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1.1 ! root 1: This is Info file gcc.info, produced by Makeinfo-1.54 from the input ! 2: file gcc.texi. ! 3: ! 4: This file documents the use and the internals of the GNU compiler. ! 5: ! 6: Published by the Free Software Foundation 675 Massachusetts Avenue ! 7: Cambridge, MA 02139 USA ! 8: ! 9: Copyright (C) 1988, 1989, 1992, 1993 Free Software Foundation, Inc. ! 10: ! 11: Permission is granted to make and distribute verbatim copies of this ! 12: manual provided the copyright notice and this permission notice are ! 13: preserved on all copies. ! 14: ! 15: Permission is granted to copy and distribute modified versions of ! 16: this manual under the conditions for verbatim copying, provided also ! 17: that the sections entitled "GNU General Public License" and "Protect ! 18: Your Freedom--Fight `Look And Feel'" are included exactly as in the ! 19: original, and provided that the entire resulting derived work is ! 20: distributed under the terms of a permission notice identical to this ! 21: one. ! 22: ! 23: Permission is granted to copy and distribute translations of this ! 24: manual into another language, under the above conditions for modified ! 25: versions, except that the sections entitled "GNU General Public ! 26: License" and "Protect Your Freedom--Fight `Look And Feel'", and this ! 27: permission notice, may be included in translations approved by the Free ! 28: Software Foundation instead of in the original English. ! 29: ! 30: ! 31: File: gcc.info, Node: Incompatibilities, Next: Fixed Headers, Prev: External Bugs, Up: Trouble ! 32: ! 33: Incompatibilities of GNU CC ! 34: =========================== ! 35: ! 36: There are several noteworthy incompatibilities between GNU C and most ! 37: existing (non-ANSI) versions of C. The `-traditional' option ! 38: eliminates many of these incompatibilities, *but not all*, by telling ! 39: GNU C to behave like the other C compilers. ! 40: ! 41: * GNU CC normally makes string constants read-only. If several ! 42: identical-looking string constants are used, GNU CC stores only one ! 43: copy of the string. ! 44: ! 45: One consequence is that you cannot call `mktemp' with a string ! 46: constant argument. The function `mktemp' always alters the string ! 47: its argument points to. ! 48: ! 49: Another consequence is that `sscanf' does not work on some systems ! 50: when passed a string constant as its format control string or ! 51: input. This is because `sscanf' incorrectly tries to write into ! 52: the string constant. Likewise `fscanf' and `scanf'. ! 53: ! 54: The best solution to these problems is to change the program to use ! 55: `char'-array variables with initialization strings for these ! 56: purposes instead of string constants. But if this is not possible, ! 57: you can use the `-fwritable-strings' flag, which directs GNU CC to ! 58: handle string constants the same way most C compilers do. ! 59: `-traditional' also has this effect, among others. ! 60: ! 61: * `-2147483648' is positive. ! 62: ! 63: This is because 2147483648 cannot fit in the type `int', so ! 64: (following the ANSI C rules) its data type is `unsigned long int'. ! 65: Negating this value yields 2147483648 again. ! 66: ! 67: * GNU CC does not substitute macro arguments when they appear inside ! 68: of string constants. For example, the following macro in GNU CC ! 69: ! 70: #define foo(a) "a" ! 71: ! 72: will produce output `"a"' regardless of what the argument A is. ! 73: ! 74: The `-traditional' option directs GNU CC to handle such cases ! 75: (among others) in the old-fashioned (non-ANSI) fashion. ! 76: ! 77: * When you use `setjmp' and `longjmp', the only automatic variables ! 78: guaranteed to remain valid are those declared `volatile'. This is ! 79: a consequence of automatic register allocation. Consider this ! 80: function: ! 81: ! 82: jmp_buf j; ! 83: ! 84: foo () ! 85: { ! 86: int a, b; ! 87: ! 88: a = fun1 (); ! 89: if (setjmp (j)) ! 90: return a; ! 91: ! 92: a = fun2 (); ! 93: /* `longjmp (j)' may occur in `fun3'. */ ! 94: return a + fun3 (); ! 95: } ! 96: ! 97: Here `a' may or may not be restored to its first value when the ! 98: `longjmp' occurs. If `a' is allocated in a register, then its ! 99: first value is restored; otherwise, it keeps the last value stored ! 100: in it. ! 101: ! 102: If you use the `-W' option with the `-O' option, you will get a ! 103: warning when GNU CC thinks such a problem might be possible. ! 104: ! 105: The `-traditional' option directs GNU C to put variables in the ! 106: stack by default, rather than in registers, in functions that call ! 107: `setjmp'. This results in the behavior found in traditional C ! 108: compilers. ! 109: ! 110: * Programs that use preprocessor directives in the middle of macro ! 111: arguments do not work with GNU CC. For example, a program like ! 112: this will not work: ! 113: ! 114: foobar ( ! 115: #define luser ! 116: hack) ! 117: ! 118: ANSI C does not permit such a construct. It would make sense to ! 119: support it when `-traditional' is used, but it is too much work to ! 120: implement. ! 121: ! 122: * Declarations of external variables and functions within a block ! 123: apply only to the block containing the declaration. In other ! 124: words, they have the same scope as any other declaration in the ! 125: same place. ! 126: ! 127: In some other C compilers, a `extern' declaration affects all the ! 128: rest of the file even if it happens within a block. ! 129: ! 130: The `-traditional' option directs GNU C to treat all `extern' ! 131: declarations as global, like traditional compilers. ! 132: ! 133: * In traditional C, you can combine `long', etc., with a typedef ! 134: name, as shown here: ! 135: ! 136: typedef int foo; ! 137: typedef long foo bar; ! 138: ! 139: In ANSI C, this is not allowed: `long' and other type modifiers ! 140: require an explicit `int'. Because this criterion is expressed by ! 141: Bison grammar rules rather than C code, the `-traditional' flag ! 142: cannot alter it. ! 143: ! 144: * PCC allows typedef names to be used as function parameters. The ! 145: difficulty described immediately above applies here too. ! 146: ! 147: * PCC allows whitespace in the middle of compound assignment ! 148: operators such as `+='. GNU CC, following the ANSI standard, does ! 149: not allow this. The difficulty described immediately above ! 150: applies here too. ! 151: ! 152: * GNU CC complains about unterminated character constants inside of ! 153: preprocessor conditionals that fail. Some programs have English ! 154: comments enclosed in conditionals that are guaranteed to fail; if ! 155: these comments contain apostrophes, GNU CC will probably report an ! 156: error. For example, this code would produce an error: ! 157: ! 158: #if 0 ! 159: You can't expect this to work. ! 160: #endif ! 161: ! 162: The best solution to such a problem is to put the text into an ! 163: actual C comment delimited by `/*...*/'. However, `-traditional' ! 164: suppresses these error messages. ! 165: ! 166: * Many user programs contain the declaration `long time ();'. In the ! 167: past, the system header files on many systems did not actually ! 168: declare `time', so it did not matter what type your program ! 169: declared it to return. But in systems with ANSI C headers, `time' ! 170: is declared to return `time_t', and if that is not the same as ! 171: `long', then `long time ();' is erroneous. ! 172: ! 173: The solution is to change your program to use `time_t' as the ! 174: return type of `time'. ! 175: ! 176: * When compiling functions that return `float', PCC converts it to a ! 177: double. GNU CC actually returns a `float'. If you are concerned ! 178: with PCC compatibility, you should declare your functions to return ! 179: `double'; you might as well say what you mean. ! 180: ! 181: * When compiling functions that return structures or unions, GNU CC ! 182: output code normally uses a method different from that used on most ! 183: versions of Unix. As a result, code compiled with GNU CC cannot ! 184: call a structure-returning function compiled with PCC, and vice ! 185: versa. ! 186: ! 187: The method used by GNU CC is as follows: a structure or union ! 188: which is 1, 2, 4 or 8 bytes long is returned like a scalar. A ! 189: structure or union with any other size is stored into an address ! 190: supplied by the caller (usually in a special, fixed register, but ! 191: on some machines it is passed on the stack). The ! 192: machine-description macros `STRUCT_VALUE' and ! 193: `STRUCT_INCOMING_VALUE' tell GNU CC where to pass this address. ! 194: ! 195: By contrast, PCC on most target machines returns structures and ! 196: unions of any size by copying the data into an area of static ! 197: storage, and then returning the address of that storage as if it ! 198: were a pointer value. The caller must copy the data from that ! 199: memory area to the place where the value is wanted. GNU CC does ! 200: not use this method because it is slower and nonreentrant. ! 201: ! 202: On some newer machines, PCC uses a reentrant convention for all ! 203: structure and union returning. GNU CC on most of these machines ! 204: uses a compatible convention when returning structures and unions ! 205: in memory, but still returns small structures and unions in ! 206: registers. ! 207: ! 208: You can tell GNU CC to use a compatible convention for all ! 209: structure and union returning with the option ! 210: `-fpcc-struct-return'. ! 211: ! 212: * GNU C complains about program fragments such as `0x74ae-0x4000' ! 213: which appear to be two hexadecimal constants separated by the minus ! 214: operator. Actually, this string is a single "preprocessing token". ! 215: Each such token must correspond to one token in C. Since this ! 216: does not, GNU C prints an error message. Although it may appear ! 217: obvious that what is meant is an operator and two values, the ANSI ! 218: C standard specifically requires that this be treated as erroneous. ! 219: ! 220: A "preprocessing token" is a "preprocessing number" if it begins ! 221: with a digit and is followed by letters, underscores, digits, ! 222: periods and `e+', `e-', `E+', or `E-' character sequences. ! 223: ! 224: To make the above program fragment valid, place whitespace in ! 225: front of the minus sign. This whitespace will end the ! 226: preprocessing number. ! 227: ! 228: ! 229: File: gcc.info, Node: Fixed Headers, Next: Disappointments, Prev: Incompatibilities, Up: Trouble ! 230: ! 231: Fixed Header Files ! 232: ================== ! 233: ! 234: GNU CC needs to install corrected versions of some system header ! 235: files. This is because most target systems have some header files that ! 236: won't work with GNU CC unless they are changed. Some have bugs, some ! 237: are incompatible with ANSI C, and some depend on special features of ! 238: other compilers. ! 239: ! 240: Installing GNU CC automatically creates and installs the fixed header ! 241: files, by running a program called `fixincludes' (or for certain ! 242: targets an alternative such as `fixinc.svr4'). Normally, you don't ! 243: need to pay attention to this. But there are cases where it doesn't do ! 244: the right thing automatically. ! 245: ! 246: * If you update the system's header files, such as by installing a ! 247: new system version, the fixed header files of GNU CC are not ! 248: automatically updated. The easiest way to update them is to ! 249: reinstall GNU CC. (If you want to be clever, look in the makefile ! 250: and you can find a shortcut.) ! 251: ! 252: * On some systems, in particular SunOS 4, header file directories ! 253: contain machine-specific symbolic links in certain places. This ! 254: makes it possible to share most of the header files among hosts ! 255: running the same version of SunOS 4 on different machine models. ! 256: ! 257: The programs that fix the header files do not understand this ! 258: special way of using symbolic links; therefore, the directory of ! 259: fixed header files is good only for the machine model used to ! 260: build it. ! 261: ! 262: In SunOS 4, only programs that look inside the kernel will notice ! 263: the difference between machine models. Therefore, for most ! 264: purposes, you need not be concerned about this. ! 265: ! 266: It is possible to make separate sets of fixed header files for the ! 267: different machine models, and arrange a structure of symbolic ! 268: links so as to use the proper set, but you'll have to do this by ! 269: hand. ! 270: ! 271: ! 272: File: gcc.info, Node: Disappointments, Next: C++ Misunderstandings, Prev: Fixed Headers, Up: Trouble ! 273: ! 274: Disappointments and Misunderstandings ! 275: ===================================== ! 276: ! 277: These problems are perhaps regrettable, but we don't know any ! 278: practical way around them. ! 279: ! 280: * Certain local variables aren't recognized by debuggers when you ! 281: compile with optimization. ! 282: ! 283: This occurs because sometimes GNU CC optimizes the variable out of ! 284: existence. There is no way to tell the debugger how to compute the ! 285: value such a variable "would have had", and it is not clear that ! 286: would be desirable anyway. So GNU CC simply does not mention the ! 287: eliminated variable when it writes debugging information. ! 288: ! 289: You have to expect a certain amount of disagreement between the ! 290: executable and your source code, when you use optimization. ! 291: ! 292: * Users often think it is a bug when GNU CC reports an error for code ! 293: like this: ! 294: ! 295: int foo (struct mumble *); ! 296: ! 297: struct mumble { ... }; ! 298: ! 299: int foo (struct mumble *x) ! 300: { ... } ! 301: ! 302: This code really is erroneous, because the scope of `struct ! 303: mumble' in the prototype is limited to the argument list ! 304: containing it. It does not refer to the `struct mumble' defined ! 305: with file scope immediately below--they are two unrelated types ! 306: with similar names in different scopes. ! 307: ! 308: But in the definition of `foo', the file-scope type is used ! 309: because that is available to be inherited. Thus, the definition ! 310: and the prototype do not match, and you get an error. ! 311: ! 312: This behavior may seem silly, but it's what the ANSI standard ! 313: specifies. It is easy enough for you to make your code work by ! 314: moving the definition of `struct mumble' above the prototype. ! 315: It's not worth being incompatible with ANSI C just to avoid an ! 316: error for the example shown above. ! 317: ! 318: * Accesses to bitfields even in volatile objects works by accessing ! 319: larger objects, such as a byte or a word. You cannot rely on what ! 320: size of object is accessed in order to read or write the bitfield; ! 321: it may even vary for a given bitfield according to the precise ! 322: usage. ! 323: ! 324: If you care about controlling the amount of memory that is ! 325: accessed, use volatile but do not use bitfields. ! 326: ! 327: * GNU CC comes with shell scripts to fix certain known problems in ! 328: system header files. They install corrected copies of various ! 329: header files in a special directory where only GNU CC will ! 330: normally look for them. The scripts adapt to various systems by ! 331: searching all the system header files for the problem cases that ! 332: we know about. ! 333: ! 334: If new system header files are installed, nothing automatically ! 335: arranges to update the corrected header files. You will have to ! 336: reinstall GNU CC to fix the new header files. More specifically, ! 337: go to the build directory and delete the files `stmp-fixinc' and ! 338: `stmp-headers', and the subdirectory `include'; then do `make ! 339: install' again. ! 340: ! 341: * On 68000 systems, you can get paradoxical results if you test the ! 342: precise values of floating point numbers. For example, you can ! 343: find that a floating point value which is not a NaN is not equal ! 344: to itself. This results from the fact that the the floating point ! 345: registers hold a few more bits of precision than fit in a `double' ! 346: in memory. Compiled code moves values between memory and floating ! 347: point registers at its convenience, and moving them into memory ! 348: truncates them. ! 349: ! 350: You can partially avoid this problem by using the `-ffloat-store' ! 351: option (*note Optimize Options::.). ! 352: ! 353: * On the MIPS, variable argument functions using `varargs.h' cannot ! 354: have a floating point value for the first argument. The reason ! 355: for this is that in the absence of a prototype in scope, if the ! 356: first argument is a floating point, it is passed in a floating ! 357: point register, rather than an integer register. ! 358: ! 359: If the code is rewritten to use the ANSI standard `stdarg.h' ! 360: method of variable arguments, and the prototype is in scope at the ! 361: time of the call, everything will work fine. ! 362: ! 363: ! 364: File: gcc.info, Node: C++ Misunderstandings, Next: Protoize Caveats, Prev: Disappointments, Up: Trouble ! 365: ! 366: Common Misunderstandings with GNU C++ ! 367: ===================================== ! 368: ! 369: C++ is a complex language and an evolving one, and its standard ! 370: definition (the ANSI C++ draft standard) is also evolving. As a result, ! 371: your C++ compiler may occasionally surprise you, even when its behavior ! 372: is correct. This section discusses some areas that frequently give ! 373: rise to questions of this sort. ! 374: ! 375: * Menu: ! 376: ! 377: * Static Definitions:: Static member declarations are not definitions ! 378: * Temporaries:: Temporaries may vanish before you expect ! 379: ! 380: ! 381: File: gcc.info, Node: Static Definitions, Next: Temporaries, Up: C++ Misunderstandings ! 382: ! 383: Declare *and* Define Static Members ! 384: ----------------------------------- ! 385: ! 386: When a class has static data members, it is not enough to *declare* ! 387: the static member; you must also *define* it. For example: ! 388: ! 389: class Foo ! 390: { ! 391: ... ! 392: void method(); ! 393: static int bar; ! 394: }; ! 395: ! 396: This declaration only establishes that the class `Foo' has an `int' ! 397: named `Foo::bar', and a member function named `Foo::method'. But you ! 398: still need to define *both* `method' and `bar' elsewhere. According to ! 399: the draft ANSI standard, you must supply an initializer in one (and ! 400: only one) source file, such as: ! 401: ! 402: int Foo::bar = 0; ! 403: ! 404: Other C++ compilers may not correctly implement the standard ! 405: behavior. As a result, when you switch to `g++' from one of these ! 406: compilers, you may discover that a program that appeared to work ! 407: correctly in fact does not conform to the standard: `g++' reports as ! 408: undefined symbols any static data members that lack definitions. ! 409: ! 410: ! 411: File: gcc.info, Node: Temporaries, Prev: Static Definitions, Up: C++ Misunderstandings ! 412: ! 413: Temporaries May Vanish Before You Expect ! 414: ---------------------------------------- ! 415: ! 416: It is dangerous to use pointers or references to *portions* of a ! 417: temporary object. The compiler may very well delete the object before ! 418: you expect it to, leaving a pointer to garbage. The most common place ! 419: where this problem crops up is in classes like the libg++ `String' ! 420: class, that define a conversion function to type `char *' or `const ! 421: char *'. However, any class that returns a pointer to some internal ! 422: structure is potentially subject to this problem. ! 423: ! 424: For example, a program may use a function `strfunc' that returns ! 425: `String' objects, and another function `charfunc' that operates on ! 426: pointers to `char': ! 427: ! 428: String strfunc (); ! 429: void charfunc (const char *); ! 430: ! 431: In this situation, it may seem natural to write ! 432: `charfunc (strfunc ());' based on the knowledge that class `String' has ! 433: an explicit conversion to `char' pointers. However, what really ! 434: happens is akin to `charfunc (strfunc ().convert ());', where the ! 435: `convert' method is a function to do the same data conversion normally ! 436: performed by a cast. Since the last use of the temporary `String' ! 437: object is the call to the conversion function, the compiler may delete ! 438: that object before actually calling `charfunc'. The compiler has no ! 439: way of knowing that deleting the `String' object will invalidate the ! 440: pointer. The pointer then points to garbage, so that by the time ! 441: `charfunc' is called, it gets an invalid argument. ! 442: ! 443: Code like this may run successfully under some other compilers, ! 444: especially those that delete temporaries relatively late. However, the ! 445: GNU C++ behavior is also standard-conformant, so if your program depends ! 446: on late destruction of temporaries it is not portable. ! 447: ! 448: If you think this is surprising, you should be aware that the ANSI ! 449: C++ committee continues to debate the lifetime-of-temporaries problem. ! 450: ! 451: For now, at least, the safe way to write such code is to give the ! 452: temporary a name, which forces it to remain until the end of the scope ! 453: of the name. For example: ! 454: ! 455: String& tmp = strfunc (); ! 456: charfunc (tmp); ! 457: ! 458: ! 459: File: gcc.info, Node: Protoize Caveats, Next: Non-bugs, Prev: C++ Misunderstandings, Up: Trouble ! 460: ! 461: Caveats of using `protoize' ! 462: =========================== ! 463: ! 464: The conversion programs `protoize' and `unprotoize' can sometimes ! 465: change a source file in a way that won't work unless you rearrange it. ! 466: ! 467: * `protoize' can insert references to a type name or type tag before ! 468: the definition, or in a file where they are not defined. ! 469: ! 470: If this happens, compiler error messages should show you where the ! 471: new references are, so fixing the file by hand is straightforward. ! 472: ! 473: * There are some C constructs which `protoize' cannot figure out. ! 474: For example, it can't determine argument types for declaring a ! 475: pointer-to-function variable; this you must do by hand. `protoize' ! 476: inserts a comment containing `???' each time it finds such a ! 477: variable; so you can find all such variables by searching for this ! 478: string. ANSI C does not require declaring the argument types of ! 479: pointer-to-function types. ! 480: ! 481: * Using `unprotoize' can easily introduce bugs. If the program ! 482: relied on prototypes to bring about conversion of arguments, these ! 483: conversions will not take place in the program without prototypes. ! 484: One case in which you can be sure `unprotoize' is safe is when you ! 485: are removing prototypes that were made with `protoize'; if the ! 486: program worked before without any prototypes, it will work again ! 487: without them. ! 488: ! 489: You can find all the places where this problem might occur by ! 490: compiling the program with the `-Wconversion' option. It prints a ! 491: warning whenever an argument is converted. ! 492: ! 493: * Both conversion programs can be confused if there are macro calls ! 494: in and around the text to be converted. In other words, the ! 495: standard syntax for a declaration or definition must not result ! 496: from expanding a macro. This problem is inherent in the design of ! 497: C and cannot be fixed. If only a few functions have confusing ! 498: macro calls, you can easily convert them manually. ! 499: ! 500: * `protoize' cannot get the argument types for a function whose ! 501: definition was not actually compiled due to preprocessor ! 502: conditionals. When this happens, `protoize' changes nothing in ! 503: regard to such a function. `protoize' tries to detect such ! 504: instances and warn about them. ! 505: ! 506: You can generally work around this problem by using `protoize' step ! 507: by step, each time specifying a different set of `-D' options for ! 508: compilation, until all of the functions have been converted. ! 509: There is no automatic way to verify that you have got them all, ! 510: however. ! 511: ! 512: * Confusion may result if there is an occasion to convert a function ! 513: declaration or definition in a region of source code where there ! 514: is more than one formal parameter list present. Thus, attempts to ! 515: convert code containing multiple (conditionally compiled) versions ! 516: of a single function header (in the same vicinity) may not produce ! 517: the desired (or expected) results. ! 518: ! 519: If you plan on converting source files which contain such code, it ! 520: is recommended that you first make sure that each conditionally ! 521: compiled region of source code which contains an alternative ! 522: function header also contains at least one additional follower ! 523: token (past the final right parenthesis of the function header). ! 524: This should circumvent the problem. ! 525: ! 526: * `unprotoize' can become confused when trying to convert a function ! 527: definition or declaration which contains a declaration for a ! 528: pointer-to-function formal argument which has the same name as the ! 529: function being defined or declared. We recommand you avoid such ! 530: choices of formal parameter names. ! 531: ! 532: * You might also want to correct some of the indentation by hand and ! 533: break long lines. (The conversion programs don't write lines ! 534: longer than eighty characters in any case.) ! 535: ! 536: ! 537: File: gcc.info, Node: Non-bugs, Next: Warnings and Errors, Prev: Protoize Caveats, Up: Trouble ! 538: ! 539: Certain Changes We Don't Want to Make ! 540: ===================================== ! 541: ! 542: This section lists changes that people frequently request, but which ! 543: we do not make because we think GNU CC is better without them. ! 544: ! 545: * Checking the number and type of arguments to a function which has ! 546: an old-fashioned definition and no prototype. ! 547: ! 548: Such a feature would work only occasionally--only for calls that ! 549: appear in the same file as the called function, following the ! 550: definition. The only way to check all calls reliably is to add a ! 551: prototype for the function. But adding a prototype eliminates the ! 552: motivation for this feature. So the feature is not worthwhile. ! 553: ! 554: * Warning about using an expression whose type is signed as a shift ! 555: count. ! 556: ! 557: Shift count operands are probably signed more often than unsigned. ! 558: Warning about this would cause far more annoyance than good. ! 559: ! 560: * Warning about assigning a signed value to an unsigned variable. ! 561: ! 562: Such assignments must be very common; warning about them would ! 563: cause more annoyance than good. ! 564: ! 565: * Warning about unreachable code. ! 566: ! 567: It's very common to have unreachable code in machine-generated ! 568: programs. For example, this happens normally in some files of GNU ! 569: C itself. ! 570: ! 571: * Warning when a non-void function value is ignored. ! 572: ! 573: Coming as I do from a Lisp background, I balk at the idea that ! 574: there is something dangerous about discarding a value. There are ! 575: functions that return values which some callers may find useful; ! 576: it makes no sense to clutter the program with a cast to `void' ! 577: whenever the value isn't useful. ! 578: ! 579: * Assuming (for optimization) that the address of an external symbol ! 580: is never zero. ! 581: ! 582: This assumption is false on certain systems when `#pragma weak' is ! 583: used. ! 584: ! 585: * Making `-fshort-enums' the default. ! 586: ! 587: This would cause storage layout to be incompatible with most other ! 588: C compilers. And it doesn't seem very important, given that you ! 589: can get the same result in other ways. The case where it matters ! 590: most is when the enumeration-valued object is inside a structure, ! 591: and in that case you can specify a field width explicitly. ! 592: ! 593: * Making bitfields unsigned by default on particular machines where ! 594: "the ABI standard" says to do so. ! 595: ! 596: The ANSI C standard leaves it up to the implementation whether a ! 597: bitfield declared plain `int' is signed or not. This in effect ! 598: creates two alternative dialects of C. ! 599: ! 600: The GNU C compiler supports both dialects; you can specify the ! 601: signed dialect with `-fsigned-bitfields' and the unsigned dialect ! 602: with `-funsigned-bitfields'. However, this leaves open the ! 603: question of which dialect to use by default. ! 604: ! 605: Currently, the preferred dialect makes plain bitfields signed, ! 606: because this is simplest. Since `int' is the same as `signed int' ! 607: in every other context, it is cleanest for them to be the same in ! 608: bitfields as well. ! 609: ! 610: Some computer manufacturers have published Application Binary ! 611: Interface standards which specify that plain bitfields should be ! 612: unsigned. It is a mistake, however, to say anything about this ! 613: issue in an ABI. This is because the handling of plain bitfields ! 614: distinguishes two dialects of C. Both dialects are meaningful on ! 615: every type of machine. Whether a particular object file was ! 616: compiled using signed bitfields or unsigned is of no concern to ! 617: other object files, even if they access the same bitfields in the ! 618: same data structures. ! 619: ! 620: A given program is written in one or the other of these two ! 621: dialects. The program stands a chance to work on most any machine ! 622: if it is compiled with the proper dialect. It is unlikely to work ! 623: at all if compiled with the wrong dialect. ! 624: ! 625: Many users appreciate the GNU C compiler because it provides an ! 626: environment that is uniform across machines. These users would be ! 627: inconvenienced if the compiler treated plain bitfields differently ! 628: on certain machines. ! 629: ! 630: Occasionally users write programs intended only for a particular ! 631: machine type. On these occasions, the users would benefit if the ! 632: GNU C compiler were to support by default the same dialect as the ! 633: other compilers on that machine. But such applications are rare. ! 634: And users writing a program to run on more than one type of ! 635: machine cannot possibly benefit from this kind of compatibility. ! 636: ! 637: This is why GNU CC does and will treat plain bitfields in the same ! 638: fashion on all types of machines (by default). ! 639: ! 640: There are some arguments for making bitfields unsigned by default ! 641: on all machines. If, for example, this becomes a universal de ! 642: facto standard, it would make sense for GNU CC to go along with ! 643: it. This is something to be considered in the future. ! 644: ! 645: (Of course, users strongly concerned about portability should ! 646: indicate explicitly in each bitfield whether it is signed or not. ! 647: In this way, they write programs which have the same meaning in ! 648: both C dialects.) ! 649: ! 650: * Undefining `__STDC__' when `-ansi' is not used. ! 651: ! 652: Currently, GNU CC defines `__STDC__' as long as you don't use ! 653: `-traditional'. This provides good results in practice. ! 654: ! 655: Programmers normally use conditionals on `__STDC__' to ask whether ! 656: it is safe to use certain features of ANSI C, such as function ! 657: prototypes or ANSI token concatenation. Since plain `gcc' supports ! 658: all the features of ANSI C, the correct answer to these questions ! 659: is "yes". ! 660: ! 661: Some users try to use `__STDC__' to check for the availability of ! 662: certain library facilities. This is actually incorrect usage in ! 663: an ANSI C program, because the ANSI C standard says that a ! 664: conforming freestanding implementation should define `__STDC__' ! 665: even though it does not have the library facilities. `gcc -ansi ! 666: -pedantic' is a conforming freestanding implementation, and it is ! 667: therefore required to define `__STDC__', even though it does not ! 668: come with an ANSI C library. ! 669: ! 670: Sometimes people say that defining `__STDC__' in a compiler that ! 671: does not completely conform to the ANSI C standard somehow ! 672: violates the standard. This is illogical. The standard is a ! 673: standard for compilers that claim to support ANSI C, such as `gcc ! 674: -ansi'--not for other compilers such as plain `gcc'. Whatever the ! 675: ANSI C standard says is relevant to the design of plain `gcc' ! 676: without `-ansi' only for pragmatic reasons, not as a requirement. ! 677: ! 678: * Undefining `__STDC__' in C++. ! 679: ! 680: Programs written to compile with C++-to-C translators get the ! 681: value of `__STDC__' that goes with the C compiler that is ! 682: subsequently used. These programs must test `__STDC__' to ! 683: determine what kind of C preprocessor that compiler uses: whether ! 684: they should concatenate tokens in the ANSI C fashion or in the ! 685: traditional fashion. ! 686: ! 687: These programs work properly with GNU C++ if `__STDC__' is defined. ! 688: They would not work otherwise. ! 689: ! 690: In addition, many header files are written to provide prototypes ! 691: in ANSI C but not in traditional C. Many of these header files ! 692: can work without change in C++ provided `__STDC__' is defined. If ! 693: `__STDC__' is not defined, they will all fail, and will all need ! 694: to be changed to test explicitly for C++ as well. ! 695: ! 696: * Deleting "empty" loops. ! 697: ! 698: GNU CC does not delete "empty" loops because the most likely reason ! 699: you would put one in a program is to have a delay. Deleting them ! 700: will not make real programs run any faster, so it would be ! 701: pointless. ! 702: ! 703: It would be different if optimization of a nonempty loop could ! 704: produce an empty one. But this generally can't happen. ! 705: ! 706: * Making side effects happen in the same order as in some other ! 707: compiler. ! 708: ! 709: It is never safe to depend on the order of evaluation of side ! 710: effects. For example, a function call like this may very well ! 711: behave differently from one compiler to another: ! 712: ! 713: void func (int, int); ! 714: ! 715: int i = 2; ! 716: func (i++, i++); ! 717: ! 718: There is no guarantee (in either the C or the C++ standard language ! 719: definitions) that the increments will be evaluated in any ! 720: particular order. Either increment might happen first. `func' ! 721: might get the arguments `3, 4', or it might get `4, 3', or even ! 722: `3, 3'. ! 723: ! 724: * Using the "canonical" form of the target configuration name as the ! 725: directory for installation. ! 726: ! 727: This would be an improvement in some respects, but it would also ! 728: cause problems. For one thing, users might expect to use in the ! 729: `-b' option the same name specified at installation; if ! 730: installation used the canonical form, that would not work. What's ! 731: more, the canonical name might be too long for certain file ! 732: systems. ! 733: ! 734: We suggest you make a link to the installation directory under the ! 735: canonical name, if you want to use that name in the `-b' option. ! 736: ! 737: ! 738: File: gcc.info, Node: Warnings and Errors, Prev: Non-bugs, Up: Trouble ! 739: ! 740: Warning Messages and Error Messages ! 741: =================================== ! 742: ! 743: The GNU compiler can produce two kinds of diagnostics: errors and ! 744: warnings. Each kind has a different purpose: ! 745: ! 746: *Errors* report problems that make it impossible to compile your ! 747: program. GNU CC reports errors with the source file name and line ! 748: number where the problem is apparent. ! 749: ! 750: *Warnings* report other unusual conditions in your code that *may* ! 751: indicate a problem, although compilation can (and does) proceed. ! 752: Warning messages also report the source file name and line number, ! 753: but include the text `warning:' to distinguish them from error ! 754: messages. ! 755: ! 756: Warnings may indicate danger points where you should check to make ! 757: sure that your program really does what you intend; or the use of ! 758: obsolete features; or the use of nonstandard features of GNU C or C++. ! 759: Many warnings are issued only if you ask for them, with one of the `-W' ! 760: options (for instance, `-Wall' requests a variety of useful warnings). ! 761: ! 762: GNU CC always tries to compile your program if possible; it never ! 763: gratuituously rejects a program whose meaning is clear merely because ! 764: (for instance) it fails to conform to a standard. In some cases, ! 765: however, the C and C++ standards specify that certain extensions are ! 766: forbidden, and a diagnostic *must* be issued by a conforming compiler. ! 767: The `-pedantic' option tells GNU CC to issue warnings in such cases; ! 768: `-pedantic-errors' says to make them errors instead. This does not ! 769: mean that *all* non-ANSI constructs get warnings or errors. ! 770: ! 771: *Note Options to Request or Suppress Warnings: Warning Options, for ! 772: more detail on these and related command-line options. ! 773: ! 774: ! 775: File: gcc.info, Node: Bugs, Next: Service, Prev: Trouble, Up: Top ! 776: ! 777: Reporting Bugs ! 778: ************** ! 779: ! 780: Your bug reports play an essential role in making GNU CC reliable. ! 781: ! 782: When you encounter a problem, the first thing to do is to see if it ! 783: is already known. *Note Trouble::. If it isn't known, then you should ! 784: report the problem. ! 785: ! 786: Reporting a bug may help you by bringing a solution to your problem, ! 787: or it may not. (If it does not, look in the service directory; see ! 788: *Note Service::.) In any case, the principal function of a bug report ! 789: is to help the entire community by making the next version of GNU CC ! 790: work better. Bug reports are your contribution to the maintenance of ! 791: GNU CC. ! 792: ! 793: Since the maintainers are very overloaded, we cannot respond to every ! 794: bug report. However, if the bug has not been fixed, we are likely to ! 795: send you a patch and ask you to tell us whether it works. ! 796: ! 797: In order for a bug report to serve its purpose, you must include the ! 798: information that makes for fixing the bug. ! 799: ! 800: * Menu: ! 801: ! 802: * Criteria: Bug Criteria. Have you really found a bug? ! 803: * Where: Bug Lists. Where to send your bug report. ! 804: * Reporting: Bug Reporting. How to report a bug effectively. ! 805: * Patches: Sending Patches. How to send a patch for GNU CC. ! 806: * Known: Trouble. Known problems. ! 807: * Help: Service. Where to ask for help. ! 808: ! 809: ! 810: File: gcc.info, Node: Bug Criteria, Next: Bug Lists, Up: Bugs ! 811: ! 812: Have You Found a Bug? ! 813: ===================== ! 814: ! 815: If you are not sure whether you have found a bug, here are some ! 816: guidelines: ! 817: ! 818: * If the compiler gets a fatal signal, for any input whatever, that ! 819: is a compiler bug. Reliable compilers never crash. ! 820: ! 821: * If the compiler produces invalid assembly code, for any input ! 822: whatever (except an `asm' statement), that is a compiler bug, ! 823: unless the compiler reports errors (not just warnings) which would ! 824: ordinarily prevent the assembler from being run. ! 825: ! 826: * If the compiler produces valid assembly code that does not ! 827: correctly execute the input source code, that is a compiler bug. ! 828: ! 829: However, you must double-check to make sure, because you may have ! 830: run into an incompatibility between GNU C and traditional C (*note ! 831: Incompatibilities::.). These incompatibilities might be considered ! 832: bugs, but they are inescapable consequences of valuable features. ! 833: ! 834: Or you may have a program whose behavior is undefined, which ! 835: happened by chance to give the desired results with another C or ! 836: C++ compiler. ! 837: ! 838: For example, in many nonoptimizing compilers, you can write `x;' ! 839: at the end of a function instead of `return x;', with the same ! 840: results. But the value of the function is undefined if `return' ! 841: is omitted; it is not a bug when GNU CC produces different results. ! 842: ! 843: Problems often result from expressions with two increment ! 844: operators, as in `f (*p++, *p++)'. Your previous compiler might ! 845: have interpreted that expression the way you intended; GNU CC might ! 846: interpret it another way. Neither compiler is wrong. The bug is ! 847: in your code. ! 848: ! 849: After you have localized the error to a single source line, it ! 850: should be easy to check for these things. If your program is ! 851: correct and well defined, you have found a compiler bug. ! 852: ! 853: * If the compiler produces an error message for valid input, that is ! 854: a compiler bug. ! 855: ! 856: * If the compiler does not produce an error message for invalid ! 857: input, that is a compiler bug. However, you should note that your ! 858: idea of "invalid input" might be my idea of "an extension" or ! 859: "support for traditional practice". ! 860: ! 861: * If you are an experienced user of C or C++ compilers, your ! 862: suggestions for improvement of GNU CC or GNU C++ are welcome in ! 863: any case. ! 864: ! 865: ! 866: File: gcc.info, Node: Bug Lists, Next: Bug Reporting, Prev: Bug Criteria, Up: Bugs ! 867: ! 868: Where to Report Bugs ! 869: ==================== ! 870: ! 871: Send bug reports for GNU C to one of these addresses: ! 872: ! 873: [email protected] ! 874: {ucbvax|mit-eddie|uunet}!prep.ai.mit.edu!bug-gcc ! 875: ! 876: Send bug reports for GNU C++ to one of these addresses: ! 877: ! 878: [email protected] ! 879: {ucbvax|mit-eddie|uunet}!prep.ai.mit.edu!bug-g++ ! 880: ! 881: If your bug involves the C++ class library libg++, send mail to ! 882: `[email protected]'. If you're not sure, you can send the ! 883: bug report to both lists. ! 884: ! 885: *Do not send bug reports to the mailing list `help-gcc', or to the ! 886: newsgroup `gnu.gcc.help'.* Most users of GNU CC do not want to receive ! 887: bug reports. Those that do, have asked to be on `bug-gcc' and/or ! 888: `bug-g++'. ! 889: ! 890: The mailing lists `bug-gcc' and `bug-g++' both have newsgroups which ! 891: serve as repeaters: `gnu.gcc.bug' and `gnu.g++.bug'. Each mailing list ! 892: and its newsgroup carry exactly the same messages. ! 893: ! 894: Often people think of posting bug reports to the newsgroup instead of ! 895: mailing them. This appears to work, but it has one problem which can be ! 896: crucial: a newsgroup posting does not contain a mail path back to the ! 897: sender. Thus, if maintainers need more information, they may be unable ! 898: to reach you. For this reason, you should always send bug reports by ! 899: mail to the proper mailing list. ! 900: ! 901: As a last resort, send bug reports on paper to: ! 902: ! 903: GNU Compiler Bugs ! 904: Free Software Foundation ! 905: 675 Mass Ave ! 906: Cambridge, MA 02139 ! 907:
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