https://fedoraproject.org/wiki/Changes/PortingToModernC. This document represents a proposed Change. As part of the Changes process, proposals are publicly announced in order to receive community feedback. This proposal will only be implemented if approved by the Fedora Engineering Steering Committee. == Summary == Back in 1999, a new revision of the C standard removed several backwards compatibility features. However, GCC still accepts these obsolete constructs by default. Support for these constructs is confusing to programmers and potentially affect GCC's ability to implement features from future C standards. == Owner == * Name: [[User:fweimer| Florian Weimer]] * Email: [mailto:fweimer@xxxxxxxxxx| fweimer@xxxxxxxxxx] == Detailed Description == The most important change is the <b>removal of implicit function declarations</b>. This legacy compatibility feature causes the compiler to automatically supply a declaration of type `int function()` if `function` is undeclared, but called as a function. However, the implicit return type of `int` is incompatible with pointer-returning functions, which can lead to difficult debugging sessions if the compiler warning is missed, as described here: * [https://developers.redhat.com/blog/2019/04/22/implicit-function-declarations-flexs-use-of-reallocarray| Implicit function declarations: flex's use of "reallocarray"] On x86-64, functions returning `_Bool` called with an implicit declaration will also not work correctly because the return register value for `_Bool` functions is partially undefined (not all 32 bits in the implicitly declared `int` are defined). Another change for C99 is the <b>removal of implicit `int` type declarations</b>. For example, in the following fragment, both `i` and `j` are defined as `int` variables: <pre> static i = 1.0; auto j = 2.0; </pre> Support for this obsolete constructs is incompatible with adoption of type inference for `auto` definitions (which are part of C++). Neither change is trivial to implement because introducing errors for these constructs (as required by C99) alters the result of autoconf configure checks. Quite a few such checks use an implicitly declared `exit` function, for instance. These failures are not really related to the feature under test. If the build system is well written, the build still succeeds, the relevant features are automatically disabled in the test suite and removed from reference ABI lists, and it's not immediately apparent that feature is gone. Therefore, some care is needed that no such alterations happen, and packages need to be ported to C99. Various tools for this porting activity are being developed to support this proposal. Cross-distribution collaboration will help as well, sharing patches and insights. GCC 14 may also make other changes by default, as described below. These changes will also require extensive testing, and some adoption of configure checks. ==== Removal of old-style function definitions ==== An old style function definition looks like this: <pre> int sum (a, b) char *a; int b; { return atoi (a) + b; } </pre> It is equivalent to this definition with a prototype: <pre> int sum (char *a, int b) { return atoi (a) + b; } </pre> This legacy feature is very close to being incompatible with the C2X standard, which introduces unnamed function arguments. But due to a quirk in GCC's implementation, it should be possible to support both at the same time. ==== New keywords <code>bool</code>, <code>true</code>, <code>false</code> ==== Packages which supply their own definitions instead of including `<stdbool.h>` (which remains available) might fail to build. ==== Change of meaning of <code>()</code> in function declarators ==== In earlier C versions, a declaration `int function()` declares `function` as accepting an unspecified number of arguments of unknown type. This means that both `function(1)` and `function("one")` compile, even though they might not work correctly. In a future C standard, `int function()` will mean that `function` does not accept any parameters (like in C++, or as if written as `int function(void)` in current C). Calls that specify parameters will therefore result in compilation errors. We believe that this change is ABI-compatible, even on ppc64le (with its parameter save area), although it comes very close to an implicit ABI break. ==== Rejecting implicit conversions between integers and pointers as errors ==== Currently, GCC does not treat conversion between integers at pointers as errors, so this compiles: <pre> int ptr = "string"; </pre> However, this is very unlikely to work on 64-bit architectures (it may work by accident without PIE/PIC). == Feedback == The transition plan has been discussed at various GNU Tools Cauldrons. Feedback has been generally positive, except a general worry about the work required. Without a deliberate porting effort, a lot of breakage occurs, [https://lists.fedoraproject.org/archives/list/devel@xxxxxxxxxxxxxxxxxxxxxxx/message/AL3PXQ3HX26PCF5PYDZ5RU7VJZSBZEPQ/|as seen in 2016 in Fedora with an uncoordinated change], and more recently [https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213/32|with an uncoordinated Clang update]. == Benefit to Fedora == Programmers will no longer waste time tracking down things that look eerily like compiler or ABI bugs because in several cases, builds will fail with a clear error message, instead of producing a warning that is easily missed. Potential blockers to adoption of further C language changes are removed. == Scope == * Proposal owners: Update rawhide over time to be compatible with strict C99 compilers. * Other developers: Help out with non-obvious porting issues, and with upstreaming patches in case active upstreams cannot be easily identified. Tolerate early backports of upstream-submitted fixes in Fedora dist-git. * Release engineering: [https://pagure.io/releng/issues #Releng issue number] * Policies and guidelines: Fedora compiler policy will likely change due to the adoption of GCC 14 in Fedora 40. * Trademark approval: N/A (not needed for this Change) * Alignment with Objectives: N/A == Upgrade/compatibility impact == The change is expected to be transparent to those users who do not use C compilers. No features are supposed to be added or removed as a result. In fact, most of the porting effort focuses on avoiding configuration changes. == How To Test == General regression testing is sufficient. == User Experience == User experience does not change. == Dependencies == To avoid regressing the porting effort, GCC as the system compiler needs to reject obsolete constructs by default. This is expected for GCC 14, to be released as part of Fedora 40 in Spring 2024. == Contingency Plan == * Contingency mechanism: Upstream GCC will probably accept obsolete constructs longer in case distributions like Fedora cannot port to modern C in time. * Blocks release? No if GCC doesn't switch. If GCC switches, we have to complete the port. == Documentation == This section will be updated once more documentation of the porting effort will become available. == Release Notes == Probably not needed because no user visible impact is intended. -- Ben Cotton He / Him / His Fedora Program Manager Red Hat TZ=America/Indiana/Indianapolis _______________________________________________ devel mailing list -- devel@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to devel-leave@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/devel@xxxxxxxxxxxxxxxxxxxxxxx Do not reply to spam, report it: https://pagure.io/fedora-infrastructure/new_issue
