Aaron Denney
Does open("/dev/fd/n") or ("/proc/self/fd/n") act as dup() or a fresh open() to underlying file?)
As a dup(), with a side effect of resetting the file pointer to the beginning. It would not help anyway: if it's a terminal or pipe, it *has* to act as a dup() (in this case the file pointer is obviously not reset), it's not seekable, and thus pread/pwrite fail. "wc -c" tries to measure the size in bytes by seeking, and proceeds to actually read the contents only if the file is not seekable (or if it's also told to count words or lines, obviously). This applies to stdin too. In this case it must remember to subtract the starting position from the size (and to bump the result to 0 if it would be negative), in order to give consistent results whether it manages to skip reading the contents or not.
I actually don't see the problem with interacting with other processes that we've forked.
File positions are not evil. They allow to treat files and devices in a uniform way. If you run a program which - writes some opening words to stdout - forks a subprocess, which executes another program, which writes something to stdout - the parent waits for the subprocess to finish - writes closing words to stdout then all output will appear in order: the opening words, then the part from the other program, and finally the closing words. This applies *also* when stdout has been redirected to a file. In this case it's essential that the file position is shared between all processes! So even if stdout is a regular file, output should be done using write(), not pwrite(). If processes don't synchronize their writes and write at the same time, then output will be intermixed. They can choose to synchronize themselves; in particular one process can avoid writing anything while the other process is running. So it's not true that allowing them to share a file position will necessarily be unsafe and will risk mangled data. Unix doesn't guarantee that programs will behave well, but it allows well-behaving programs to cooperate. -- __("< Marcin Kowalczyk \__/ qrczak@knm.org.pl ^^ http://qrnik.knm.org.pl/~qrczak/