Python call to system program not working

Question

I wrote a little python script, intending to automate non-default options for gcc (on Kubuntu 14.04); the python runs without error now, and inserting a debug print statement (or changing the system command to 'echo') verifies the correct information is being passed, but I get an error from gcc saying

$ python gccm prog16
gcc: fatal error: no input files 
compilation terminated.

Here's the script I wrote:

#!/usr/bin/python
from sys import argv #get incoming argument
import subprocess #function to call an OS program

script, target = argv

# massage received argument into form needed for math.h linkage
target = "-o " + target + " " + target + ".c -lm"
subprocess.call (['gcc', target], shell=False)`

There are other additions I'd make to the gcc call (compile version options, stricter code checking, etc.), if I can get this to work correctly. Based on the error message, it appears to be invoking gcc correctly, but the target source file isn't being found; could this not be running in the directory from which I invoke it? If so, how can I get it to run from the correct directory (where I'm keeping my C source code files); if not, what else could cause this?

Jonathon Reinhart · Accepted Answer · 2014-11-10 02:17:48Z

1

If you're using shell=False, your arguments to the sub-processes shouldn't be concatenated together. Instead, they should each be their own element in the args list:

subprocess.call(['gcc', '-o', target, target+'.c', '-lm'], shell=False)

On a related note, any reason why you're writing something like this yourself? If you're looking to use a Python-based build system, have a look at SCons.

answered Nov 10, 2014 at 2:17

Jonathon Reinhart

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2 Comments

Zeiss Ikon Over a year ago

Well, that would have been a useful thing to mention in the documentation for subprocess (at least the version I found and read). :p I'm writing this myself because a) I can use the practice with python (it's only half a dozen lines), and b) I'm not really looking for a python-based build system, or any build system, to add another layer of things I have to learn while I'm trying to learn python itself, and relearn C, as well as c) I had no idea there was such a tool and didn't know to or how to look for it.

Jonathon Reinhart Over a year ago

No problem - just thought I would point out a related application. Enjoy your project!

b4hand · Accepted Answer · 2014-11-10 02:18:39Z

1

If you have shell=False, then you must pass each argument separately into the subprocess.call.

Try this instead:

subprocess.call (['gcc', '-o', target, target + '.c', '-lm'], shell=False)

answered Nov 10, 2014 at 2:18

b4hand

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