man write
write() writes up to count bytes from the buffer starting at buf
to the file referred to by the file descriptor fd.
The number of bytes written may be less than count if, for
example, there is insufficient space on the underlying physical
medium, or the RLIMIT_FSIZE resource limit is encountered (see
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.