piping binary data between python and c++

Question

I have been working on a plugin to QGIS using Python 2.7 which works fine until I actually go to do some image processing on the mapped layers. Even simple tasks like collec

Answer 1

The problem probably is buffering: by default, the C stdio buffers everything written to stdout and flushes the buffer back into the pipe only when a newline is written (line buffering). The problem disappears when you call fflush(stdout) after writing. You can also disable (or control) buffering via the setvbuf function defined in <stdio.h>, e.g. use setvbuf(stdout, NULL, _IONBF, 0) to disable buffering completely.

I have tested the first variant with the following two programs:

import subprocess

proc = subprocess.Popen("./echotest",
    stdin=subprocess.PIPE,
    stdout=subprocess.PIPE)

proc.stdin.write('abc')
message = proc.stdout.read(3)
print "return message ->" + message + " written by python \n" 
proc.stdin.write('q')
proc.wait()

and a little C program:

#include <stdio.h>

int main (int argc, char **argv) {
    for (;;) {
        char buf;
        fread(&buf, 1, 1, stdin);
        if ('q' == buf)
            break;
        fwrite(&buf, 1, 1, stdout);
        fflush(stdout);
    }

    return 0;
}

Note that you have to specify how many bytes you want to read back from the subprocess or the program will block, waiting for more output. If this bothers you, try one of the solutions to this question.