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Thu, 5 Dec 2019 13:14:15 +0000 From: Wilco Dijkstra To: GCC Patches CC: "sandra@codesourcery.com" , Joseph Myers Subject: [wwwdocs] Document -fcommon default change Date: Thu, 5 Dec 2019 13:14:15 +0000 Message-ID: Authentication-Results-Original: spf=none (sender IP is ) smtp.mailfrom=Wilco.Dijkstra@arm.com; x-checkrecipientrouted: true x-ms-oob-tlc-oobclassifiers: OLM:8273;OLM:8273; X-Forefront-Antispam-Report-Untrusted: SFV:NSPM; SFS:(10009020)(4636009)(396003)(136003)(346002)(39860400002)(366004)(376002)(189003)(199004)(81166006)(6916009)(81156014)(316002)(8676002)(2906002)(14454004)(86362001)(54906003)(66446008)(66556008)(71200400001)(71190400001)(8936002)(64756008)(33656002)(52536014)(76116006)(66476007)(66946007)(6506007)(26005)(4326008)(102836004)(55016002)(7696005)(5660300002)(74316002)(305945005)(9686003)(186003)(478600001)(25786009)(99286004); DIR:OUT; SFP:1101; SCL:1; SRVR:VI1PR0801MB2126; H:VI1PR0801MB2127.eurprd08.prod.outlook.com; FPR:; SPF:None; LANG:en; PTR:InfoNoRecords; 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Passes the W3 validator. Cheers, Wilco diff --git a/htdocs/gcc-10/changes.html b/htdocs/gcc-10/changes.html index e02966460450b7aad884b2d45190b9ecd8c7a5d8..304e1e8ccd38795104156e86b92062696fa5aa8b 100644 --- a/htdocs/gcc-10/changes.html +++ b/htdocs/gcc-10/changes.html @@ -102,6 +102,11 @@ a work-in-progress.

  • In C2X mode, -fno-fp-int-builtin-inexact is enabled by default.
    • + +
  • GCC now defaults to -fno-common. In C, global variables with + multiple tentative definitions will result in linker errors. + Global variable accesses are also more efficient on various targets. +

    • C++

    diff --git a/htdocs/gcc-10/porting_to.html b/htdocs/gcc-10/porting_to.html index 3256e8a35d00ce1352c169a1c6df6d8f120889ee..e2c7e226a83b7720fe6ed40061cdddbc27659664 100644 --- a/htdocs/gcc-10/porting_to.html +++ b/htdocs/gcc-10/porting_to.html @@ -29,9 +29,25 @@ and provide solutions. Let us know if you have suggestions for improvements!

    Preprocessor issues

    --> - + +

    Default to -fno-common

    + +

    + A common mistake in C is omitting extern when declaring a global + variable in a header file. If the header is included by several files it + results in multiple definitions of the same variable. In previous GCC + versions this error is ignored. GCC 10 defaults to -fno-common, + which means a linker error will now be reported. + To fix this, use extern in header files when declaring global + variables, and ensure each global is defined in exactly one C file. + As a workaround, legacy C code can be compiled with -fcommon. +

    + 
    
+      int x;  // tentative definition - avoid in header files
+
+      extern int y;  // correct declaration in a header file
+

    Fortran language issues