Why is cout outputting immediately?

Question

cout is a buffered stream. This means that the data will be written to the buffer and will be printed when the stream is flushed, program terminated or when the buffer is completely filled.

I made a small program to test how this works, but I don't understand why it prints even before any of the above conditions are met.

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

int main()
{
    cout << "Test";
    float secs = 5;
    clock_t delay = secs * CLOCKS_PER_SEC;
    clock_t start = clock();
    while (clock() - start < delay) { }
    return 0;
}

When run, "Test" is output before the loop begins.

Why is my output not buffered until the program terminates?

Answer 1

There's a great discussion on this over here.

From one of the answers:

Every C++ stream uses an associated stream buffer object to perform buffering.

When std::cout is constructed, it uses the stream buffer associated with the object stdout, declared in <cstdio>. By default, operations on std::cout can be freely mixed with <cstdio> output functions like std::printf().

In practical terms, synchronization usually means that a standard iostream object and a standard stdio object share a buffer. - IS

If std::ios_base::sync_with_stdio(false) is called (before any input or output operations on the standard streams), the standard C++ streams operate independently of the standard C streams (ie. they switch to their own separate stream buffers).

For more, see the sync_with_stdio function reference page on cppreference.

From that page, the function...

Sets whether the standard C++ streams are synchronized to the standard C streams after each input/output operation.

...In practice, this means that the synchronized C++ streams are unbuffered, and each I/O operation on a C++ stream is immediately applied to the corresponding C stream's buffer. This makes it possible to freely mix C++ and C I/O.

However, beware of calling this function after there have already been reads or writes:

If this function is called after I/O has occurred on the standard stream, the behavior is implementation-defined: implementations range from no effect to destroying the read buffer.