system("/bin/sh") closes immediately when input given through pipe

Question

I wrote a c program as below:

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

int main() {
        int a;
        scanf("%d",&a);
        system("/bin/sh");
        return 0;
}

compiled it and when i execute the a.out file it shows something like this:

user@system:~/dir$ ./a.out
123                           <- this is the input that I gave
$ whoami
user

so here I get a proper sub shell called through c
now if I execute the a.out file using input through pipe it shows like this:

user@system:~/dir$ echo 123 | ./a.out
user@system:~/dir$

it simply executes the command, creates the shell, terminates the shell and exits.

I am sure that it executes the sub shell because on doing echo 123 | ltrace ./a.out it shows like this:

system("/bin/sh" <no return ...>
--- SIGCHLD (Child exited) ---
<... system resumed> )                                                              = 0
+++ exited (status 0) +++

which means there is a sub shell created
I cant understand what difference does the two methods make

Answer 1

sh will read the stdin till it is encountering the EOF. In the first case you do not send EOF (which can be done by pressing Ctrl-D depending on your environment). In the second case the EOF is happening after the piped input is exhausted (that is after the echo 123 is done). To understand better you can simulate the sh behavior with a simple program:

#include <stdio.h>
int main(void)
{
    char input[100];
    while(fgets(input, 100, stdin)) {
        printf("Echoing back: %s\n", input);
    }
    return 0;
}

You can compile it and run with your system() call instead of sh and observe a similar behavior.