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.
36
38 On success, the number of bytes written is returned (zero indicates
39 nothing was written). It is not an error if this number is smaller
40 than the number of bytes requested; this may happen for example because
41 the disk device was filled. See also NOTES.
42 On error, -1 is returned, and errno is set appropriately.
44 If count is zero and fd refers to a regular file, then write() may
46 return a failure status if one of the errors below is detected. If no
47 errors are detected, or error detection is not performed, 0 will be
48 returned without causing any other effect. If count is zero and fd
49 refers to a file other than a regular file, the results are not speci‐
50 fied.
51
53 EAGAIN The file descriptor fd refers to a file other than a socket and
54 has been marked nonblocking (O_NONBLOCK), and the write would
55 block. See open(2) for further details on the O_NONBLOCK flag.
56 EAGAIN or EWOULDBLOCK
58 The file descriptor fd refers to a socket and has been marked
59 nonblocking (O_NONBLOCK), and the write would block.
60 POSIX.1-2001 allows either error to be returned for this case,
61 and does not require these constants to have the same value, so
62 a portable application should check for both possibilities.
63 EBADF fd is not a valid file descriptor or is not open for writing.
65 EDESTADDRREQ
67 fd refers to a datagram socket for which a peer address has not
68 been set using connect(2).
69 EDQUOT The user's quota of disk blocks on the filesystem containing the
71 file referred to by fd has been exhausted.
72 EFAULT buf is outside your accessible address space.
74 EFBIG An attempt was made to write a file that exceeds the implementa‐
76 tion-defined maximum file size or the process's file size limit,
77 or to write at a position past the maximum allowed offset.
78 EINTR The call was interrupted by a signal before any data was writ‐
80 ten; see signal(7).
81 EINVAL fd is attached to an object which is unsuitable for writing; or
83 the file was opened with the O_DIRECT flag, and either the
84 address specified in buf, the value specified in count, or the
85 file offset is not suitably aligned.
86 EIO A low-level I/O error occurred while modifying the inode. This
88 error may relate to the write-back of data written by an earlier
89 write(2), which may have been issued to a different file
90 descriptor on the same file. Since Linux 4.13, errors from
91 write-back come with a promise that they may be reported by sub‐
92 sequent. write(2) requests, and will be reported by a subse‐
93 quent fsync(2) (whether or not they were also reported by
94 write(2)). An alternate cause of EIO on networked filesystems
95 is when an advisory lock had been taken out on the file descrip‐
96 tor and this lock has been lost. See the Lost locks section of
97 fcntl(2) for further details.
98 ENOSPC The device containing the file referred to by fd has no room for
100 the data.
101 EPERM The operation was prevented by a file seal; see fcntl(2).
103 EPIPE fd is connected to a pipe or socket whose reading end is closed.
105 When this happens the writing process will also receive a SIG‐
106 PIPE signal. (Thus, the write return value is seen only if the
107 program catches, blocks or ignores this signal.)
108 Other errors may occur, depending on the object connected to fd.
110
112 SVr4, 4.3BSD, POSIX.1-2001.
113 Under SVr4 a write may be interrupted and return EINTR at any point,
115 not just before any data is written.
116
118 The types size_t and ssize_t are, respectively, unsigned and signed
119 integer data types specified by POSIX.1.
120 A successful return from write() does not make any guarantee that data
122 has been committed to disk. On some filesystems, including NFS, it
123 does not even guarantee that space has successfully been reserved for
124 the data. In this case, some errors might be delayed until a future
125 write(2), fsync(2), or even close(2). The only way to be sure is to
126 call fsync(2) after you are done writing all your data.
127 If a write() is interrupted by a signal handler before any bytes are
129 written, then the call fails with the error EINTR; if it is interrupted
130 after at least one byte has been written, the call succeeds, and
131 returns the number of bytes written.
132 On Linux, write() (and similar system calls) will transfer at most
134 0x7ffff000 (2,147,479,552) bytes, returning the number of bytes actu‐
135 ally transferred. (This is true on both 32-bit and 64-bit systems.)
136
138 According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread Interactions
139 with Regular File Operations"):
140 All of the following functions shall be atomic with respect to each
142 other in the effects specified in POSIX.1-2008 when they operate on
143 regular files or symbolic links: ...
144 Among the APIs subsequently listed are write() and writev(2). And
146 among the effects that should be atomic across threads (and processes)
147 are updates of the file offset. However, on Linux before version 3.14,
148 this was not the case: if two processes that share an open file
149 description (see open(2)) perform a write() (or writev(2)) at the same
150 time, then the I/O operations were not atomic with respect updating the
151 file offset, with the result that the blocks of data output by the two
152 processes might (incorrectly) overlap. This problem was fixed in Linux
153 3.14.
154
156 close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2), pwrite(2),
157 read(2), select(2), writev(2), fwrite(3)
158
160 This page is part of release 4.15 of the Linux man-pages project. A
161 description of the project, information about reporting bugs, and the
162 latest version of this page, can be found at
163 https://www.kernel.org/doc/man-pages/.
164Linux 2018-02-02 WRITE(2)