read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf.
If count is zero, read() returns zero and has no other results. If count is greater than SSIZE_MAX, the result is
write() writes up to count bytes from the buffer pointed 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 phys-
ical 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 current 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) which can be proved to occur after a write() has returned returns the new data. Note
that not all file systems are POSIX conforming.