How Can I Detect Whether stdin is a Terminal or a Pipe in C, C , and Qt?

Detecting stdin Type in C, C , and Qt

When executing "python" without arguments in a terminal, the Python interactive shell is launched. However, when "cat | python" is executed from the terminal, interactive mode is not enabled, indicating that stdin is connected to a pipe. This article explores how to detect stdin type in C, C , and Qt using the isatty function.

Function Overview

The isatty function in stdio.h determines whether a file descriptor is connected to a terminal. When applied to fileno(stdin), it checks if stdin is a terminal (e.g., a console or terminal emulator) or not (e.g., a file or pipe).

C / C Example

In C or C , use the following code to detect stdin type:

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

int main() {
  if (isatty(fileno(stdin)))
    printf("stdin is a terminal\n");
  else
    printf("stdin is a file or a pipe\n");
  return 0;
}

Qt Example

Qt provides a KStdin class that inherits from QIODevice. It has a isTerminal() method that can be used to check if stdin is connected to a terminal:

#include <QStdin>

int main() {
  KStdin stdinDevice;
  if (stdinDevice.isTerminal())
    qDebug() << "stdin is a terminal";
  else
    qDebug() << "stdin is a file or a pipe";
  return 0;
}

By utilizing isatty in C or C or KStdin in Qt, it is possible to detect whether stdin is connected to a terminal or a pipe, providing valuable information for controlling program behavior based on input source.

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