|WRITE(2)||Linux Programmer's Manual||WRITE(2)|
setrlimit(2)), or the call was interrupted by a signal handler after having written less than count bytes. (See also pipe(7).) For a seekable file (i.e., one to which lseek(2) may be applied, for example, a regular file) writing takes place at the file offset, and the file offset is incremented by the number of bytes actually written. If the file was open(2)ed with O_APPEND, the file offset is first set to the end of the file before writing. The adjustment of the file offset and the write operation are performed as an atomic step. POSIX requires that a read(2) that can be proved to occur after a write() has returned will return the new data. Note that not all filesystems are POSIX conforming. According to POSIX.1, if count is greater than SSIZE_MAX, the result is implementation-defined; see NOTES for the upper limit on Linux.
- The file descriptor fd refers to a file other than a socket and has been marked nonblocking (O_NONBLOCK), and the write would block. See open(2) for further details on the O_NONBLOCK flag.
- EAGAIN or EWOULDBLOCK
- The file descriptor fd refers to a socket and has been marked nonblocking (O_NONBLOCK), and the write would block. POSIX.1-2001 allows either error to be returned for this case, and does not require these constants to have the same value, so a portable application should check for both possibilities.
- fd is not a valid file descriptor or is not open for writing.
- fd refers to a datagram socket for which a peer address has not been set using connect(2).
- The user's quota of disk blocks on the filesystem containing the file referred to by fd has been exhausted.
- buf is outside your accessible address space.
- An attempt was made to write a file that exceeds the implementation-defined maximum file size or the process's file size limit, or to write at a position past the maximum allowed offset.
- The call was interrupted by a signal before any data was written; see signal(7).
- fd is attached to an object which is unsuitable for writing; or the file was opened with the O_DIRECT flag, and either the address specified in buf, the value specified in count, or the file offset is not suitably aligned.
- A low-level I/O error occurred while modifying the inode. This error may relate to the write-back of data written by an earlier write(2), which may have been issued to a different file descriptor on the same file. Since Linux 4.13, errors from write-back come with a promise that they may be reported by subsequent. write(2) requests, and will be reported by a subsequent fsync(2) (whether or not they were also reported by write(2)).
- The device containing the file referred to by fd has no room for the data.
- The operation was prevented by a file seal; see fcntl(2).
- fd is connected to a pipe or socket whose reading end is closed. When this happens the writing process will also receive a SIGPIPE signal. (Thus, the write return value is seen only if the program catches, blocks or ignores this signal.)
All of the following functions shall be atomic with respect to each other in the effects specified in POSIX.1-2008 when they operate on regular files or symbolic links: ...Among the APIs subsequently listed are write() and writev(2). And among the effects that should be atomic across threads (and processes) are updates of the file offset. However, on Linux before version 3.14, this was not the case: if two processes that share an open file description (see open(2)) perform a write() (or writev(2)) at the same time, then the I/O operations were not atomic with respect updating the file offset, with the result that the blocks of data output by the two processes might (incorrectly) overlap. This problem was fixed in Linux 3.14. close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2), pwrite(2), read(2), select(2), writev(2), fwrite(3)