How Can I Determine if Standard Input (stdin) is a Terminal or a Pipe in C/C /Qt?

Knowing Whether Stdin is a Terminal or Pipe in C/C /Qt

In certain scenarios, it's important to differentiate between stdin connected to a terminal or a pipe. For instance, when launching Python from the terminal, its interactive mode is influenced by whether stdin is a terminal or a pipe.

Detection in C/C

To determine the nature of stdin, use the isatty function:

#include <stdio.h>
#include <io.h>

if (isatty(fileno(stdin))) {
  printf("stdin is a terminal\n");
} else {
  printf("stdin is a file or a pipe\n");
}

On Windows, use the corresponding prefixed functions: _isatty, _fileno.

Detection in Qt

Qt provides the QFileDevice::isSequential method:

#include <QFileDevice>

if (stdin->isSequential()) {
  // stdin is a pipe
} else {
  // stdin is a terminal
}

This approach leverages the fact that sequential devices are normally pipes or other unseekable sources.

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