1readv(2)                      System Calls Manual                     readv(2)
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3
4

NAME

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

LIBRARY

10       Standard C library (libc, -lc)
11

SYNOPSIS

13       #include <sys/uio.h>
14
15       ssize_t readv(int fd, const struct iovec *iov, int iovcnt);
16       ssize_t writev(int fd, const struct iovec *iov, int iovcnt);
17
18       ssize_t preadv(int fd, const struct iovec *iov, int iovcnt,
19                       off_t offset);
20       ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt,
21                       off_t offset);
22
23       ssize_t preadv2(int fd, const struct iovec *iov, int iovcnt,
24                       off_t offset, int flags);
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, described in
45       iovec(3type).
46
47       The readv() system call works just like read(2)  except  that  multiple
48       buffers are filled.
49
50       The  writev() system call works just like write(2) except that multiple
51       buffers are written out.
52
53       Buffers are processed in array order.  This  means  that  readv()  com‐
54       pletely fills iov[0] before proceeding to iov[1], and so on.  (If there
55       is insufficient data, then not all buffers pointed to  by  iov  may  be
56       filled.)   Similarly, writev() writes out the entire contents of iov[0]
57       before proceeding to iov[1], and so on.
58
59       The data transfers performed by readv() and writev()  are  atomic:  the
60       data  written  by writev() is written as a single block that is not in‐
61       termingled with output from writes  in  other  processes;  analogously,
62       readv() is guaranteed to read a contiguous block of data from the file,
63       regardless of read operations performed in other threads  or  processes
64       that  have file descriptors referring to the same open file description
65       (see open(2)).
66
67   preadv() and pwritev()
68       The preadv() system call combines  the  functionality  of  readv()  and
69       pread(2).   It performs the same task as readv(), but adds a fourth ar‐
70       gument, offset, which specifies the file offset at which the input  op‐
71       eration is to be performed.
72
73       The  pwritev()  system  call combines the functionality of writev() and
74       pwrite(2).  It performs the same task as writev(), but  adds  a  fourth
75       argument,  offset,  which specifies the file offset at which the output
76       operation is to be performed.
77
78       The file offset is not changed by these system  calls.   The  file  re‐
79       ferred to by fd must be capable of seeking.
80
81   preadv2() and pwritev2()
82       These system calls are similar to preadv() and pwritev() calls, but add
83       a fifth argument, flags, which modifies the behavior on a per-call  ba‐
84       sis.
85
86       Unlike  preadv()  and pwritev(), if the offset argument is -1, then the
87       current file offset is used and updated.
88
89       The flags argument contains a bitwise OR of zero or more of the follow‐
90       ing flags:
91
92       RWF_DSYNC (since Linux 4.7)
93              Provide  a  per-write  equivalent  of  the O_DSYNC open(2) flag.
94              This flag is meaningful only for pwritev2(), and its effect  ap‐
95              plies only to the data range written by the system call.
96
97       RWF_HIPRI (since Linux 4.6)
98              High priority read/write.  Allows block-based filesystems to use
99              polling of the device, which provides lower latency, but may use
100              additional  resources.   (Currently, this feature is usable only
101              on a file descriptor opened using the O_DIRECT flag.)
102
103       RWF_SYNC (since Linux 4.7)
104              Provide a per-write equivalent of the O_SYNC open(2) flag.  This
105              flag  is  meaningful only for pwritev2(), and its effect applies
106              only to the data range written by the system call.
107
108       RWF_NOWAIT (since Linux 4.14)
109              Do not wait for data which is  not  immediately  available.   If
110              this  flag  is  specified, the preadv2() system call will return
111              instantly if it would have to read data from the backing storage
112              or wait for a lock.  If some data was successfully read, it will
113              return the number of bytes read.  If no bytes were read, it will
114              return  -1  and  set errno to EAGAIN (but see BUGS).  Currently,
115              this flag is meaningful only for preadv2().
116
117       RWF_APPEND (since Linux 4.16)
118              Provide a per-write equivalent of  the  O_APPEND  open(2)  flag.
119              This  flag is meaningful only for pwritev2(), and its effect ap‐
120              plies only to the data range written by the  system  call.   The
121              offset argument does not affect the write operation; the data is
122              always appended to the end of the file.  However, if the  offset
123              argument is -1, the current file offset is updated.
124

RETURN VALUE

126       On success, readv(), preadv(), and preadv2() return the number of bytes
127       read; writev(), pwritev(), and pwritev2() return the  number  of  bytes
128       written.
129
130       Note  that  it  is not an error for a successful call to transfer fewer
131       bytes than requested (see read(2) and write(2)).
132
133       On error, -1 is returned, and errno is set to indicate the error.
134

ERRORS

136       The errors  are  as  given  for  read(2)  and  write(2).   Furthermore,
137       preadv(),  preadv2(),  pwritev(),  and pwritev2() can also fail for the
138       same reasons as lseek(2).  Additionally, the following errors  are  de‐
139       fined:
140
141       EINVAL The sum of the iov_len values overflows an ssize_t value.
142
143       EINVAL The  vector count, iovcnt, is less than zero or greater than the
144              permitted maximum.
145
146       EOPNOTSUPP
147              An unknown flag is specified in flags.
148

VERSIONS

150   C library/kernel differences
151       The raw preadv() and pwritev() system calls have call  signatures  that
152       differ  slightly  from  that of the corresponding GNU C library wrapper
153       functions shown in the SYNOPSIS.  The final argument,  offset,  is  un‐
154       packed by the wrapper functions into two arguments in the system calls:
155
156           unsigned long pos_l, unsigned long pos
157
158       These  arguments contain, respectively, the low order and high order 32
159       bits of offset.
160

STANDARDS

162       readv()
163       writev()
164              POSIX.1-2008.
165
166       preadv()
167       pwritev()
168              BSD.
169
170       preadv2()
171       pwritev2()
172              Linux.
173

HISTORY

175       readv()
176       writev()
177              POSIX.1-2001, 4.4BSD (first appeared in 4.2BSD).
178
179       preadv(), pwritev(): Linux 2.6.30, glibc 2.10.
180
181       preadv2(), pwritev2(): Linux 4.6, glibc 2.26.
182
183   Historical C library/kernel differences
184       To deal with the fact that IOV_MAX was so  low  on  early  versions  of
185       Linux,  the  glibc  wrapper functions for readv() and writev() did some
186       extra work if they detected that  the  underlying  kernel  system  call
187       failed  because  this  limit was exceeded.  In the case of readv(), the
188       wrapper function allocated a temporary buffer large enough for  all  of
189       the  items  specified  by iov, passed that buffer in a call to read(2),
190       copied data from the buffer to the locations specified by the  iov_base
191       fields  of the elements of iov, and then freed the buffer.  The wrapper
192       function for writev() performed the analogous task  using  a  temporary
193       buffer and a call to write(2).
194
195       The need for this extra effort in the glibc wrapper functions went away
196       with Linux 2.2 and later.  However, glibc continued to provide this be‐
197       havior  until  glibc  2.10.  Starting with glibc 2.9, the wrapper func‐
198       tions provide this behavior only if the library detects that the system
199       is  running  a Linux kernel older than Linux 2.6.18 (an arbitrarily se‐
200       lected kernel version).  And since glibc 2.20 (which requires a minimum
201       of  Linux 2.6.32), the glibc wrapper functions always just directly in‐
202       voke the system calls.
203

NOTES

205       POSIX.1 allows an implementation to place a  limit  on  the  number  of
206       items  that  can be passed in iov.  An implementation can advertise its
207       limit by defining IOV_MAX in <limits.h> or at run time via  the  return
208       value from sysconf(_SC_IOV_MAX).  On modern Linux systems, the limit is
209       1024.  Back in Linux 2.0 days, this limit was 16.
210

BUGS

212       Linux 5.9 and Linux 5.10 have a bug where preadv2() with the RWF_NOWAIT
213       flag may return 0 even when not at end of file.
214

EXAMPLES

216       The following code sample demonstrates the use of writev():
217
218           char          *str0 = "hello ";
219           char          *str1 = "world\n";
220           ssize_t       nwritten;
221           struct iovec  iov[2];
222
223           iov[0].iov_base = str0;
224           iov[0].iov_len = strlen(str0);
225           iov[1].iov_base = str1;
226           iov[1].iov_len = strlen(str1);
227
228           nwritten = writev(STDOUT_FILENO, iov, 2);
229

SEE ALSO

231       pread(2), read(2), write(2)
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233
234
235Linux man-pages 6.05              2023-05-03                          readv(2)
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