Trouble running a C code with arguments from python's subprocess.call

Question

I'm kind of new to Python and trying to figure something out. I am trying to write a quick script to do parameter testing for a C code that I've written.

The part that is getting to me is when I try to use subprocess.call in order to run my C code. The C code has one argument that is the name of a file that is opened within the code itself.

For example:

filename = myclass.path + myclass.inputname
subprocess.call(["./code", filename])

This will run the code, and it passes the correct filename, but the C code will not read in any of the information from the file at all. If I pass something like:

./code "filename"

to the shell, where "filename" is actually what is printed when I use the print command in python, it works just fine.

Just for the sake of being complete, here are the lines that are relevant within my C code:

in = fopen(argv[1], "r");
fscanf(in, "%s %d %d", variable1, &variable2, &variable3);

Any idea what is happening?

is "filename" printed with or without the quotes? The shell will get rid of them, but subprocess won't. — mgilson
– mgilson, Commented Jun 13, 2012 at 3:29
Agreeing with @mgilson: what does filename look like? It might be a special character issue or something. (You also might want to use os.path.join instead of myclass.path + myclass.inputname). — Danica
– Danica, Commented Jun 13, 2012 at 3:33
Do myclass.path have a trailing slash? You might need to add a slash between the path and the filename. — Some programmer dude
– Some programmer dude, Commented Jun 13, 2012 at 4:29

cdarke · Accepted Answer · 2012-06-13 08:09:37Z

1

Aside from the obvious, that you are not checking the result of fopen (are you using absolute path names?), your fscanf is wrong. Try:

fscanf(in, "%s %d %d", variable1, &variable2, &variable3);

Another possibility is that the file is not in the format that fscanf expects - it is particularly picky about that and can easily crash your program if it is wrong.

answered Jun 13, 2012 at 8:09

cdarke

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mhawke · Accepted Answer · 2012-06-13 03:40:10Z

0

Adding printf("%s\n", argv[1]); to your C code will let you see what it is receiving. Does the filename contain spaces? You might also like to check argv[2] in case it does.

Also, check the return value from fopen(). Is it NULL? (in which case it is failing to open the file).

answered Jun 13, 2012 at 3:40

mhawke

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