Well, that is some interesting stuff. So, in POSIX, the child gets the same FD in the same place and it is actually a second reference to the kernels open file table. The same entry as the parent uses (via FD) to determine the offset, flags, etc. That would explain why the child calling exit() flushes the parents input file as well as its own. They are the same file as far as the kernel maintains it. As well, the buffering on the file determines how much of the file the parent loses when the child flushes the buffer. For a standard 4K buffer, if the input file is less than a full buffer, the parent would see an EOF even though it had last read one or more short lines of input. Now, if the input file had been unbuffered at the kernel level, this would not cause the problem we see. Perhaps we'd lose the next character read and buffered. I must remember to call _exit() when I use fork. The above makes sense when you consider stdout and stderr, for they keep the parent and child from clobbering each other's output. I'm not so sure it's useful on input files so much, though. JMHO.
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