前言：
此刻看官们对“ubuntu防火墙设置防cc攻击”大体比较注意，同学们都需要了解一些“ubuntu防火墙设置防cc攻击”的相关文章。那么小编在网络上汇集了一些关于“ubuntu防火墙设置防cc攻击””的相关内容，希望看官们能喜欢，咱们一起来学习一下吧！GCC是每个从事Linux，以及嵌入式开发等必备的编译器，由于帮助手册较多，这里使用了分页的形式进行分享，如下为Ubuntu Server 22.04操作系统平台和GCC编译器的基本信息。
老韩Linux DevOps系列
GCC帮助手册的第11小节，第2288~3288行，如下。
2288 from 'map<[...],vector<double>>' to 'map<[...],vector<float>>2289 the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common and2290 differing parts of the types, such as:2291 map<2292 [...],2293 vector<2294 [double != float]>>2295 The parts that differ are highlighted with color ("double" and "float" in this case).2296 -fno-elide-type2297 By default when the C++ frontend prints diagnostics showing mismatching template types, common parts of the2298 types are printed as "[...]" to simplify the error message. For example:2299 could not convert 'std::map<int, std::vector<double> >()'2300 from 'map<[...],vector<double>>' to 'map<[...],vector<float>>2301 Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the2302 -fdiagnostics-show-template-tree flag.2303 -fdiagnostics-path-format=KIND2304 Specify how to print paths of control-flow events for diagnostics that have such a path associated with2305 them.2306 KIND is none, separate-events, or inline-events, the default.2307 none means to not print diagnostic paths.2308 separate-events means to print a separate "note" diagnostic for each event within the diagnostic. For2309 example:2310 test.c:29:5: error: passing NULL as argument 1 to 'PyList_Append' which requires a non-NULL parameter2311 test.c:25:10: note: (1) when 'PyList_New' fails, returning NULL2312 test.c:27:3: note: (2) when 'i < count'2313 test.c:29:5: note: (3) when calling 'PyList_Append', passing NULL from (1) as argument 12314 inline-events means to print the events "inline" within the source code. This view attempts to consolidate2315 the events into runs of sufficiently-close events, printing them as labelled ranges within the source.2316 For example, the same events as above might be printed as:2317 'test': events 1-32318 |2319 | 25 | list = PyList_New(0);2320 | | ^~~~~~~~~~~~~2321 | | |2322 | | (1) when 'PyList_New' fails, returning NULL2323 | 26 |2324 | 27 | for (i = 0; i < count; i++) {2325 | | ~~~2326 | | |2327 | | (2) when 'i < count'2328 | 28 | item = PyLong_FromLong(random());2329 | 29 | PyList_Append(list, item);2330 | | ~~~~~~~~~~~~~~~~~~~~~~~~~2331 | | |2332 | | (3) when calling 'PyList_Append', passing NULL from (1) as argument 12333 |2334 Interprocedural control flow is shown by grouping the events by stack frame, and using indentation to show2335 how stack frames are nested, pushed, and popped.2336 For example:2337 'test': events 1-22338 |2339 | 133 | {2340 | | ^2341 | | |2342 | | (1) entering 'test'2343 | 134 | boxed_int *obj = make_boxed_int (i);2344 | | ~~~~~~~~~~~~~~~~~~2345 | | |2346 | | (2) calling 'make_boxed_int'2347 |2348 +--> 'make_boxed_int': events 3-42349 |2350 | 120 | {2351 | | ^2352 | | |2353 | | (3) entering 'make_boxed_int'2354 | 121 | boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));2355 | | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~2356 | | |2357 | | (4) calling 'wrapped_malloc'2358 |2359 +--> 'wrapped_malloc': events 5-62360 |2361 | 7 | {2362 | | ^2363 | | |2364 | | (5) entering 'wrapped_malloc'2365 | 8 | return malloc (size);2366 | | ~~~~~~~~~~~~~2367 | | |2368 | | (6) calling 'malloc'2369 |2370 <-------------+2371 |2372 'test': event 72373 |2374 | 138 | free_boxed_int (obj);2375 | | ^~~~~~~~~~~~~~~~~~~~2376 | | |2377 | | (7) calling 'free_boxed_int'2378 |2379 (etc)2380 -fdiagnostics-show-path-depths2381 This option provides additional information when printing control-flow paths associated with a diagnostic.2382 If this is option is provided then the stack depth will be printed for each run of events within2383 -fdiagnostics-path-format=separate-events.2384 This is intended for use by GCC developers and plugin developers when debugging diagnostics that report2385 interprocedural control flow.2386 -fno-show-column2387 Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a2388 program that does not understand the column numbers, such as dejagnu.2389 -fdiagnostics-column-unit=UNIT2390 Select the units for the column number. This affects traditional diagnostics (in the absence of2391 -fno-show-column), as well as JSON format diagnostics if requested.2392 The default UNIT, display, considers the number of display columns occupied by each character. This may be2393 larger than the number of bytes required to encode the character, in the case of tab characters, or it may2394 be smaller, in the case of multibyte characters. For example, the character "GREEK SMALL LETTER PI2395 (U+03C0)" occupies one display column, and its UTF-8 encoding requires two bytes; the character "SLIGHTLY2396 SMILING FACE (U+1F642)" occupies two display columns, and its UTF-8 encoding requires four bytes.2397 Setting UNIT to byte changes the column number to the raw byte count in all cases, as was traditionally2398 output by GCC prior to version 11.1.0.2399 -fdiagnostics-column-origin=ORIGIN2400 Select the origin for column numbers, i.e. the column number assigned to the first column. The default2401 value of 1 corresponds to traditional GCC behavior and to the GNU style guide. Some utilities may perform2402 better with an origin of 0; any non-negative value may be specified.2403 -fdiagnostics-format=FORMAT2404 Select a different format for printing diagnostics. FORMAT is text or json. The default is text.2405 The json format consists of a top-level JSON array containing JSON objects representing the diagnostics.2406 The JSON is emitted as one line, without formatting; the examples below have been formatted for clarity.2407 Diagnostics can have child diagnostics. For example, this error and note:2408 misleading-indentation.c:15:3: warning: this 'if' clause does not2409 guard... [-Wmisleading-indentation]2410 15 | if (flag)2411 | ^~2412 misleading-indentation.c:17:5: note: ...this statement, but the latter2413 is misleadingly indented as if it were guarded by the 'if'2414 17 | y = 2;2415 | ^2416 might be printed in JSON form (after formatting) like this:2417 [2418 {2419 "kind": "warning",2420 "locations": [2421 {2422 "caret": {2423 "display-column": 3,2424 "byte-column": 3,2425 "column": 3,2426 "file": "misleading-indentation.c",2427 "line": 152428 },2429 "finish": {2430 "display-column": 4,2431 "byte-column": 4,2432 "column": 4,2433 "file": "misleading-indentation.c",2434 "line": 152435 }2436 }2437 ],2438 "message": "this \u2018if\u2019 clause does not guard...",2439 "option": "-Wmisleading-indentation",2440 "option_url": ";,2441 "children": [2442 {2443 "kind": "note",2444 "locations": [2445 {2446 "caret": {2447 "display-column": 5,2448 "byte-column": 5,2449 "column": 5,2450 "file": "misleading-indentation.c",2451 "line": 172452 }2453 }2454 ],2455 "message": "...this statement, but the latter is ..."2456 }2457 ]2458 "column-origin": 1,2459 },2460 ...2461 ]2462 where the "note" is a child of the "warning".2463 A diagnostic has a "kind". If this is "warning", then there is an "option" key describing the command-line2464 option controlling the warning.2465 A diagnostic can contain zero or more locations. Each location has an optional "label" string and up to2466 three positions within it: a "caret" position and optional "start" and "finish" positions. A position is2467 described by a "file" name, a "line" number, and three numbers indicating a column position:2468 * "display-column" counts display columns, accounting for tabs and multibyte characters.2469 * "byte-column" counts raw bytes.2470 * "column" is equal to one of the previous two, as dictated by the -fdiagnostics-column-unit option.2471 All three columns are relative to the origin specified by -fdiagnostics-column-origin, which is typically2472 equal to 1 but may be set, for instance, to 0 for compatibility with other utilities that number columns2473 from 0. The column origin is recorded in the JSON output in the "column-origin" tag. In the remaining2474 examples below, the extra column number outputs have been omitted for brevity.2475 For example, this error:2476 bad-binary-ops.c:64:23: error: invalid operands to binary + (have 'S' {aka2477 'struct s'} and 'T' {aka 'struct t'})2478 64 | return callee_4a () + callee_4b ();2479 | ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~2480 | | |2481 | | T {aka struct t}2482 | S {aka struct s}2483 has three locations. Its primary location is at the "+" token at column 23. It has two secondary2484 locations, describing the left and right-hand sides of the expression, which have labels. It might be2485 printed in JSON form as:2486 {2487 "children": [],2488 "kind": "error",2489 "locations": [2490 {2491 "caret": {2492 "column": 23, "file": "bad-binary-ops.c", "line": 642493 }2494 },2495 {2496 "caret": {2497 "column": 10, "file": "bad-binary-ops.c", "line": 642498 },2499 "finish": {2500 "column": 21, "file": "bad-binary-ops.c", "line": 642501 },2502 "label": "S {aka struct s}"2503 },2504 {2505 "caret": {2506 "column": 25, "file": "bad-binary-ops.c", "line": 642507 },2508 "finish": {2509 "column": 36, "file": "bad-binary-ops.c", "line": 642510 },2511 "label": "T {aka struct t}"2512 }2513 ],2514 "message": "invalid operands to binary + ..."2515 }2516 If a diagnostic contains fix-it hints, it has a "fixits" array, consisting of half-open intervals, similar2517 to the output of -fdiagnostics-parseable-fixits. For example, this diagnostic with a replacement fix-it2518 hint:2519 demo.c:8:15: error: 'struct s' has no member named 'colour'; did you2520 mean 'color'?2521 8 | return ptr->colour;2522 | ^~~~~~2523 | color2524 might be printed in JSON form as:2525 {2526 "children": [],2527 "fixits": [2528 {2529 "next": {2530 "column": 21,2531 "file": "demo.c",2532 "line": 82533 },2534 "start": {2535 "column": 15,2536 "file": "demo.c",2537 "line": 82538 },2539 "string": "color"2540 }2541 ],2542 "kind": "error",2543 "locations": [2544 {2545 "caret": {2546 "column": 15,2547 "file": "demo.c",2548 "line": 82549 },2550 "finish": {2551 "column": 20,2552 "file": "demo.c",2553 "line": 82554 }2555 }2556 ],2557 "message": "\u2018struct s\u2019 has no member named ..."2558 }2559 where the fix-it hint suggests replacing the text from "start" up to but not including "next" with2560 "string"'s value. Deletions are expressed via an empty value for "string", insertions by having "start"2561 equal "next".2562 If the diagnostic has a path of control-flow events associated with it, it has a "path" array of objects2563 representing the events. Each event object has a "description" string, a "location" object, along with a2564 "function" string and a "depth" number for representing interprocedural paths. The "function" represents2565 the current function at that event, and the "depth" represents the stack depth relative to some baseline:2566 the higher, the more frames are within the stack.2567 For example, the intraprocedural example shown for -fdiagnostics-path-format= might have this JSON for its2568 path:2569 "path": [2570 {2571 "depth": 0,2572 "description": "when 'PyList_New' fails, returning NULL",2573 "function": "test",2574 "location": {2575 "column": 10,2576 "file": "test.c",2577 "line": 252578 }2579 },2580 {2581 "depth": 0,2582 "description": "when 'i < count'",2583 "function": "test",2584 "location": {2585 "column": 3,2586 "file": "test.c",2587 "line": 272588 }2589 },2590 {2591 "depth": 0,2592 "description": "when calling 'PyList_Append', passing NULL from (1) as argument 1",2593 "function": "test",2594 "location": {2595 "column": 5,2596 "file": "test.c",2597 "line": 292598 }2599 }2600 ]2601 Options to Request or Suppress Warnings2602 Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky2603 or suggest there may have been an error.2604 The following language-independent options do not enable specific warnings but control the kinds of diagnostics2605 produced by GCC.2606 -fsyntax-only2607 Check the code for syntax errors, but don't do anything beyond that.2608 -fmax-errors=n2609 Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to2610 continue processing the source code. If n is 0 (the default), there is no limit on the number of error2611 messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this2612 option.2613 -w Inhibit all warning messages.2614 -Werror2615 Make all warnings into errors.2616 -Werror=2617 Make the specified warning into an error. The specifier for a warning is appended; for example2618 -Werror=switch turns the warnings controlled by -Wswitch into errors. This switch takes a negative form,2619 to be used to negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings2620 not be errors, even when -Werror is in effect.2621 The warning message for each controllable warning includes the option that controls the warning. That2622 option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the2623 warning message can be disabled using the -fno-diagnostics-show-option flag.)2624 Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply2625 anything.2626 -Wfatal-errors2627 This option causes the compiler to abort compilation on the first error occurred rather than trying to keep2628 going and printing further error messages.2629 You can request many specific warnings with options beginning with -W, for example -Wimplicit to request2630 warnings on implicit declarations. Each of these specific warning options also has a negative form beginning2631 -Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two forms, whichever2632 is not the default. For further language-specific options also refer to C++ Dialect Options and Objective-C2633 and Objective-C++ Dialect Options. Additional warnings can be produced by enabling the static analyzer;2634 Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on further2635 options, such as -Wunused-value. The combined effect of positive and negative forms is that more specific2636 options have priority over less specific ones, independently of their position in the command-line. For options2637 of the same specificity, the last one takes effect. Options enabled or disabled via pragmas take effect as if2638 they appeared at the end of the command-line.2639 When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC emits a diagnostic stating that2640 the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no2641 diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the2642 use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an2643 unrecognized option is present.2644 The effectiveness of some warnings depends on optimizations also being enabled. For example2645 -Wsuggest-final-types is more effective with link-time optimization and -Wmaybe-uninitialized does not warn at2646 all unless optimization is enabled.2647 -Wpedantic2648 -pedantic2649 Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden2650 extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version2651 of the ISO C standard specified by any -std option used.2652 Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few2653 require -ansi or a -std option specifying the required version of ISO C). However, without this option,2654 certain GNU extensions and traditional C and C++ features are supported as well. With this option, they2655 are rejected.2656 -Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with2657 __. This alternate format can also be used to disable warnings for non-ISO __intN types, i.e. __intN__.2658 Pedantic warnings are also disabled in the expression that follows "__extension__". However, only system2659 header files should use these escape routes; application programs should avoid them.2660 Some users try to use -Wpedantic to check programs for strict ISO C conformance. They soon find that it2661 does not do quite what they want: it finds some non-ISO practices, but not all---only those for which ISO C2662 requires a diagnostic, and some others for which diagnostics have been added.2663 A feature to report any failure to conform to ISO C might be useful in some instances, but would require2664 considerable additional work and would be quite different from -Wpedantic. We don't have plans to support2665 such a feature in the near future.2666 Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or gnu99,2667 there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based.2668 Warnings from -Wpedantic are given where they are required by the base standard. (It does not make sense2669 for such warnings to be given only for features not in the specified GNU C dialect, since by definition the2670 GNU dialects of C include all features the compiler supports with the given option, and there would be2671 nothing to warn about.)2672 -pedantic-errors2673 Give an error whenever the base standard (see -Wpedantic) requires a diagnostic, in some cases where there2674 is undefined behavior at compile-time and in some other cases that do not prevent compilation of programs2675 that are valid according to the standard. This is not equivalent to -Werror=pedantic, since there are2676 errors enabled by this option and not enabled by the latter and vice versa.2677 -Wall2678 This enables all the warnings about constructions that some users consider questionable, and that are easy2679 to avoid (or modify to prevent the warning), even in conjunction with macros. This also enables some2680 language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect2681 Options.2682 -Wall turns on the following warning flags:2683 -Waddress -Warray-bounds=1 (only with -O2) -Warray-parameter=2 (C and Objective-C only) -Wbool-compare2684 -Wbool-operation -Wc++11-compat -Wc++14-compat -Wcatch-value (C++ and Objective-C++ only)2685 -Wchar-subscripts -Wcomment -Wduplicate-decl-specifier (C and Objective-C only) -Wenum-compare (in C/ObjC;2686 this is on by default in C++) -Wformat -Wformat-overflow -Wformat-truncation -Wint-in-bool-context2687 -Wimplicit (C and Objective-C only) -Wimplicit-int (C and Objective-C only) -Wimplicit-function-declaration2688 (C and Objective-C only) -Winit-self (only for C++) -Wlogical-not-parentheses -Wmain (only for C/ObjC and2689 unless -ffreestanding) -Wmaybe-uninitialized -Wmemset-elt-size -Wmemset-transposed-args2690 -Wmisleading-indentation (only for C/C++) -Wmissing-attributes -Wmissing-braces (only for C/ObjC)2691 -Wmultistatement-macros -Wnarrowing (only for C++) -Wnonnull -Wnonnull-compare -Wopenmp-simd -Wparentheses2692 -Wpessimizing-move (only for C++) -Wpointer-sign -Wrange-loop-construct (only for C++) -Wreorder -Wrestrict2693 -Wreturn-type -Wsequence-point -Wsign-compare (only in C++) -Wsizeof-array-div -Wsizeof-pointer-div2694 -Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-overflow=1 -Wswitch -Wtautological-compare2695 -Wtrigraphs -Wuninitialized -Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value2696 -Wunused-variable -Wvla-parameter (C and Objective-C only) -Wvolatile-register-var -Wzero-length-bounds2697 Note that some warning flags are not implied by -Wall. Some of them warn about constructions that users2698 generally do not consider questionable, but which occasionally you might wish to check for; others warn2699 about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify2700 the code to suppress the warning. Some of them are enabled by -Wextra but many of them must be enabled2701 individually.2702 -Wextra2703 This enables some extra warning flags that are not enabled by -Wall. (This option used to be called -W.2704 The older name is still supported, but the newer name is more descriptive.)2705 -Wclobbered -Wcast-function-type -Wdeprecated-copy (C++ only) -Wempty-body -Wenum-conversion (C only)2706 -Wignored-qualifiers -Wimplicit-fallthrough=3 -Wmissing-field-initializers -Wmissing-parameter-type (C2707 only) -Wold-style-declaration (C only) -Woverride-init -Wsign-compare (C only) -Wstring-compare2708 -Wredundant-move (only for C++) -Wtype-limits -Wuninitialized -Wshift-negative-value (in C++11 to C++17 and2709 in C99 and newer) -Wunused-parameter (only with -Wunused or -Wall) -Wunused-but-set-parameter (only with2710 -Wunused or -Wall)2711 The option -Wextra also prints warning messages for the following cases:2712 * A pointer is compared against integer zero with "<", "<=", ">", or ">=".2713 * (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.2714 * (C++ only) Ambiguous virtual bases.2715 * (C++ only) Subscripting an array that has been declared "register".2716 * (C++ only) Taking the address of a variable that has been declared "register".2717 * (C++ only) A base class is not initialized in the copy constructor of a derived class.2718 -Wabi (C, Objective-C, C++ and Objective-C++ only)2719 Warn about code affected by ABI changes. This includes code that may not be compatible with the vendor-2720 neutral C++ ABI as well as the psABI for the particular target.2721 Since G++ now defaults to updating the ABI with each major release, normally -Wabi warns only about C++ ABI2722 compatibility problems if there is a check added later in a release series for an ABI issue discovered2723 since the initial release. -Wabi warns about more things if an older ABI version is selected (with2724 -fabi-version=n).2725 -Wabi can also be used with an explicit version number to warn about C++ ABI compatibility with a2726 particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.2727 If an explicit version number is provided and -fabi-compat-version is not specified, the version number2728 from this option is used for compatibility aliases. If no explicit version number is provided with this2729 option, but -fabi-compat-version is specified, that version number is used for C++ ABI warnings.2730 Although an effort has been made to warn about all such cases, there are probably some cases that are not2731 warned about, even though G++ is generating incompatible code. There may also be cases where warnings are2732 emitted even though the code that is generated is compatible.2733 You should rewrite your code to avoid these warnings if you are concerned about the fact that code2734 generated by G++ may not be binary compatible with code generated by other compilers.2735 Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:2736 * A template with a non-type template parameter of reference type was mangled incorrectly:2737 extern int N;2738 template <int &> struct S {};2739 void n (S<N>) {2}2740 This was fixed in -fabi-version=3.2741 * SIMD vector types declared using "__attribute ((vector_size))" were mangled in a non-standard way that2742 does not allow for overloading of functions taking vectors of different sizes.2743 The mangling was changed in -fabi-version=4.2744 * "__attribute ((const))" and "noreturn" were mangled as type qualifiers, and "decltype" of a plain2745 declaration was folded away.2746 These mangling issues were fixed in -fabi-version=5.2747 * Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators,2748 causing "va_arg" to complain. On most targets this does not actually affect the parameter passing ABI,2749 as there is no way to pass an argument smaller than "int".2750 Also, the ABI changed the mangling of template argument packs, "const_cast", "static_cast", prefix2751 increment/decrement, and a class scope function used as a template argument.2752 These issues were corrected in -fabi-version=6.2753 * Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of2754 "nullptr_t".2755 These issues were corrected in -fabi-version=7.2756 * When mangling a function type with function-cv-qualifiers, the un-qualified function type was2757 incorrectly treated as a substitution candidate.2758 This was fixed in -fabi-version=8, the default for GCC 5.1.2759 * "decltype(nullptr)" incorrectly had an alignment of 1, leading to unaligned accesses. Note that this2760 did not affect the ABI of a function with a "nullptr_t" parameter, as parameters have a minimum2761 alignment.2762 This was fixed in -fabi-version=9, the default for GCC 5.2.2763 * Target-specific attributes that affect the identity of a type, such as ia32 calling conventions on a2764 function type (stdcall, regparm, etc.), did not affect the mangled name, leading to name collisions2765 when function pointers were used as template arguments.2766 This was fixed in -fabi-version=10, the default for GCC 6.1.2767 This option also enables warnings about psABI-related changes. The known psABI changes at this point2768 include:2769 * For SysV/x86-64, unions with "long double" members are passed in memory as specified in psABI. Prior2770 to GCC 4.4, this was not the case. For example:2771 union U {2772 long double ld;2773 int i;2774 };2775 "union U" is now always passed in memory.2776 -Wchar-subscripts2777 Warn if an array subscript has type "char". This is a common cause of error, as programmers often forget2778 that this type is signed on some machines. This warning is enabled by -Wall.2779 -Wno-coverage-mismatch2780 Warn if feedback profiles do not match when using the -fprofile-use option. If a source file is changed2781 between compiling with -fprofile-generate and with -fprofile-use, the files with the profile feedback can2782 fail to match the source file and GCC cannot use the profile feedback information. By default, this2783 warning is enabled and is treated as an error. -Wno-coverage-mismatch can be used to disable the warning2784 or -Wno-error=coverage-mismatch can be used to disable the error. Disabling the error for this warning can2785 result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to2786 an existing code-base. Completely disabling the warning is not recommended.2787 -Wno-cpp2788 (C, Objective-C, C++, Objective-C++ and Fortran only) Suppress warning messages emitted by "#warning"2789 directives.2790 -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)2791 Give a warning when a value of type "float" is implicitly promoted to "double". CPUs with a 32-bit2792 "single-precision" floating-point unit implement "float" in hardware, but emulate "double" in software. On2793 such a machine, doing computations using "double" values is much more expensive because of the overhead2794 required for software emulation.2795 It is easy to accidentally do computations with "double" because floating-point literals are implicitly of2796 type "double". For example, in:2797 float area(float radius)2798 {2799 return 3.14159 * radius * radius;2800 }2801 the compiler performs the entire computation with "double" because the floating-point literal is a2802 "double".2803 -Wduplicate-decl-specifier (C and Objective-C only)2804 Warn if a declaration has duplicate "const", "volatile", "restrict" or "_Atomic" specifier. This warning2805 is enabled by -Wall.2806 -Wformat2807 -Wformat=n2808 Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied have types appropriate2809 to the format string specified, and that the conversions specified in the format string make sense. This2810 includes standard functions, and others specified by format attributes, in the "printf", "scanf",2811 "strftime" and "strfmon" (an X/Open extension, not in the C standard) families (or other target-specific2812 families). Which functions are checked without format attributes having been specified depends on the2813 standard version selected, and such checks of functions without the attribute specified are disabled by2814 -ffreestanding or -fno-builtin.2815 The formats are checked against the format features supported by GNU libc version 2.2. These include all2816 ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU2817 extensions. Other library implementations may not support all these features; GCC does not support warning2818 about features that go beyond a particular library's limitations. However, if -Wpedantic is used with2819 -Wformat, warnings are given about format features not in the selected standard version (but not for2820 "strfmon" formats, since those are not in any version of the C standard).2821 -Wformat=12822 -Wformat2823 Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since2824 -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.2825 Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul,2826 -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall.2827 -Wformat=22828 Enable -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral2829 -Wformat-security -Wformat-y2k.2830 -Wno-format-contains-nul2831 If -Wformat is specified, do not warn about format strings that contain NUL bytes.2832 -Wno-format-extra-args2833 If -Wformat is specified, do not warn about excess arguments to a "printf" or "scanf" format function. The2834 C standard specifies that such arguments are ignored.2835 Where the unused arguments lie between used arguments that are specified with $ operand number2836 specifications, normally warnings are still given, since the implementation could not know what type to2837 pass to "va_arg" to skip the unused arguments. However, in the case of "scanf" formats, this option2838 suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says2839 that such unused arguments are allowed.2840 -Wformat-overflow2841 -Wformat-overflow=level2842 Warn about calls to formatted input/output functions such as "sprintf" and "vsprintf" that might overflow2843 the destination buffer. When the exact number of bytes written by a format directive cannot be determined2844 at compile-time it is estimated based on heuristics that depend on the level argument and on optimization.2845 While enabling optimization will in most cases improve the accuracy of the warning, it may also result in2846 false positives.2847 -Wformat-overflow2848 -Wformat-overflow=12849 Level 1 of -Wformat-overflow enabled by -Wformat employs a conservative approach that warns only about2850 calls that most likely overflow the buffer. At this level, numeric arguments to format directives with2851 unknown values are assumed to have the value of one, and strings of unknown length to be empty.2852 Numeric arguments that are known to be bounded to a subrange of their type, or string arguments whose2853 output is bounded either by their directive's precision or by a finite set of string literals, are2854 assumed to take on the value within the range that results in the most bytes on output. For example,2855 the call to "sprintf" below is diagnosed because even with both a and b equal to zero, the terminating2856 NUL character ('\0') appended by the function to the destination buffer will be written past its end.2857 Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may2858 not be sufficient to avoid the overflow.2859 void f (int a, int b)2860 {2861 char buf [13];2862 sprintf (buf, "a = %i, b = %i\n", a, b);2863 }2864 -Wformat-overflow=22865 Level 2 warns also about calls that might overflow the destination buffer given an argument of2866 sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to have the minimum2867 representable value for signed types with a precision greater than 1, and the maximum representable2868 value otherwise. Unknown string arguments whose length cannot be assumed to be bounded either by the2869 directive's precision, or by a finite set of string literals they may evaluate to, or the character2870 array they may point to, are assumed to be 1 character long.2871 At level 2, the call in the example above is again diagnosed, but this time because with a equal to a2872 32-bit "INT_MIN" the first %i directive will write some of its digits beyond the end of the destination2873 buffer. To make the call safe regardless of the values of the two variables, the size of the2874 destination buffer must be increased to at least 34 bytes. GCC includes the minimum size of the buffer2875 in an informational note following the warning.2876 An alternative to increasing the size of the destination buffer is to constrain the range of formatted2877 values. The maximum length of string arguments can be bounded by specifying the precision in the2878 format directive. When numeric arguments of format directives can be assumed to be bounded by less2879 than the precision of their type, choosing an appropriate length modifier to the format specifier will2880 reduce the required buffer size. For example, if a and b in the example above can be assumed to be2881 within the precision of the "short int" type then using either the %hi format directive or casting the2882 argument to "short" reduces the maximum required size of the buffer to 24 bytes.2883 void f (int a, int b)2884 {2885 char buf [23];2886 sprintf (buf, "a = %hi, b = %i\n", a, (short)b);2887 }2888 -Wno-format-zero-length2889 If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length2890 formats are allowed.2891 -Wformat-nonliteral2892 If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked,2893 unless the format function takes its format arguments as a "va_list".2894 -Wformat-security2895 If -Wformat is specified, also warn about uses of format functions that represent possible security2896 problems. At present, this warns about calls to "printf" and "scanf" functions where the format string is2897 not a string literal and there are no format arguments, as in "printf (foo);". This may be a security hole2898 if the format string came from untrusted input and contains %n. (This is currently a subset of what2899 -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not2900 included in -Wformat-nonliteral.)2901 -Wformat-signedness2902 If -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is2903 signed and vice versa.2904 -Wformat-truncation2905 -Wformat-truncation=level2906 Warn about calls to formatted input/output functions such as "snprintf" and "vsnprintf" that might result2907 in output truncation. When the exact number of bytes written by a format directive cannot be determined at2908 compile-time it is estimated based on heuristics that depend on the level argument and on optimization.2909 While enabling optimization will in most cases improve the accuracy of the warning, it may also result in2910 false positives. Except as noted otherwise, the option uses the same logic -Wformat-overflow.2911 -Wformat-truncation2912 -Wformat-truncation=12913 Level 1 of -Wformat-truncation enabled by -Wformat employs a conservative approach that warns only2914 about calls to bounded functions whose return value is unused and that will most likely result in2915 output truncation.2916 -Wformat-truncation=22917 Level 2 warns also about calls to bounded functions whose return value is used and that might result in2918 truncation given an argument of sufficient length or magnitude.2919 -Wformat-y2k2920 If -Wformat is specified, also warn about "strftime" formats that may yield only a two-digit year.2921 -Wnonnull2922 Warn about passing a null pointer for arguments marked as requiring a non-null value by the "nonnull"2923 function attribute.2924 -Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.2925 -Wnonnull-compare2926 Warn when comparing an argument marked with the "nonnull" function attribute against null inside the2927 function.2928 -Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.2929 -Wnull-dereference2930 Warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null2931 pointer. This option is only active when -fdelete-null-pointer-checks is active, which is enabled by2932 optimizations in most targets. The precision of the warnings depends on the optimization options used.2933 -Winit-self (C, C++, Objective-C and Objective-C++ only)2934 Warn about uninitialized variables that are initialized with themselves. Note this option can only be used2935 with the -Wuninitialized option.2936 For example, GCC warns about "i" being uninitialized in the following snippet only when -Winit-self has2937 been specified:2938 int f()2939 {2940 int i = i;2941 return i;2942 }2943 This warning is enabled by -Wall in C++.2944 -Wno-implicit-int (C and Objective-C only)2945 This option controls warnings when a declaration does not specify a type. This warning is enabled by2946 default in C99 and later dialects of C, and also by -Wall.2947 -Wno-implicit-function-declaration (C and Objective-C only)2948 This option controls warnings when a function is used before being declared. This warning is enabled by2949 default in C99 and later dialects of C, and also by -Wall. The warning is made into an error by2950 -pedantic-errors.2951 -Wimplicit (C and Objective-C only)2952 Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.2953 -Wimplicit-fallthrough2954 -Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and -Wno-implicit-fallthrough is the same as2955 -Wimplicit-fallthrough=0.2956 -Wimplicit-fallthrough=n2957 Warn when a switch case falls through. For example:2958 switch (cond)2959 {2960 case 1:2961 a = 1;2962 break;2963 case 2:2964 a = 2;2965 case 3:2966 a = 3;2967 break;2968 }2969 This warning does not warn when the last statement of a case cannot fall through, e.g. when there is a2970 return statement or a call to function declared with the noreturn attribute. -Wimplicit-fallthrough= also2971 takes into account control flow statements, such as ifs, and only warns when appropriate. E.g.2972 switch (cond)2973 {2974 case 1:2975 if (i > 3) {2976 bar (5);2977 break;2978 } else if (i < 1) {2979 bar (0);2980 } else2981 return;2982 default:2983 ...2984 }2985 Since there are occasions where a switch case fall through is desirable, GCC provides an attribute,2986 "__attribute__ ((fallthrough))", that is to be used along with a null statement to suppress this warning2987 that would normally occur:2988 switch (cond)2989 {2990 case 1:2991 bar (0);2992 __attribute__ ((fallthrough));2993 default:2994 ...2995 }2996 C++17 provides a standard way to suppress the -Wimplicit-fallthrough warning using "[[fallthrough]];"2997 instead of the GNU attribute. In C++11 or C++14 users can use "[[gnu::fallthrough]];", which is a GNU2998 extension. Instead of these attributes, it is also possible to add a fallthrough comment to silence the2999 warning. The whole body of the C or C++ style comment should match the given regular expressions listed3000 below. The option argument n specifies what kind of comments are accepted:3001 *<-Wimplicit-fallthrough=0 disables the warning altogether.>3002 *<-Wimplicit-fallthrough=1 matches ".*" regular>3003 expression, any comment is used as fallthrough comment.3004 *<-Wimplicit-fallthrough=2 case insensitively matches>3005 ".*falls?[ \t-]*thr(ough|u).*" regular expression.3006 *<-Wimplicit-fallthrough=3 case sensitively matches one of the>3007 following regular expressions:3008 *<"-fallthrough">3009 *<"@fallthrough@">3010 *<"lint -fallthrough[ \t]*">3011 *<"[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?">3012 *<"[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?">3013 *<"[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?">3014 *<-Wimplicit-fallthrough=4 case sensitively matches one of the>3015 following regular expressions:3016 *<"-fallthrough">3017 *<"@fallthrough@">3018 *<"lint -fallthrough[ \t]*">3019 *<"[ \t]*FALLTHR(OUGH|U)[ \t]*">3020 *<-Wimplicit-fallthrough=5 doesn't recognize any comments as>3021 fallthrough comments, only attributes disable the warning.3022 The comment needs to be followed after optional whitespace and other comments by "case" or "default"3023 keywords or by a user label that precedes some "case" or "default" label.3024 switch (cond)3025 {3026 case 1:3027 bar (0);3028 /* FALLTHRU */3029 default:3030 ...3031 }3032 The -Wimplicit-fallthrough=3 warning is enabled by -Wextra.3033 -Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only)3034 Control if warnings triggered by the "warn_if_not_aligned" attribute should be issued. These warnings are3035 enabled by default.3036 -Wignored-qualifiers (C and C++ only)3037 Warn if the return type of a function has a type qualifier such as "const". For ISO C such a type3038 qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the warning is3039 only emitted for scalar types or "void". ISO C prohibits qualified "void" return types on function3040 definitions, so such return types always receive a warning even without this option.3041 This warning is also enabled by -Wextra.3042 -Wno-ignored-attributes (C and C++ only)3043 This option controls warnings when an attribute is ignored. This is different from the -Wattributes option3044 in that it warns whenever the compiler decides to drop an attribute, not that the attribute is either3045 unknown, used in a wrong place, etc. This warning is enabled by default.3046 -Wmain3047 Warn if the type of "main" is suspicious. "main" should be a function with external linkage, returning3048 int, taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled3049 by default in C++ and is enabled by either -Wall or -Wpedantic.3050 -Wmisleading-indentation (C and C++ only)3051 Warn when the indentation of the code does not reflect the block structure. Specifically, a warning is3052 issued for "if", "else", "while", and "for" clauses with a guarded statement that does not use braces,3053 followed by an unguarded statement with the same indentation.3054 In the following example, the call to "bar" is misleadingly indented as if it were guarded by the "if"3055 conditional.3056 if (some_condition ())3057 foo ();3058 bar (); /* Gotcha: this is not guarded by the "if". */3059 In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the statements3060 line up (defaulting to 8).3061 The warning is not issued for code involving multiline preprocessor logic such as the following example.3062 if (flagA)3063 foo (0);3064 #if SOME_CONDITION_THAT_DOES_NOT_HOLD3065 if (flagB)3066 #endif3067 foo (1);3068 The warning is not issued after a "#line" directive, since this typically indicates autogenerated code, and3069 no assumptions can be made about the layout of the file that the directive references.3070 This warning is enabled by -Wall in C and C++.3071 -Wmissing-attributes3072 Warn when a declaration of a function is missing one or more attributes that a related function is declared3073 with and whose absence may adversely affect the correctness or efficiency of generated code. For example,3074 the warning is issued for declarations of aliases that use attributes to specify less restrictive3075 requirements than those of their targets. This typically represents a potential optimization opportunity.3076 By contrast, the -Wattribute-alias=2 option controls warnings issued when the alias is more restrictive3077 than the target, which could lead to incorrect code generation. Attributes considered include3078 "alloc_align", "alloc_size", "cold", "const", "hot", "leaf", "malloc", "nonnull", "noreturn", "nothrow",3079 "pure", "returns_nonnull", and "returns_twice".3080 In C++, the warning is issued when an explicit specialization of a primary template declared with attribute3081 "alloc_align", "alloc_size", "assume_aligned", "format", "format_arg", "malloc", or "nonnull" is declared3082 without it. Attributes "deprecated", "error", and "warning" suppress the warning..3083 You can use the "copy" attribute to apply the same set of attributes to a declaration as that on another3084 declaration without explicitly enumerating the attributes. This attribute can be applied to declarations of3085 functions, variables, or types.3086 -Wmissing-attributes is enabled by -Wall.3087 For example, since the declaration of the primary function template below makes use of both attribute3088 "malloc" and "alloc_size" the declaration of the explicit specialization of the template is diagnosed3089 because it is missing one of the attributes.3090 template <class T>3091 T* __attribute__ ((malloc, alloc_size (1)))3092 allocate (size_t);3093 template <>3094 void* __attribute__ ((malloc)) // missing alloc_size3095 allocate<void> (size_t);3096 -Wmissing-braces3097 Warn if an aggregate or union initializer is not fully bracketed. In the following example, the3098 initializer for "a" is not fully bracketed, but that for "b" is fully bracketed.3099 int a[2][2] = { 0, 1, 2, 3 };3100 int b[2][2] = { { 0, 1 }, { 2, 3 } };3101 This warning is enabled by -Wall.3102 -Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)3103 Warn if a user-supplied include directory does not exist.3104 -Wno-missing-profile3105 This option controls warnings if feedback profiles are missing when using the -fprofile-use option. This3106 option diagnoses those cases where a new function or a new file is added between compiling with3107 -fprofile-generate and with -fprofile-use, without regenerating the profiles. In these cases, the profile3108 feedback data files do not contain any profile feedback information for the newly added function or file3109 respectively. Also, in the case when profile count data (.gcda) files are removed, GCC cannot use any3110 profile feedback information. In all these cases, warnings are issued to inform you that a profile3111 generation step is due. Ignoring the warning can result in poorly optimized code. -Wno-missing-profile3112 can be used to disable the warning, but this is not recommended and should be done only when non-existent3113 profile data is justified.3114 -Wno-mismatched-dealloc3115 Warn for calls to deallocation functions with pointer arguments returned from from allocations functions3116 for which the former isn't a suitable deallocator. A pair of functions can be associated as matching3117 allocators and deallocators by use of attribute "malloc". Unless disabled by the -fno-builtin option the3118 standard functions "calloc", "malloc", "realloc", and "free", as well as the corresponding forms of C++3119 "operator new" and "operator delete" are implicitly associated as matching allocators and deallocators. In3120 the following example "mydealloc" is the deallocator for pointers returned from "myalloc".3121 void mydealloc (void*);3122 __attribute__ ((malloc (mydealloc, 1))) void*3123 myalloc (size_t);3124 void f (void)3125 {3126 void *p = myalloc (32);3127 // ...use p...3128 free (p); // warning: not a matching deallocator for myalloc3129 mydealloc (p); // ok3130 }3131 In C++, the related option -Wmismatched-new-delete diagnoses mismatches involving either "operator new" or3132 "operator delete".3133 Option -Wmismatched-dealloc is enabled by default.3134 -Wmultistatement-macros3135 Warn about unsafe multiple statement macros that appear to be guarded by a clause such as "if", "else",3136 "for", "switch", or "while", in which only the first statement is actually guarded after the macro is3137 expanded.3138 For example:3139 #define DOIT x++; y++3140 if (c)3141 DOIT;3142 will increment "y" unconditionally, not just when "c" holds. The can usually be fixed by wrapping the3143 macro in a do-while loop:3144 #define DOIT do { x++; y++; } while (0)3145 if (c)3146 DOIT;3147 This warning is enabled by -Wall in C and C++.3148 -Wparentheses3149 Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where3150 a truth value is expected, or when operators are nested whose precedence people often get confused about.3151 Also warn if a comparison like "x<=y<=z" appears; this is equivalent to "(x<=y ? 1 : 0) <= z", which is a3152 different interpretation from that of ordinary mathematical notation.3153 Also warn for dangerous uses of the GNU extension to "?:" with omitted middle operand. When the condition3154 in the "?": operator is a boolean expression, the omitted value is always 1. Often programmers expect it3155 to be a value computed inside the conditional expression instead.3156 For C++ this also warns for some cases of unnecessary parentheses in declarations, which can indicate an3157 attempt at a function call instead of a declaration:3158 {3159 // Declares a local variable called mymutex.3160 std::unique_lock<std::mutex> (mymutex);3161 // User meant std::unique_lock<std::mutex> lock (mymutex);3162 }3163 This warning is enabled by -Wall.3164 -Wsequence-point3165 Warn about code that may have undefined semantics because of violations of sequence point rules in the C3166 and C++ standards.3167 The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of3168 sequence points, which represent a partial ordering between the execution of parts of the program: those3169 executed before the sequence point, and those executed after it. These occur after the evaluation of a3170 full expression (one which is not part of a larger expression), after the evaluation of the first operand3171 of a "&&", "||", "? :" or "," (comma) operator, before a function is called (but after the evaluation of3172 its arguments and the expression denoting the called function), and in certain other places. Other than as3173 expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not3174 specified. All these rules describe only a partial order rather than a total order, since, for example, if3175 two functions are called within one expression with no sequence point between them, the order in which the3176 functions are called is not specified. However, the standards committee have ruled that function calls do3177 not overlap.3178 It is not specified when between sequence points modifications to the values of objects take effect.3179 Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that3180 "Between the previous and next sequence point an object shall have its stored value modified at most once3181 by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value3182 to be stored.". If a program breaks these rules, the results on any particular implementation are entirely3183 unpredictable.3184 Examples of code with undefined behavior are "a = a++;", "a[n] = b[n++]" and "a[i++] = i;". Some more3185 complicated cases are not diagnosed by this option, and it may give an occasional false positive result,3186 but in general it has been found fairly effective at detecting this sort of problem in programs.3187 The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires3188 that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are3189 no longer undefined. But this option will still warn about them, to help people avoid writing code that is3190 undefined in C and earlier revisions of C++.3191 The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence3192 point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions,3193 may be found on the GCC readings page, at <;.3194 This warning is enabled by -Wall for C and C++.3195 -Wno-return-local-addr3196 Do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after3197 the function returns.3198 -Wreturn-type3199 Warn whenever a function is defined with a return type that defaults to "int". Also warn about any3200 "return" statement with no return value in a function whose return type is not "void" (falling off the end3201 of the function body is considered returning without a value).3202 For C only, warn about a "return" statement with an expression in a function whose return type is "void",3203 unless the expression type is also "void". As a GNU extension, the latter case is accepted without a3204 warning unless -Wpedantic is used. Attempting to use the return value of a non-"void" function other than3205 "main" that flows off the end by reaching the closing curly brace that terminates the function is3206 undefined.3207 Unlike in C, in C++, flowing off the end of a non-"void" function other than "main" results in undefined3208 behavior even when the value of the function is not used.3209 This warning is enabled by default in C++ and by -Wall otherwise.3210 -Wno-shift-count-negative3211 Controls warnings if a shift count is negative. This warning is enabled by default.3212 -Wno-shift-count-overflow3213 Controls warnings if a shift count is greater than or equal to the bit width of the type. This warning is3214 enabled by default.3215 -Wshift-negative-value3216 Warn if left shifting a negative value. This warning is enabled by -Wextra in C99 (and newer) and C++11 to3217 C++17 modes.3218 -Wno-shift-overflow3219 -Wshift-overflow=n3220 These options control warnings about left shift overflows.3221 -Wshift-overflow=13222 This is the warning level of -Wshift-overflow and is enabled by default in C99 and C++11 modes (and3223 newer). This warning level does not warn about left-shifting 1 into the sign bit. (However, in C,3224 such an overflow is still rejected in contexts where an integer constant expression is required.) No3225 warning is emitted in C++20 mode (and newer), as signed left shifts always wrap.3226 -Wshift-overflow=23227 This warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is3228 active.3229 -Wswitch3230 Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of3231 the named codes of that enumeration. (The presence of a "default" label prevents this warning.) "case"3232 labels outside the enumeration range also provoke warnings when this option is used (even if there is a3233 "default" label). This warning is enabled by -Wall.3234 -Wswitch-default3235 Warn whenever a "switch" statement does not have a "default" case.3236 -Wswitch-enum3237 Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of3238 the named codes of that enumeration. "case" labels outside the enumeration range also provoke warnings3239 when this option is used. The only difference between -Wswitch and this option is that this option gives a3240 warning about an omitted enumeration code even if there is a "default" label.3241 -Wno-switch-bool3242 Do not warn when a "switch" statement has an index of boolean type and the case values are outside the3243 range of a boolean type. It is possible to suppress this warning by casting the controlling expression to3244 a type other than "bool". For example:3245 switch ((int) (a == 4))3246 {3247 ...3248 }3249 This warning is enabled by default for C and C++ programs.3250 -Wno-switch-outside-range3251 This option controls warnings when a "switch" case has a value that is outside of its respective type3252 range. This warning is enabled by default for C and C++ programs.3253 -Wno-switch-unreachable3254 Do not warn when a "switch" statement contains statements between the controlling expression and the first3255 case label, which will never be executed. For example:3256 switch (cond)3257 {3258 i = 15;3259 ...3260 case 5:3261 ...3262 }3263 -Wswitch-unreachable does not warn if the statement between the controlling expression and the first case3264 label is just a declaration:3265 switch (cond)3266 {3267 int i;3268 ...3269 case 5:3270 i = 5;3271 ...3272 }3273 This warning is enabled by default for C and C++ programs.3274 -Wsync-nand (C and C++ only)3275 Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch" built-in functions are used. These functions3276 changed semantics in GCC 4.4.3277 -Wunused-but-set-parameter3278 Warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).3279 To suppress this warning use the "unused" attribute.3280 This warning is also enabled by -Wunused together with -Wextra.3281 -Wunused-but-set-variable3282 Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This3283 warning is enabled by -Wall.3284 To suppress this warning use the "unused" attribute.3285 This warning is also enabled by -Wunused, which is enabled by -Wall.3286 -Wunused-function3287 Warn whenever a static function is declared but not defined or a non-inline static function is unused.3288 This warning is enabled by -Wall. 版权声明：
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