when calling open() for writing with flag O_WRONLY set, and all flags O_CREAT, O_EXCL, and O_TRUNC not set, what is the behavior of the first write() call. what will be seen as happening to the file if it is written to without truncating?

unfortunately, the man pages tend to describe flag behavior when set, but do not describe the effect when not set, or the behavior of combinations with other flags.

I believe the intent is if you have an existing file you can "seek()" to a location and overwrite any portion of an existing file.

if the first operation after the file is open without O_TRUNC is write() (not seek()), where is the data written to? in other words, where is the implied first "seek()" position? is it position 0, the beginning of the file?

what is supposed to happen if O_TRUNC is used with O_RDONLY? i have seen this actually truncate the file to 0 length.

If the file is opened with O_APPEND, the write will append after the last byte.

Otherwise, unless you seek first, it will overwrite the first.

> what is supposed to happen if O_TRUNC is used with O_RDONLY? i have seen this actually truncate the file to 0 length.

that sounds like a bug...

From man 2 open...

O_TRUNC
If the file already exists and is a regular file and the access mode allows writing (i.e., is O_RDWR
or O_WRONLY) it will be truncated to length 0. If the file is a FIFO or terminal device file, the
O_TRUNC flag is ignored. Otherwise, the effect of O_TRUNC is unspecified.

unspecified could mean anything. it could cause my laptop to vanish. i better be more careful. i will try to make a minimal reproduction.

.. or worse
https://progforperf.github.io/Expert_C_Programming.pdf