From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=BRl/=4C=gentoo.org=soap@sourceware.org>
Received: from smtp.gentoo.org (woodpecker.gentoo.org [140.211.166.183])
	by sourceware.org (Postfix) with ESMTP id D46F538432E8
	for <libc-alpha@sourceware.org>; Sun,  4 Dec 2022 21:34:37 +0000 (GMT)
DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org D46F538432E8
Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=gentoo.org
Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=gentoo.org
Message-ID: <08776f09871ef817669410c7b1a26122e3c2c75b.camel@gentoo.org>
Subject: Resource use fopen-vs-freopen
From: David Seifert <soap@gentoo.org>
To: libc-alpha@sourceware.org
Date: Sun, 04 Dec 2022 22:34:32 +0100
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
User-Agent: Evolution 3.46.2 
MIME-Version: 1.0
X-Spam-Status: No, score=-5.5 required=5.0 tests=BAYES_00,JMQ_SPF_NEUTRAL,KAM_DMARC_STATUS,RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL,SPF_HELO_PASS,SPF_PASS,TXREP autolearn=no autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org
List-Id: <libc-alpha.sourceware.org>

Hi glibc,
this came up recently as a point of interest. Say we have snippet 1:

  #include <stdio.h>

  void reset_std_streams(void) {
    stdin =3D fopen("/dev/null", "r");
  }

  int main(void) {
    reset_std_streams();
    // A
    /*
    -- actual program code --
    */
  }

and snippet 2:

  #include <stdio.h>

  void reset_std_streams(void) {
    freopen("/dev/null", "r", stdin);
  }

  int main(void) {
    reset_std_streams();
    // B
    /*
    -- actual program code --
    */
  }

When comparing the total tally of all resources used at point A in
snippet 1 and point B in snippet 2, is there an actual difference? Are
there any resources in terms of buffers, memory mapped pages or other
static or dynamic resources that freopen() variant would use that the
fopen() variant would not?