使用管道将数据从C进程发送到Python进程

Question

I'm trying to send data from a C program to a python program (both are running simultaneously). I'm attempting to use the popen function in C. The way I understand it, whatever I write to the file descriptor in C can be read from stdin in the python program. However, the python program appears to hang at the point where it reads from stdin. Any suggestions?

Here's the C program:

test.c

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

int main(void) {

    FILE * fp = popen("sudo python display.py", "w");
    if (fp == NULL) {
        printf("popen error\n");
        exit(1);
    }

    int inc = 0;
    char buf[10];

    while(1) {
        sprintf(buf, "%d", inc);
        fputs(buf, fp);
        inc++;
        sleep(1);
    }


    return (0);
}

Here is the python program, which should read the value of "inc" and output it

display.py

import sys

value = 0

while(True):
    value = sys.stdin.read()
    print value

The program seems to hang at value = sys.stdin.read() . I have tried readline() and readlines() as well with the same result.

Answer 1

readline() would work if you were outputting lines from your C program.

Since you never print a newline, readline will wait forever.

read() with no parameters will wait until the C program exits (or otherwise closes stdout). Again this is never going to happen with your program

Answer 2

You rather should use fprintf(3) instead of mixing sprintf (which is unsafe, prefer snprintf(3) ) with fputs .

You really want to output whole lines (because textual protocols are so convenient to debug). And use readline on the Python side.

At last, a FILE* obtained by popen(3) is not even guaranteed to be line-buffered (but could have a much larger buffer, perhaps 4Kbytes or more). You might set its buffer using setvbuf(3) but you really should flush explicitly the FILE* buffer with fflush(3) . If you don't, the output could stay in the buffer for a long time (without any actual write(2) done by stdio functions like fprintf or fputs ) so the pipe(7) stays empty.

^{(so you have problems even on the C side)}

The program seems to hang at value = sys.stdin.read().

I guess that not a single byte was actually written by your C code (you could check by using strace(1) ) because your fputs stayed in the buffer. You might need to wait a long time (perhaps 1000 seconds, to fill a buffer of a few kilobytes) to have it actually write something on the pipe, because you forgot the fflush .

A popen -ed file should absolutely be pclose d.

So you should replace your (incorrect) while loop with:

while(1) {
    fprintf(fp, "%d\n", inc);
    inc++;
    fflush(fp);
    sleep(1);
}

And your loop should be exited somehow. Perhaps you might catch some signal(7) -probably SIGTERM - (but be sure to read signal-safety(7) ). Be sure to pclose (to avoid zombie processes).

^{(I am assuming you are on Linux or some other POSIX system)}