1WRITEV(3P) POSIX Programmer's Manual WRITEV(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 writev — write a vector
13
15 #include <sys/uio.h>
16 ssize_t writev(int fildes, const struct iovec *iov, int iovcnt);
18
20 The writev() function shall be equivalent to write(), except as
21 described below. The writev() function shall gather output data from
22 the iovcnt buffers specified by the members of the iov array: iov[0],
23 iov[1], ..., iov[iovcnt-1]. The iovcnt argument is valid if greater
24 than 0 and less than or equal to {IOV_MAX}, as defined in <limits.h>.
25 Each iovec entry specifies the base address and length of an area in
27 memory from which data should be written. The writev() function shall
28 always write a complete area before proceeding to the next.
29 If fildes refers to a regular file and all of the iov_len members in
31 the array pointed to by iov are 0, writev() shall return 0 and have no
32 other effect. For other file types, the behavior is unspecified.
33 If the sum of the iov_len values is greater than {SSIZE_MAX}, the oper‐
35 ation shall fail and no data shall be transferred.
36
38 Upon successful completion, writev() shall return the number of bytes
39 actually written. Otherwise, it shall return a value of -1, the file-
40 pointer shall remain unchanged, and errno shall be set to indicate an
41 error.
42
44 Refer to write().
45 In addition, the writev() function shall fail if:
47 EINVAL The sum of the iov_len values in the iov array would overflow an
49 ssize_t.
50 The writev() function may fail and set errno to:
52 EINVAL The iovcnt argument was less than or equal to 0, or greater than
54 {IOV_MAX}.
55 The following sections are informative.
57
59 Writing Data from an Array
60 The following example writes data from the buffers specified by members
61 of the iov array to the file associated with the file descriptor fd.
62 #include <sys/types.h>
65 #include <sys/uio.h>
66 #include <unistd.h>
67 ...
68 ssize_t bytes_written;
69 int fd;
70 char *buf0 = "short string\n";
71 char *buf1 = "This is a longer string\n";
72 char *buf2 = "This is the longest string in this example\n";
73 int iovcnt;
74 struct iovec iov[3];
75 iov[0].iov_base = buf0;
77 iov[0].iov_len = strlen(buf0);
78 iov[1].iov_base = buf1;
79 iov[1].iov_len = strlen(buf1);
80 iov[2].iov_base = buf2;
81 iov[2].iov_len = strlen(buf2);
82 ...
83 iovcnt = sizeof(iov) / sizeof(struct iovec);
84 bytes_written = writev(fd, iov, iovcnt);
86 ...
87
89 None.
90
92 Refer to write().
93
95 None.
96
98 readv(), write()
99 The Base Definitions volume of POSIX.1‐2017, <limits.h>, <sys_uio.h>
101
103 Portions of this text are reprinted and reproduced in electronic form
104 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
105 table Operating System Interface (POSIX), The Open Group Base Specifi‐
106 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
107 Electrical and Electronics Engineers, Inc and The Open Group. In the
108 event of any discrepancy between this version and the original IEEE and
109 The Open Group Standard, the original IEEE and The Open Group Standard
110 is the referee document. The original Standard can be obtained online
111 at http://www.opengroup.org/unix/online.html .
112 Any typographical or formatting errors that appear in this page are
114 most likely to have been introduced during the conversion of the source
115 files to man page format. To report such errors, see https://www.ker‐
116 nel.org/doc/man-pages/reporting_bugs.html .
117IEEE/The Open Group 2017 WRITEV(3P)