[GRASS-dev] Continuos output from grass python script of subprocess

Michel Wortmann wortmann at pik-potsdam.de
Mon Aug 11 04:17:07 PDT 2014

Dear Glynn,
thank you ever so much for this detailed answer. Yes the program's 
buffering was the problem, I'm running a fortran executable and the 
flush() command did the trick.
Best regards,

On 08/11/2014 10:52 AM, Glynn Clements wrote:
> Michel Wortmann wrote:
>> could you tell me why this simple example script doesn�t show me the
>> output of my program that I�m launching through subprocess? It�s
>> only giving me the output once the program has finished. As the
>> program runs for quite some time, I would like to see the output
>> while the script is running through the grass module interface. (In
>> the actual script, I�m preparing some input and postprocess output
>> before/after the program has run.)
> The most likely reason is that the child program's stdout is
> fully-buffered, meaning that the stdio functions will buffer the data
> and only pass it to the OS when the buffer is full (for GNU libc, the
> default buffer size is 8192 bytes).
> Typically, stdout is line-buffered if it is associated with a
> terminal, and fully buffered otherwise (e.g. if it is associated with
> a file or pipe).
> The exact wording of the C standard is (§7.19.3p7)
> 	As initially opened, the standard error stream is not fully
> 	buffered; the standard input and standard output streams are
> 	fully buffered if and only if the stream can be determined not
> 	to refer to an interactive device.
> If the program is one you can modify, you can force line buffering
> using the setvbuf() function, or you can explicitly flush the buffer
> after each line with the fflush() function. Or you could write
> progress output to stderr, which is typically unbuffered by default.
> If the script doesn't need to process the program's output you could
> just remove the stdout=subprocess.PIPE, allowing the child process to
> inherit the script's stdout.
> If the script needs to process the program's output, you can't modify
> the program, and you're using Linux, you could use Python's "pty"
> module (which is only available on Unix and only supported on Linux)
> to create a pseudo-tty (pty), and set the child process' stdout to the
> pty slave. This will cause the stdio library to make stdout
> line-buffered.

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