1READV(2)                   Linux Programmer's Manual                  READV(2)
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NAME

6       readv,  writev, preadv, pwritev, preadv2, pwritev2 - read or write data
7       into multiple buffers
8

SYNOPSIS

10       #include <sys/uio.h>
11
12       ssize_t readv(int fd, const struct iovec *iov, int iovcnt);
13       ssize_t writev(int fd, const struct iovec *iov, int iovcnt);
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15       ssize_t preadv(int fd, const struct iovec *iov, int iovcnt,
16                       off_t offset);
17       ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt,
18                       off_t offset);
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20       ssize_t preadv2(int fd, const struct iovec *iov, int iovcnt,
21                       off_t offset, int flags);
22       ssize_t pwritev2(int fd, const struct iovec *iov, int iovcnt,
23                       off_t offset, int flags);
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25   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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27       preadv(), pwritev():
28           Since glibc 2.19:
29               _DEFAULT_SOURCE
30           Glibc 2.19 and earlier:
31               _BSD_SOURCE
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DESCRIPTION

34       The readv() system call reads iovcnt buffers from the  file  associated
35       with the file descriptor fd into the buffers described by iov ("scatter
36       input").
37
38       The writev() system call writes iovcnt buffers of data described by iov
39       to the file associated with the file descriptor fd ("gather output").
40
41       The  pointer  iov  points  to  an array of iovec structures, defined in
42       <sys/uio.h> as:
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44           struct iovec {
45               void  *iov_base;    /* Starting address */
46               size_t iov_len;     /* Number of bytes to transfer */
47           };
48
49       The readv() system call works just like read(2)  except  that  multiple
50       buffers are filled.
51
52       The  writev() system call works just like write(2) except that multiple
53       buffers are written out.
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55       Buffers are processed in array order.  This  means  that  readv()  com‐
56       pletely fills iov[0] before proceeding to iov[1], and so on.  (If there
57       is insufficient data, then not all buffers pointed to  by  iov  may  be
58       filled.)   Similarly, writev() writes out the entire contents of iov[0]
59       before proceeding to iov[1], and so on.
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61       The data transfers performed by readv() and writev()  are  atomic:  the
62       data  written  by writev() is written as a single block that is not in‐
63       termingled with output from writes  in  other  processes;  analogously,
64       readv() is guaranteed to read a contiguous block of data from the file,
65       regardless of read operations performed in other threads  or  processes
66       that  have file descriptors referring to the same open file description
67       (see open(2)).
68
69   preadv() and pwritev()
70       The preadv() system call combines  the  functionality  of  readv()  and
71       pread(2).   It performs the same task as readv(), but adds a fourth ar‐
72       gument, offset, which specifies the file offset at which the input  op‐
73       eration is to be performed.
74
75       The  pwritev()  system  call combines the functionality of writev() and
76       pwrite(2).  It performs the same task as writev(), but  adds  a  fourth
77       argument,  offset,  which specifies the file offset at which the output
78       operation is to be performed.
79
80       The file offset is not changed by these