Writing to C program using GO but C program doesn't stop at scanf()

Question

In these snippets the go program tries to write to C program's stdin. The problem is that the c program starts an infinite loop after the go loop finishes.

main.c

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


int main(){
    int age;
    char name[8]; 

    for(;;)
    {
    scanf(" %s : %d\n",&name, &age);
    printf("%s : %d\n",name, age);
    }
    return 0;
}

run/main.go

func main() {

    proc := exec.Command("../main")
    stdin, err := proc.StdinPipe()
    if err != nil {
        fmt.Println(err) 
    }

    defer stdin.Close()

    proc.Stdout = os.Stdout
    proc.Stderr = os.Stderr

    fmt.Println("START")                      
    if err = proc.Start(); err != nil { 
        log.Fatal("An error occured: ", err)
    }

    for i := 0; i < 500; i++ {
        io.WriteString(stdin, fmt.Sprintf("hello : %d\n", i))
        //How to wait here and read from printf()?
    }

    //proc.Process.Kill() Here proc gets killed too fast

    fmt.Println("END")
}

When killed, process doesn't complete it's output

Output

START
END
hello : 497
hello : 498
hello : 499
hello : 499

Output Expected

START
....
hello : 497
hello : 498
hello : 499
END

You forwarded the stdout of the child process to the standard output of your process. If you don't want that, don't do that and read from the proc.Stdout. Or have the child program send a signal when it's done. — Cheatah
– Cheatah, Commented Feb 9, 2022 at 19:42
Remove the \n from the scanf: stackoverflow.com/questions/15443483/using-n-in-scanf-in-c — kaylum
– kaylum, Commented Feb 9, 2022 at 19:43
You need to coordinate with the child process in some way, read stdout, have it exit if you close stdin, etc. The C program enters an infinite loop when you close stdin without the Go program too, because that's what you wrote in the C program. — Mr_Pink
– Mr_Pink, Commented Feb 9, 2022 at 19:44
for(;;) is an infinite loop since there's nothing in it to break out. Use the return value from scanf to determine that it failed and break there. — Retired Ninja
– Retired Ninja, Commented Feb 9, 2022 at 19:47

ned · Accepted Answer · 2022-02-10 17:02:01Z

After reading the comments and few more searching I have worked it out. On C program I removed \n from scanf() and added fflush(stdout) after printf. According to this page I changed them to these:

int main(){
    int age;
    char name[8]; 

    int ret;

    for(;;)
    {
    ret= scanf("%s : %d",&name, &age);
    if (ret == -1) {
        break;
    }
    printf("%s : %d\n",name, age);

    fflush(stdout);
    }
    
    return 0;
}

func main(){
    proc := exec.Command("../main")

    stdin, err := proc.StdinPipe()
    if err != nil {
        fmt.Println(err)
    }

    reader, err := proc.StdoutPipe()
    if err != nil {
        fmt.Println(err)
        return
    }

    scanner := bufio.NewScanner(reader)
    resp := make(chan string)

    go func() {
        for scanner.Scan() {
            resp <- scanner.Text()
        }
    }()

    err = proc.Start()
    if err != nil {
        fmt.Fprintln(err)
        return
    }
    fmt.Println("START")

    for i := 0; i < 500; i++ {
        io.WriteString(stdin, fmt.Sprintf("hello : %d\n", i))

        v := <-resp
        print(v+"\n")

    }

    err = proc.Process.Kill()

    if err != nil {
        fmt.Fprintln(err)
        return
    }
    fmt.Println("END")

}

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