STDIN、STDOUT 重定向与 printf scanf 挂起

Question

Can anyone find out why this code hang at fgets() for parent and scanf() for child?
If I turn child process's printf/scanf into write/read it doesn't hang.
Can any one give a reason to this problem?

int main(){
    int pfd1[2], pfd2[2];
    pipe(pfd1); pipe(pfd2);

    if (fork() == 0){
        dup2(pfd1[1], STDOUT_FILENO);
        dup2(pfd2[0], STDIN_FILENO);
        close(pfd1[0]); close(pfd1[1]);
        close(pfd2[1]); close(pfd2[0]);
        char buf[] = "Hello world\n";
        int n;
        printf("%s", buf);
        fflush(stdin);
        scanf("%d", &n);
        fprintf(stderr, "get n = %d\n", n);
    }    
    else {
        char buf[128];
        FILE *fp_w = fdopen(pfd2[1], "w");
        FILE *fp_r = fdopen(pfd1[0], "r");
        fgets(buf, 128, fp_r);
        printf("%s", buf);
        fprintf(fp_w, "%d\n", 10);

    }

    return 0;
}

Answer 1

Output to stdout is by default line-buffered, meaning the stdout internal buffer is flushed when you print a newline ( '\\n' ). But that is only the default, when stdout is connected to a terminal or console.

Since you make stdout write to a pipe, it's no longer line-buffered, but fully buffered. That means the output will only be flushed when you fill up the internal buffer, or when explicitly doing by calling fflush(stdout) .

Since your code as shown in the question doesn't flush the buffers, nothing will ever be written, which means the parent process will wait forever on input from fp_r which never comes. Since the parent process is locked, it will in turn not write to fp_w and the reading in the child-process will then also be blocked.

The solution, as you have discovered, is to explicitly flush stdout after writing to it in the child-process.