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 in‐
66       termingled with output from writes in other processes (but see  pipe(7)
67       for  an  exception);  analogously, readv() is guaranteed to read a con‐
68       tiguous block of data from the file, regardless of read operations per‐
69       formed  in other threads or processes that have file descriptors refer‐
70       ring 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 ar‐
75       gument, offset, which specifies the file offset at which the input  op‐
76       eration 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  re‐
84       ferred 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  ba‐
89       sis.
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  ap‐
100              plies 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 ap‐
125              plies 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  de‐
144       fined:
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 un‐
180       packed by the wrapper functions into two arguments in the system calls:
181
182           unsigned long pos_l, unsigned long pos
183
184       These arguments contain, respectively, the low order and high order  32
185       bits of offset.
186
187   Historical C library/kernel differences
188       To  deal  with  the  fact  that IOV_MAX was so low on early versions of
189       Linux, the glibc wrapper functions for readv() and  writev()  did  some
190       extra  work  if  they  detected  that the underlying kernel system call
191       failed because this limit was exceeded.  In the case  of  readv(),  the
192       wrapper  function  allocated a temporary buffer large enough for all of
193       the items specified by iov, passed that buffer in a  call  to  read(2),
194       copied  data from the buffer to the locations specified by the iov_base
195       fields of the elements of iov, and then freed the buffer.  The  wrapper
196       function  for  writev()  performed the analogous task using a temporary
197       buffer and a call to write(2).
198
199       The need for this extra effort in the glibc wrapper functions went away
200       with Linux 2.2 and later.  However, glibc continued to provide this be‐
201       havior until version 2.10.  Starting with glibc version 2.9, the  wrap‐
202       per  functions  provide  this behavior only if the library detects that
203       the system is running a Linux kernel older than version 2.6.18 (an  ar‐
204       bitrarily  selected  kernel  version).  And since glibc 2.20 (which re‐
205       quires a minimum Linux kernel version of  2.6.32),  the  glibc  wrapper
206       functions always just directly invoke the system calls.
207

EXAMPLES

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

SEE ALSO

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

COLOPHON

227       This  page  is  part of release 5.10 of the Linux man-pages project.  A
228       description of the project, information about reporting bugs,  and  the
229       latest     version     of     this    page,    can    be    found    at
230       https://www.kernel.org/doc/man-pages/.
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234Linux                             2020-12-21                          READV(2)
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