On 26.11.2011 07:41, Bart Schaefer wrote:
On Fri, Nov 25, 2011 at 6:34 PM, Bart Schaefer barton.schaefer@gmail.com wrote:
Next you create wget #2, which (because it was forked from the parent shell) shares all the file descriptors that the shell had open to wget #1, e.g., including the input to the fifo. Repeat for all the rest of the wget. By the time you have created the last one, each of them has a set of descriptors shared with every other that was created ahead of them.
Thus, even though you write to the fifo for wget #2 and close it from the parent shell, it doesn't actually see EOF and begin processing the input until the corresponding descriptor shared by wget #1 is closed when wget #1 exits.
I wrote that backwards. Actually I think the *last* one (#20) exits first, and then #19, and so on down to #1 ... but the descriptor management issue is the same.
Wow ! You guys are so great !
That is exactly how my script behaves. The POSIX (actually SuS) text on the matter said that the implementation may or may not inherit the descriptors in the forked child processes, and that a portable script should close the descriptors to be sure they are not inherited. Somehow I did not pay to much attention to the issue and I just assumed I need not worry about it.
But you are right, in my loop every new wget process inherits the file descriptors opened by the script for the previous childs (for their pipes, to be accurate), and as such keeps the files open even if my script later closes its copies of the descriptors. And I though I am the toughest programmer ever ... !
The solution was simple, no file descriptor wrangling: just use two loops: one to start all the wget processes and connect them to the pipes, and the other to open all those fd's. In this way the wgets have nothing special to inherit.
Sorry about the wrong group. After asking in a Unix shell group without much success, I suspected it must be CentOS doing something strange with the FIFOs at the system level.
Thank you, Timothy Madden