How to pass a C struct to Python to get data?

Question

I have code for both Python and C that need to communicate to each other through a pipe created by Popen. I have a test struct in C that needs to be passed back to Python but I can't seem to reconstruct that struct on the Python side. This is a much more complicated project but the struct I created below is just an example to get the code to work, and I can try to figure out the more advanced things later. I am not an expert in C, pointers and piping, and I do not have a clear understanding of it. Most of the C code below is just from my readings.

Python:

testStruct = struct.Struct('< i') 
cProg = Popen("./cProg.out", stdin=PIPE, stdout=PIPE)
data = ""
dataRead = cProg.stdout.read(1)
while dataRead != "\n":
    data += dataRead
    dataRead = cProg.stdout.read(1)

myStruct = testStruct.unpack(data)
print myStruct.i

C:

typedef struct{
    int i;
} TestStruct;

int main(void)
{
    int wfd = fileno(stdout);
    TestStruct t;
    t.i = 5;
    char sendBack[sizeof(t)];
    memcpy(sendBack, &t, sizeof(t));
    write(wfd, sendBack, sizeof(sendBack));
    write(wfd, "\n", 1);

}

But when I run the Python code I get the error:

unpack requires a string argument of length 4

Like I said I do not understand how structs and C. If there's any suggestion on refining this code, or better yet another suggestion on passing a C struct back to Python to unpack and grab the data. I can read and write through the pipe, the code I have posted are just snippets from my actual code. I know that the issue has to do with sending of the struct back to Python through stdout.

print the length of data. What do you get? Also, is this python 2 or 3? String handling changes. — salezica
– salezica, Commented Jun 7, 2014 at 1:41
You're reading 1 byte, but the error message clearly indicates that you need to give it 4 bytes. — Jonathon Reinhart
– Jonathon Reinhart, Commented Jun 7, 2014 at 3:43

ooga · Accepted Answer · 2014-06-07 03:48:50Z

2

Here's an example of reading data in Python from a C program through a pipe.

C Program

#include <stdio.h>

typedef struct{
    int i;
    int j;
} TestStruct;

int main() {
    TestStruct ts = {11111, 22222};
    fwrite(&ts, sizeof ts, 1, stdout);
    return 0;
}

Python 2.7 Program

from subprocess import Popen, PIPE
from struct import calcsize, unpack

cprog = Popen("cprog", stdout=PIPE)

fmt = "@ii"
str = cprog.stdout.read(calcsize(fmt))
cprog.stdout.close()

(i, j) = unpack(fmt, str)
print i, j

edited Jun 7, 2014 at 3:48

answered Jun 7, 2014 at 2:51

ooga

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4 Comments

Jonathon Reinhart Over a year ago

Don't use from xxx import *.

ooga Over a year ago

@JonathonReinhart I agree. I used it for "simplicity", but probably shouldn't have. I'll change it.

user3716881 Over a year ago

Thanks @ooga, this is what I needed. I just had to change my format to native (@) and it fixed the problem. Would you happen to know how to pass a char array in the TestStruct of an unknown length back?

ooga Over a year ago

That probably wasn't the problem since your machine is most likely little-endian and therefore < should have worked (as it does for me). Don't focus on one little detail; note every difference from your attempt. In particular, note that you used unpack incorrectly. Now to read a string of unknown length from the C program, if it ends with a newline (and doesn't contain embedded newlines) you could read it with cprog.stdout.readline().

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