CODEWITHSUNDEEP
c++
aseembits93
aseembits93

Reputation: 83

how does the function system() work in c++?

I am trying to understand system calls made in c++ using system("some command"). here's the code 

#include <iostream>
#include <cstdlib>
using namespace std;

int main()
{
  cout << "Hello ";
  system("./pause");
  cout << "World";
  cout << endl;

  return 0;
}

the executable "pause" is created from the following code

#include <iostream>
using namespace std;

int main()
{
    cout<<"enter any key to continue\n";
    cin.get();
    return 0;

}

I get the following output 

enter any key to continue
1
Hello World

Can someone please explain the output to me? I was expecting this - 

Hello
enter any key to continue
1
World

Upvotes: 6

Views: 884

Answers (4)

Petr
Petr

Reputation: 10007

The reason for the particular behavior that you observe seems to be just cout buffering: the Hello is not printed immediately, instead held in a buffer until endl is output (or the buffer is filled completely, or you explicitly call flush()). This is in no way related to a system() call.

A simpler example:

cout << "Hello";
sleep(10);
cout << "World";

Both words will appear at the same time, not with a 10 seconds delay.

Upvotes: 5

Hcorg
Hcorg

Reputation: 12178

It's probably not a case of system call but output stream buffering.

cout << "xxx" does not necessary outputs something, so program called by system can be executed before cout flushes it buffer to console.

try adding cout.flush() after cout << "Hello" or write cout << "Hello" << flush

also: cout << endl automagically calls flush

Upvotes: 6

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

The answer on "how does system library function works?" is usually operating system specific. See here for a Linux point of view. Notice that system is not a system call and there is a priori no relation between using system(3) and having cout buffer being flushed.

You should flush the stdout before calling system

 cout << "Hello " << flush;

or preferably

 cout << "Hello " << endl;

The behavior you are observing is because cout is buffered and you forgot to flush the buffer.

Upvotes: 4

StenSoft
StenSoft

Reputation: 9617

system runs a command in a shell. But your problem is not with system but with cout. cout is line-buffered, ie. it won't flush (write out) its data until a new line character is encountered. You need to flush it explicitely with cout << flush.

Upvotes: 6

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