1WRITE(2) Linux Programmer's Manual WRITE(2)
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6 write - write to a file descriptor
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9 #include <unistd.h>
10 ssize_t write(int fd, const void *buf, size_t count);
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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.
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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 fl