`_fileno`

Article
12/02/2022

Gets the file descriptor associated with a stream.

Syntax

int _fileno(
   FILE *stream
);

Parameters

stream
Pointer to the FILE structure.

Return value

_fileno returns the file descriptor. There's no error return. The result is undefined if stream doesn't specify an open file. If stream is NULL, _fileno invokes the invalid parameter handler, as described in Parameter validation. If execution is allowed to continue, this function returns -1 and sets errno to EINVAL.

For more information about these and other error codes, see errno, _doserrno, _sys_errlist, and _sys_nerr.

Note

If stdout or stderr is not associated with an output stream (for example, in a Windows application without a console window), the file descriptor returned is -2. In previous versions, the file descriptor returned was -1. This change allows applications to distinguish this condition from an error.

Remarks

The _fileno routine returns the file descriptor currently associated with stream. This routine is implemented both as a function and as a macro. For information about choosing either implementation, see Recommendations for choosing between functions and macros.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.

Requirements

Function	Required header
`_fileno`	`<stdio.h>`

For more compatibility information, see Compatibility.

Example

// crt_fileno.c
// This program uses _fileno to obtain
// the file descriptor for some standard C streams.
//

#include <stdio.h>

int main( void )
{
   printf( "The file descriptor for stdin is %d\n", _fileno( stdin ) );
   printf( "The file descriptor for stdout is %d\n", _fileno( stdout ) );
   printf( "The file descriptor for stderr is %d\n", _fileno( stderr ) );
}

The file descriptor for stdin is 0
The file descriptor for stdout is 1
The file descriptor for stderr is 2

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