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
14       ssize_t writev(int fd, const struct iovec *iov, int iovcnt);
15
16       ssize_t preadv(int fd, const struct iovec *iov, int iovcnt,
17                      off_t offset);
18
19       ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt,
20                       off_t offset);
21
22       ssize_t preadv2(int fd, const struct iovec *iov, int iovcnt,
23                       off_t offset, int flags);
24
25       ssize_t pwritev2(int fd, const struct iovec *iov, int iovcnt,
26                        off_t offset, int flags);
27
28   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
29
30       preadv(), pwritev():
31           Since glibc 2.19:
32               _DEFAULT_SOURCE
33           Glibc 2.19 and earlier:
34               _BSD_SOURCE
35

DESCRIPTION

37       The readv() system call reads iovcnt buffers from the  file  associated
38       with the file descriptor fd into the buffers described by iov ("scatter
39       input").
40
41       The writev() system call writes iovcnt buffers of data described by iov
42       to the file associated with the file descriptor fd ("gather output").
43
44       The  pointer  iov  points  to  an array of iovec structures, defined in
45       <sys/uio.h> as:
46
47           struct iovec {
48               void  *iov_base;    /* Starting address */
49               size_t iov_len;     /* Number of bytes to transfer */
50           };
51
52       The readv() system call works just like read(2)  except  that  multiple
53       buffers are filled.
54
55       The  writev() system call works just like write(2) except that multiple
56       buffers are written out.
57
58       Buffers are processed in array order.  This  means  that  readv()  com‐
59       pletely fills iov[0] before proceeding to iov[1], and so on.  (If there
60       is insufficient data, then not all buffers pointed to  by  iov  may  be
61       filled.)   Similarly, writev() writes out the entire contents of iov[0]
62       before proceeding to iov[1], and so on.
63
64       The data transfers performed by readv() and writev()  are  atomic:  the
65       data  written  by  writev()  is  written  as a single block that is not
66       intermingled with output  from  writes  in  other  processes  (but  see
67       pipe(7) for an exception); analogously, readv() is guaranteed to read a
68       contiguous block of data from the file, regardless of  read  operations
69       performed  in  other  threads  or  processes that have file descriptors
70       referring to the same open file description (see open(2)).
71
72   preadv() and pwritev()
73       The preadv() system call combines  the  functionality  of  readv()  and
74       pread(2).   It  performs  the  same  task as readv(), but adds a fourth
75       argument, offset, which specifies the file offset at  which  the  input
76       operation is to be performed.
77
78       The  pwritev()  system  call combines the functionality of writev() and
79       pwrite(2).  It performs the same task as writev(), but  adds  a  fourth
80       argument,  offset,  which specifies the file offset at which the output
81       operation is to be performed.
82
83       The file offset is  not  changed  by  these  system  calls.   The  file
84       referred to by fd must be capable of seeking.
85
86   preadv2() and pwritev2()
87       These system calls are similar to preadv() and pwritev() calls, but add
88       a fifth argument, flags, which modifies  the  behavior  on  a  per-call
89       basis.
90
91       Unlike  preadv()  and pwritev(), if the offset argument is -1, then the
92       current file offset is used and updated.
93
94       The flags argument contains a bitwise OR of zero or more of the follow‐
95       ing flags:
96
97       RWF_DSYNC (since Linux 4.7)
98              Provide  a  per-write  equivalent  of  the O_DSYNC open(2) flag.
99              This flag is meaningful only  for  pwritev2(),  and  its  effect
100              applies only to the data range written by the system call.
101
102       RWF_HIPRI (since Linux 4.6)
103              High priority read/write.  Allows block-based filesystems to use
104              polling of the device, which provides lower latency, but may use
105              additional  resources.   (Currently, this feature is usable only
106              on a file descriptor opened using the O_DIRECT flag.)
107
108       RWF_SYNC (since Linux 4.7)
109              Provide a per-write equivalent of the O_SYNC open(2) flag.  This
110              flag  is  meaningful only for pwritev2(), and its effect applies
111              only to the data range written by the system call.
112
113       RWF_NOWAIT (since Linux 4.14)
114              Do not wait for data which is  not  immediately  available.   If
115              this  flag  is  specified, the preadv2() system call will return
116              instantly if it would have to read data from the backing storage
117              or wait for a lock.  If some data was successfully read, it will
118              return the number of bytes read.  If no bytes were read, it will
119              return  -1  and  set  errno  to EAGAIN.  Currently, this flag is
120              meaningful only for preadv2().
121
122       RWF_APPEND (since Linux 4.16)
123              Provide a per-write equivalent of  the  O_APPEND  open(2)  flag.
124              This  flag  is  meaningful  only  for pwritev2(), and its effect
125              applies only to the data range written by the system call.   The
126              offset argument does not affect the write operation; the data is
127              always appended to the end of the file.  However, if the  offset
128              argument is -1, the current file offset is updated.
129

RETURN VALUE

131       On  success, readv(), preadv() and preadv2() return the number of bytes
132       read; writev(), pwritev() and pwritev2() return  the  number  of  bytes
133       written.
134
135       Note  that  it  is not an error for a successful call to transfer fewer
136       bytes than requested (see read(2) and write(2)).
137
138       On error, -1 is returned, and errno is set appropriately.
139

ERRORS

141       The errors  are  as  given  for  read(2)  and  write(2).   Furthermore,
142       preadv(),  preadv2(),  pwritev(),  and pwritev2() can also fail for the
143       same reasons as  lseek(2).   Additionally,  the  following  errors  are
144       defined:
145
146       EINVAL The sum of the iov_len values overflows an ssize_t value.
147
148       EINVAL The  vector count, iovcnt, is less than zero or greater than the
149              permitted maximum.
150
151       EOPNOTSUPP
152              An unknown flag is specified in flags.
153

VERSIONS

155       preadv() and pwritev() first appeared in Linux 2.6.30; library  support
156       was added in glibc 2.10.
157
158       preadv2()  and pwritev2() first appeared in Linux 4.6.  Library support
159       was added in glibc 2.26.
160

CONFORMING TO

162       readv(), writev(): POSIX.1-2001,  POSIX.1-2008,  4.4BSD  (these  system
163       calls first appeared in 4.2BSD).
164
165       preadv(), pwritev(): nonstandard, but present also on the modern BSDs.
166
167       preadv2(), pwritev2(): nonstandard Linux extension.
168

NOTES

170       POSIX.1  allows  an  implementation  to  place a limit on the number of
171       items that can be passed in iov.  An implementation can  advertise  its
172       limit  by  defining IOV_MAX in <limits.h> or at run time via the return
173       value from sysconf(_SC_IOV_MAX).  On modern Linux systems, the limit is
174       1024.  Back in Linux 2.0 days, this limit was 16.
175
176   C library/kernel differences
177       The  raw  preadv() and pwritev() system calls have call signatures that
178       differ slightly from that of the corresponding GNU  C  library  wrapper
179       functions  shown  in  the  SYNOPSIS.   The  final  argument, offset, is
180       unpacked by the wrapper functions into  two  arguments  in  the  system
181       calls:
182
183           unsigned long pos_l, unsigned long pos
184
185       These  arguments contain, respectively, the low order and high order 32
186       bits of offset.
187
188   Historical C library/kernel differences
189       To deal with the fact that IOV_MAX was so  low  on  early  versions  of
190       Linux,  the  glibc  wrapper functions for readv() and writev() did some
191       extra work if they detected that  the  underlying  kernel  system  call
192       failed  because  this  limit was exceeded.  In the case of readv(), the
193       wrapper function allocated a temporary buffer large enough for  all  of
194       the  items  specified  by iov, passed that buffer in a call to read(2),
195       copied data from the buffer to the locations specified by the  iov_base
196       fields  of the elements of iov, and then freed the buffer.  The wrapper
197       function for writev() performed the analogous task  using  a  temporary
198       buffer and a call to write(2).
199
200       The need for this extra effort in the glibc wrapper functions went away
201       with Linux 2.2 and later.  However, glibc  continued  to  provide  this
202       behavior  until  version  2.10.   Starting  with glibc version 2.9, the
203       wrapper functions provide this behavior only  if  the  library  detects
204       that the system is running a Linux kernel older than version 2.6.18 (an
205       arbitrarily selected kernel version).   And  since  glibc  2.20  (which
206       requires  a  minimum Linux kernel version of 2.6.32), the glibc wrapper
207       functions always just directly invoke the system calls.
208

EXAMPLE

210       The following code sample demonstrates the use of writev():
211
212           char *str0 = "hello ";
213           char *str1 = "world\n";
214           struct iovec iov[2];
215           ssize_t nwritten;
216
217           iov[0].iov_base = str0;
218           iov[0].iov_len = strlen(str0);
219           iov[1].iov_base = str1;
220           iov[1].iov_len = strlen(str1);
221
222           nwritten = writev(STDOUT_FILENO, iov, 2);
223

SEE ALSO

225       pread(2), read(2), write(2)
226

COLOPHON

228       This page is part of release 5.04 of the Linux  man-pages  project.   A
229       description  of  the project, information about reporting bugs, and the
230       latest    version    of    this    page,    can     be     found     at
231       https://www.kernel.org/doc/man-pages/.
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235Linux                             2018-04-30                          READV(2)
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