Under what condition does a Python subprocess get a SIGPIPE?

Question

I am reading the the Python documentation on the Popen class in the subprocess module section and I came across the following code:

p1 = Popen([\"dmesg\"], s         


        

Answer 1

SIGPIPE is a signal that would be sent if dmesg tried to write to a closed pipe. Here, dmesg ends up with two targets to write to, your Python process and the grep process.

That's because subprocess clones file handles (using the os.dup2() function). Configuring p2 to use p1.stdout triggers a os.dup2() call that asks the OS to duplicate the pipe filehandle; the duplicate is used to connect dmesg to grep.

With two open file handles for dmesg stdout, dmesg is never given a SIGPIPE signal if only one of them closes early, so grep closing would never be detected. dmesg would needlessly continue to produce output.

So by closing p1.stdout immediately, you ensure that the only remaining filehandle reading from dmesg stdout is the grep process, and if that process were to exit, dmesg receives a SIGPIPE.