1READV(2) Linux Programmer's Manual READV(2)
2
6 readv, writev, preadv, pwritev - read or write data into multiple buf‐
7 fers
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);
15 ssize_t preadv(int fd, const struct iovec *iov, int iovcnt,
17 off_t offset);
18 ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt,
20 off_t offset);
21 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
23 preadv(), pwritev(): _BSD_SOURCE
25
27 The readv() system call reads iovcnt buffers from the file associated
28 with the file descriptor fd into the buffers described by iov ("scatter
29 input").
30 The writev() system call writes iovcnt buffers of data described by iov
32 to the file associated with the file descriptor fd ("gather output").
33 The pointer iov points to an array of iovec structures, defined in
35 <sys/uio.h> as:
36 struct iovec {
38 void *iov_base; /* Starting address */
39 size_t iov_len; /* Number of bytes to transfer */
40 };
41 The readv() system call works just like read(2) except that multiple
43 buffers are filled.
44 The writev() system call works just like write(2) except that multiple
46 buffers are written out.
47 Buffers are processed in array order. This means that readv() com‐
49 pletely fills iov[0] before proceeding to iov[1], and so on. (If there
50 is insufficient data, then not all buffers pointed to by iov may be
51 filled.) Similarly, writev() writes out the entire contents of iov[0]
52 before proceeding to iov[1], and so on.
53 The data transfers performed by readv() and writev() are atomic: the
55 data written by writev() is written as a single block that is not
56 intermingled with output from writes in other processes (but see
57 pipe(7) for an exception); analogously, readv() is guaranteed to read a
58 contiguous block of data from the file, regardless of read operations
59 performed in other threads or processes that have file descriptors
60 referring to the same open file description (see open(2)).
61 preadv() and pwritev()
63 The preadv() system call combines the functionality of readv() and
64 pread(2). It performs the same task as readv(), but adds a fourth
65 argument, offset, which specifies the file offset at which the input
66 operation is to be performed.
67 The pwritev() system call combines the functionality of writev() and
69 pwrite(2). It performs the same task as writev(), but adds a fourth
70 argument, offset, which specifies the file offset at which the output
71 operation is to be performed.
72 The file offset is not changed by these system calls. The file
74 referred to by fd must be capable of seeking.
75
77 On success, readv() and preadv() return the number of bytes read;
78 writev() and pwritev() return the number of bytes written. On error,
79 -1 is returned, and errno is set appropriately.
80
82 The errors are as given for read(2) and write(2). Furthermore,
83 preadv() and pwritev() can also fail for the same reasons as lseek(2).
84 Additionally, the following error is defined:
85 EINVAL The sum of the iov_len values overflows an ssize_t value. Or,
87 the vector count iovcnt is less than zero or greater than the
88 permitted maximum.
89
91 preadv() and pwritev() first appeared in Linux 2.6.30; library support
92 was added in glibc 2.10.
93
95 readv(), writev(): 4.4BSD (these system calls first appeared in
96 4.2BSD), POSIX.1-2001. Linux libc5 used size_t as the type of the
97 iovcnt argument, and int as the return type.
98 preadv(), pwritev(): nonstandard, but present also on the modern BSDs.
100
102 Linux Notes
103 POSIX.1-2001 allows an implementation to place a limit on the number of
104 items that can be passed in iov. An implementation can advertise its
105 limit by defining IOV_MAX in <limits.h> or at run time via the return
106 value from sysconf(_SC_IOV_MAX). On Linux, the limit advertised by
107 these mechanisms is 1024, which is the true kernel limit. However, the
108 glibc wrapper functions do some extra work if they detect that the
109 underlying kernel system call failed because this limit was exceeded.
110 In the case of readv() the wrapper function allocates a temporary buf‐
111 fer large enough for all of the items specified by iov, passes that
112 buffer in a call to read(2), copies data from the buffer to the loca‐
113 tions specified by the iov_base fields of the elements of iov, and then
114 frees the buffer. The wrapper function for writev() performs the anal‐
115 ogous task using a temporary buffer and a call to write(2).
116
118 It is not advisable to mix calls to readv() or writev(), which operate
119 on file descriptors, with the functions from the stdio library; the
120 results will be undefined and probably not what you want.
121
123 The following code sample demonstrates the use of writev():
124 char *str0 = "hello ";
126 char *str1 = "world\n";
127 struct iovec iov[2];
128 ssize_t nwritten;
129 iov[0].iov_base = str0;
131 iov[0].iov_len = strlen(str0);
132 iov[1].iov_base = str1;
133 iov[1].iov_len = strlen(str1);
134 nwritten = writev(STDOUT_FILENO, iov, 2);
136
138 pread(2), read(2), write(2)
139
141 This page is part of release 3.32 of the Linux man-pages project. A
142 description of the project, and information about reporting bugs, can
143 be found at http://www.kernel.org/doc/man-pages/.
144Linux 2010-11-17 READV(2)