1READV(2) Linux Programmer's Manual READV(2)
2
6 readv, writev, preadv, pwritev, preadv2, pwritev2 - read or write data
7 into multiple buffers
8
10 #include <sys/uio.h>
11 ssize_t readv(int fd, const struct iovec *iov, int iovcnt);
13 ssize_t writev(int fd, const struct iovec *iov, int iovcnt);
14 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);
19 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);
24 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
26 preadv(), pwritev():
28 Since glibc 2.19:
29 _DEFAULT_SOURCE
30 Glibc 2.19 and earlier:
31 _BSD_SOURCE
32
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 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 The pointer iov points to an array of iovec structures, defined in
42 <sys/uio.h> as:
43 struct iovec {
45 void *iov_base; /* Starting address */
46 size_t iov_len; /* Number of bytes to transfer */
47 };
48 The readv() system call works just like read(2) except that multiple
50 buffers are filled.
51 The writev() system call works just like write(2) except that multiple
53 buffers are written out.
54 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.
60 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 (but see pipe(7)
64 for an exception); analogously, readv() is guaranteed to read a con‐
65 tiguous block of data from the file, regardless of read operations per‐
66 formed in other threads or processes that have file descriptors refer‐
67 ring to the same open file description (see open(2)).
68 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 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 The file offset is not changed by these system calls. The file re‐
81 ferred to by fd must be capable of seeking.
82 preadv2() and pwritev2()
84 These system calls are similar to preadv() and pwritev() calls, but add
85 a fifth argument, flags, which modifies the behavior on a per-call ba‐
86 sis.
87 Unlike preadv() and pwritev(), if the offset argument is -1, then the
89 current file offset is used and updated.
90 The flags argument contains a bitwise OR of zero or more of the follow‐
92 ing flags:
93 RWF_DSYNC (since Linux 4.7)
95 Provide a per-write equivalent of the O_DSYNC open(2) flag.
96 This flag is meaningful only for pwritev2(), and its effect ap‐
97 plies only to the data range written by the system call.
98 RWF_HIPRI (since Linux 4.6)
100 High priority read/write. Allows block-based filesystems to use
101 polling of the device, which provides lower latency, but may use
102 additional resources. (Currently, this feature is usable only
103 on a file descriptor opened using the O_DIRECT flag.)
104 RWF_SYNC (since Linux 4.7)
106 Provide a per-write equivalent of the O_SYNC open(2) flag. This
107 flag is meaningful only for pwritev2(), and its effect applies
108 only to the data range written by the system call.
109 RWF_NOWAIT (since Linux 4.14)
111 Do not wait for data which is not immediately available. If
112 this flag is specified, the preadv2() system call will return
113 instantly if it would have to read data from the backing storage
114 or wait for a lock. If some data was successfully read, it will
115 return the number of bytes read. If no bytes were read, it will
116 return -1 and set errno to EAGAIN. Currently, this flag is
117 meaningful only for preadv2().
118 RWF_APPEND (since Linux 4.16)
120 Provide a per-write equivalent of the O_APPEND open(2) flag.
121 This flag is meaningful only for pwritev2(), and its effect ap‐
122 plies only to the data range written by the system call. The
123 offset argument does not affect the write operation; the data is
124 always appended to the end of the file. However, if the offset
125 argument is -1, the current file offset is updated.
126
128 On success, readv(), preadv(), and preadv2() return the number of bytes
129 read; writev(), pwritev(), and pwritev2() return the number of bytes
130 written.
131 Note that it is not an error for a successful call to transfer fewer
133 bytes than requested (see read(2) and write(2)).
134 On error, -1 is returned, and errno is set to indicate the error.
136
138 The errors are as given for read(2) and write(2). Furthermore,
139 preadv(), preadv2(), pwritev(), and pwritev2() can also fail for the
140 same reasons as lseek(2). Additionally, the following errors are de‐
141 fined:
142 EINVAL The sum of the iov_len values overflows an ssize_t value.
144 EINVAL The vector count, iovcnt, is less than zero or greater than the
146 permitted maximum.
147 EOPNOTSUPP
149 An unknown flag is specified in flags.
150
152 preadv() and pwritev() first appeared in Linux 2.6.30; library support
153 was added in glibc 2.10.
154 preadv2() and pwritev2() first appeared in Linux 4.6. Library support
156 was added in glibc 2.26.
157
159 readv(), writev(): POSIX.1-2001, POSIX.1-2008, 4.4BSD (these system
160 calls first appeared in 4.2BSD).
161 preadv(), pwritev(): nonstandard, but present also on the modern BSDs.
163 preadv2(), pwritev2(): nonstandard Linux extension.
165
167 POSIX.1 allows an implementation to place a limit on the number of
168 items that can be passed in iov. An implementation can advertise its
169 limit by defining IOV_MAX in <limits.h> or at run time via the return
170 value from sysconf(_SC_IOV_MAX). On modern Linux systems, the limit is
171 1024. Back in Linux 2.0 days, this limit was 16.
172 C library/kernel differences
174 The raw preadv() and pwritev() system calls have call signatures that
175 differ slightly from that of the corresponding GNU C library wrapper
176 functions shown in the SYNOPSIS. The final argument, offset, is un‐
177 packed by the wrapper functions into two arguments in the system calls:
178 unsigned long pos_l, unsigned long pos
180 These arguments contain, respectively, the low order and high order 32
182 bits of offset.
183 Historical C library/kernel differences
185 To deal with the fact that IOV_MAX was so low on early versions of
186 Linux, the glibc wrapper functions for readv() and writev() did some
187 extra work if they detected that the underlying kernel system call
188 failed because this limit was exceeded. In the case of readv(), the
189 wrapper function allocated a temporary buffer large enough for all of
190 the items specified by iov, passed that buffer in a call to read(2),
191 copied data from the buffer to the locations specified by the iov_base
192 fields of the elements of iov, and then freed the buffer. The wrapper
193 function for writev() performed the analogous task using a temporary
194 buffer and a call to write(2).
195 The need for this extra effort in the glibc wrapper functions went away
197 with Linux 2.2 and later. However, glibc continued to provide this be‐
198 havior until version 2.10. Starting with glibc version 2.9, the wrap‐
199 per functions provide this behavior only if the library detects that
200 the system is running a Linux kernel older than version 2.6.18 (an ar‐
201 bitrarily selected kernel version). And since glibc 2.20 (which re‐
202 quires a minimum Linux kernel version of 2.6.32), the glibc wrapper
203 functions always just directly invoke the system calls.
204
206 The following code sample demonstrates the use of writev():
207 char *str0 = "hello ";
209 char *str1 = "world\n";
210 struct iovec iov[2];
211 ssize_t nwritten;
212 iov[0].iov_base = str0;
214 iov[0].iov_len = strlen(str0);
215 iov[1].iov_base = str1;
216 iov[1].iov_len = strlen(str1);
217 nwritten = writev(STDOUT_FILENO, iov, 2);
219
221 pread(2), read(2), write(2)
222
224 This page is part of release 5.12 of the Linux man-pages project. A
225 description of the project, information about reporting bugs, and the
226 latest version of this page, can be found at
227 https://www.kernel.org/doc/man-pages/.
228Linux 2021-03-22 READV(2)