捕获exec.Command的缓冲stdout输出

Question

I'm trying to catch output of external program. Example:

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

int main() {

    int i = 0;

    while(i < 10) {
        printf("i = %i
", i++);
        usleep(2000000);
    }
    return 0;
}

And here is my main.go:

package main

import (
    "bufio"
    "io"
    "log"
    "os/exec"
)

func reecho(closer io.ReadCloser)  {
    reader := bufio.NewReader(closer)

    for {
        s, e := reader.ReadString('
')
        if e != nil {
            log.Println(e)
            break
        }
        log.Println(s)
    }
}

func main() {
    cmd := exec.Command("./infcount")
    log.Println("starting ", cmd)
    stdout, err := cmd.StdoutPipe()
    stderr, _ := cmd.StderrPipe()

    if err != nil {
        log.Fatal(err)
    }
    if err := cmd.Start(); err != nil {
        log.Fatal(err)
    }

    go reecho(stdout)
    go reecho(stderr)

    if err := cmd.Wait(); err != nil {
        log.Fatal(err)
    }
}

The problem is buffering of stdout. "reecho" get data only when 4096 bytes in stdout buffer or program is exiting(for my short example). Is there way to decrease size of buffer to catch every line of output?

Update: Same binary 'infcount' works fine when ran from shell. It writes every 'i' to the screen.

Answer 1

version of your C program that will track the output as it happens

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

    int main() {

        int i = 0;

        while(i < 10) {
            printf("i = %i
", i++);
            usleep(2000000);
            fflush(stdout);
        }
        return 0;
    }

If it is an existing precompiled program then stdbuf may be able to fix it, see https://www.perkin.org.uk/posts/how-to-fix-stdio-buffering.html