a per-write equivalent of the O_APPEND open(2)
              flag.  This flag is meaningful only for pwritev2(), and
              its effect applies only to the data range written by the
              system call.  The offset argument does not affect the
              write operation; the data is always appended to the end of
              the file.  However, if the offset argument is -1, the
              current file offset is updated.

RETURN VALUE         top

       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 to indicate the error.

ERRORS         top

       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.

       EOPNOTSUPP
              An unknown flag is specified in flags.

VERSIONS         top

       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         top

       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         top

       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.

EXAMPLES         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

       pread(2), read(2), write(2)

COLOPHON         top

       This page is part of release 5.11 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
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                       READV(2)

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