when using different file descripter, why is the result different? (system programming)

Question

I am studying about file descripter and realized that if I use dup2() function, the result will be different.

The first snippet ...

int main(void){
    char buf1[BUFSIZ] = "I am low\n";

    printf("i am high\n");
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));

    return 0;
}

... produces the following result:

i am high
i am low 
i am low
i am low

But the second snippet ...

int main(void){
    int fd = open("dupout",O_CREAT | O_WRONLY, 0655);
    char buf1[BUFSIZ] = "I am low\n";
    dup2(fd, 1);

    printf("i am high\n");
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    close(fd);

    return 0;
}

... produces the following content in dupout:

i am low 
i am low
i am low
i am high

Why do the results differ?

Answer 1

explain:

IO buffer have three types full buffer, line buffer and no buffer.

First example:

Default, stdout is line buffer, means when the buffer is fulled or when meet \n, buffer will flush.

int main(void){
    char buf1[BUFSIZ] = "I am low\n";
    printf("i am high\n");
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    return 0;
}

Output :

i am high
I am low
I am low
I am low

But when we change it to :

int main(void){
    char buf1[BUFSIZ] = "I am low\n";
    printf("i am high");   // no '\n'
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    return 0;
}

Output:

I am low
I am low
I am low
i am high

continue : change to :

int main(void){
    char buf1[BUFSIZ] = "I am low\n";
    printf("i am high"); // no '\n'
    fflush(stdout);// flush
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    return 0;
}

Output:

i am highI am low
I am low
I am low

Second example:

Default, file IO is full buffer, means when the buffer is fulled, buffer will flush.

int main(void){
    int fd = open("dupout",O_CREAT | O_WRONLY, 0655);
    char buf1[BUFSIZ] = "I am low\n";
    dup2(fd, 1);

    printf("i am high\n");
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    close(fd);

    return 0;
}

Output:

I am low
I am low
I am low
i am high

But when we change it to :

int main(void){
    int fd = open("dupout",O_CREAT | O_WRONLY, 0655);
    char buf1[BUFSIZ] = "I am low\n";
    dup2(fd, 1);

    printf("i am high\n");
    fflush(stdout);
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    close(fd);

    return 0;
}

Output:

i am high
I am low
I am low
I am low

---

          old

Because of IO buffer. If you add setvbuf(stdout, NULL, _IONBF, 0); before printf, result is right. The line means set no IO buffer.

The following is all code:

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#include <unistd.h>

int main(void){
    int fd = open("dupout",O_CREAT | O_WRONLY, 0655);
    char buf1[BUFSIZ] = "I am low\n";
    dup2(fd, 1);

    setvbuf(stdout, NULL, _IONBF, 0);
    printf("i am high\n");
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    write(1, buf1, strlen(buf1));
    close(fd);

    return 0;
}