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_sys_istty()

Declared in rt_sys.h, the _sys_istty() function determines whether a file handle is attached to an interactive device.

Syntax

int _sys_istty(FILEHANDLE fh);

Usage

The Arm libraries call __sys_istty() to determine whether a file handle (that is being used for an output file stream) is attached to an interactive device.

For file streams that are attached to interactive devices, the Arm library:

  • Provides unbuffered behavior by default, in the absence of a call to setbuf() or setvbuf().
  • Prohibits seeking.

Restriction

stdin, stdout, and stderr are assumed to be interactive devices. They are line-buffered at program startup, regardless of what _sys_istty reports for them. An exception is if they have been redirected on the command line.

Note

This restriction does not apply when using microlib. In microlib, stdin, stdout, and stderr are always unbuffered.

Returns

The return value is one of the following values:

0
fh is not attached to an interactive device.
1
fh is attached to an interactive device.
other
An error occurred.
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