1WRITE(2) Linux Programmer's Manual WRITE(2)
2
6 write - write to a file descriptor
7
9 #include <unistd.h>
10 ssize_t write(int fd, const void *buf, size_t count);
12
14 write() writes up to count bytes from the buffer starting at buf to the
15 file referred to by the file descriptor fd.
16 The number of bytes written may be less than count if, for example,
18 there is insufficient space on the underlying physical medium, or the
19 RLIMIT_FSIZE resource limit is encountered (see setrlimit(2)), or the
20 call was interrupted by a signal handler after having written less than
21 count bytes. (See also pipe(7).)
22 For a seekable file (i.e., one to which lseek(2) may be applied, for
24 example, a regular file) writing takes place at the file offset, and
25 the file offset is incremented by the number of bytes actually written.
26 If the file was open(2)ed with O_APPEND, the file offset is first set
27 to the end of the file before writing. The adjustment of the file off‐
28 set and the write operation are performed as an atomic step.
29 POSIX requires that a read(2) that can be proved to occur after a
31 write() has returned will return the new data. Note that not all
32 filesystems are POSIX conforming.
33 According to POSIX.1, if count is greater than SSIZE_MAX, the result is
35 implementation-defined; see NOTES for the upper limit on Linux.
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38 On success, the number of bytes written is returned. On error, -1 is
39 returned, and errno is set to indicate the error.
40 Note that a successful write() may transfer fewer than count bytes.
42 Such partial writes can occur for various reasons; for example, because
43 there was insufficient space on the disk device to write all of the re‐
44 quested bytes, or because a blocked write() to a socket, pipe, or simi‐
45 lar was interrupted by a signal handler after it had transferred some,
46 but before it had transferred all of the requested bytes. In the event
47 of a partial write, the caller can make another write() call to trans‐
48 fer the remaining bytes. The subsequent call will either transfer fur‐
49 ther bytes or may result in an error (e.g., if the disk is now full).
50 If count is zero and fd refers to a regular file, then write() may re‐
52 turn a failure status if one of the errors below is detected. If no
53 errors are detected, or error detection is not performed, 0 is returned
54 without causing any other effect. If count is zero and fd refers to a
55 file other than a regular file, the results are not specified.
56
58 EAGAIN The file descriptor fd refers to a file other than a socket and
59 has been marked nonblocking (O_NONBLOCK), and the write would
60 block. See open(2) for further details on the O_NONBLOCK flag.
61 EAGAIN or EWOULDBLOCK
63 The file descriptor fd refers to a socket and has been marked
64 nonblocking (O_NONBLOCK), and the write would block.
65 POSIX.1-2001 allows either error to be returned for this case,
66 and does not require these constants to have the same value, so
67 a portable application should check for both possibilities.
68 EBADF fd is not a valid file descriptor or is not open for writing.
70 EDESTADDRREQ
72 fd refers to a datagram socket for which a peer address has not
73 been set using connect(2).
74 EDQUOT The user's quota of disk blocks on the filesystem containing the
76 file referred to by fd has been exhausted.
77 EFAULT buf is outside your accessible address space.
79 EFBIG An attempt was made to write a file that exceeds the implementa‐
81 tion-defined maximum file size or the process's file size limit,
82 or to write at a position past the maximum allowed offset.
83 EINTR The call was interrupted by a signal before any data was writ‐
85 ten; see signal(7).
86 EINVAL fd is attached to an object which is unsuitable for writing; or
88 the file was opened with the O_DIRECT flag, and either the ad‐
89 dress specified in buf, the value specified in count, or the
90 file offset is not suitably aligned.
91 EIO A low-level I/O error occurred while modifying the inode. This
93 error may relate to the write-back of data written by an earlier
94 write(), which may have been issued to a different file descrip‐
95 tor on the same file. Since Linux 4.13, errors from write-back
96 come with a promise that they may be reported by subsequent.
97 write() requests, and will be reported by a subsequent fsync(2)
98 (whether or not they were also reported by write()). An alter‐
99 nate cause of EIO on networked filesystems is when an advisory
100 lock had been taken out on the file descriptor and this lock has
101 been lost. See the Lost locks section of fcntl(2) for further
102 details.
103 ENOSPC The device containing the file referred to by fd has no room for
105 the data.
106 EPERM The operation was prevented by a file seal; see fcntl(2).
108 EPIPE fd is connected to a pipe or socket whose reading end is closed.
110 When this happens the writing process will also receive a SIG‐
111 PIPE signal. (Thus, the write return value is seen only if the
112 program catches, blocks or ignores this signal.)
113 Other errors may occur, depending on the object connected to fd.
115
117 SVr4, 4.3BSD, POSIX.1-2001.
118 Under SVr4 a write may be interrupted and return EINTR at any point,
120 not just before any data is written.
121
123 The types size_t and ssize_t are, respectively, unsigned and signed in‐
124 teger data types specified by POSIX.1.
125 A successful return from write() does not make any guarantee that data
127 has been committed to disk. On some filesystems, including NFS, it
128 does not even guarantee that space has successfully been reserved for
129 the data. In this case, some errors might be delayed until a future
130 write(), fsync(2), or even close(2). The only way to be sure is to
131 call fsync(2) after you are done writing all your data.
132 If a write() is interrupted by a signal handler before any bytes are
134 written, then the call fails with the error EINTR; if it is interrupted
135 after at least one byte has been written, the call succeeds, and re‐
136 turns the number of bytes written.
137 On Linux, write() (and similar system calls) will transfer at most
139 0x7ffff000 (2,147,479,552) bytes, returning the number of bytes actu‐
140 ally transferred. (This is true on both 32-bit and 64-bit systems.)
141 An error return value while performing write() using direct I/O does
143 not mean the entire write has failed. Partial data may be written and
144 the data at the file offset on which the write() was attempted should
145 be considered inconsistent.
146
148 According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread Interactions
149 with Regular File Operations"):
150 All of the following functions shall be atomic with respect to each
152 other in the effects specified in POSIX.1-2008 when they operate on
153 regular files or symbolic links: ...
154 Among the APIs subsequently listed are write() and writev(2). And
156 among the effects that should be atomic across threads (and processes)
157 are updates of the file offset. However, on Linux before version 3.14,
158 this was not the case: if two processes that share an open file de‐
159 scription (see open(2)) perform a write() (or writev(2)) at the same
160 time, then the I/O operations were not atomic with respect to updating
161 the file offset, with the result that the blocks of data output by the
162 two processes might (incorrectly) overlap. This problem was fixed in
163 Linux 3.14.
164
166 close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2), pwrite(2),
167 read(2), select(2), writev(2), fwrite(3)
168
170 This page is part of release 5.13 of the Linux man-pages project. A
171 description of the project, information about reporting bugs, and the
172 latest version of this page, can be found at
173 https://www.kernel.org/doc/man-pages/.
174Linux 2021-03-22 WRITE(2)