This is Info file gcc.info, produced by Makeinfo-1.54 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. Published by the Free Software Foundation 675 Massachusetts Avenue Cambridge, MA 02139 USA Copyright (C) 1988, 1989, 1992, 1993 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License" and "Protect Your Freedom--Fight `Look And Feel'" are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public License" and "Protect Your Freedom--Fight `Look And Feel'", and this permission notice, may be included in translations approved by the Free Software Foundation instead of in the original English.  File: gcc.info, Node: Invoking G++, Next: C Dialect Options, Prev: Overall Options, Up: Invoking GCC Compiling C++ Programs ====================== C++ source files conventionally use one of the suffixes `.C', `.cc', or `.cxx'; preprocessed C++ files use the suffix `.ii'. GNU CC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name `gcc'). However, C++ programs often require class libraries as well as a compiler that understands the C++ language--and under some circumstances, you might want to compile programs from standard input, or otherwise without a suffix that flags them as C++ programs. `g++' is a shell script that calls GNU CC with the default language set to C++, and automatically specifies linking against the GNU class library libg++. (1) On many systems, the script `g++' is also installed with the name `c++'. When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. *Note Options Controlling C Dialect: C Dialect Options, for explanations of options for languages related to C. *Note Options Controlling C++ Dialect: C++ Dialect Options, for explanations of options that are meaningful only for C++ programs. ---------- Footnotes ---------- (1) Prior to release 2 of the compiler, there was a separate `g++' compiler. That version was based on GNU CC, but not integrated with it. Versions of `g++' with a `1.XX' version number--for example, `g++' version 1.37 or 1.42--are much less reliable than the versions integrated with GCC 2. Moreover, combining G++ `1.XX' with a version 2 GCC will simply not work.  File: gcc.info, Node: C Dialect Options, Next: C++ Dialect Options, Prev: Invoking G++, Up: Invoking GCC Options Controlling C Dialect ============================= The following options control the dialect of C (or languages derived from C, such as C++ and Objective C) that the compiler accepts: `-ansi' Support all ANSI standard C programs. This turns off certain features of GNU C that are incompatible with ANSI C, such as the `asm', `inline' and `typeof' keywords, and predefined macros such as `unix' and `vax' that identify the type of system you are using. It also enables the undesirable and rarely used ANSI trigraph feature, and disallows `$' as part of identifiers. The alternate keywords `__asm__', `__extension__', `__inline__' and `__typeof__' continue to work despite `-ansi'. You would not want to use them in an ANSI C program, of course, but it useful to put them in header files that might be included in compilations done with `-ansi'. Alternate predefined macros such as `__unix__' and `__vax__' are also available, with or without `-ansi'. The `-ansi' option does not cause non-ANSI programs to be rejected gratuitously. For that, `-pedantic' is required in addition to `-ansi'. *Note Warning Options::. The macro `__STRICT_ANSI__' is predefined when the `-ansi' option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ANSI standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things. The functions `alloca', `abort', `exit', and `_exit' are not builtin functions when `-ansi' is used. `-fno-asm' Do not recognize `asm', `inline' or `typeof' as a keyword. These words may then be used as identifiers. You can use the keywords `__asm__', `__inline__' and `__typeof__' instead. `-ansi' implies `-fno-asm'. `-fno-builtin' Don't recognize builtin functions that do not begin with two leading underscores. Currently, the functions affected include `abort', `abs', `alloca', `cos', `exit', `fabs', `ffs', `labs', `memcmp', `memcpy', `sin', `sqrt', `strcmp', `strcpy', and `strlen'. GCC normally generates special code to handle certain builtin functions more efficiently; for instance, calls to `alloca' may become single instructions that adjust the stack directly, and calls to `memcpy' may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. The `-ansi' option prevents `alloca' and `ffs' from being builtin functions, since these functions do not have an ANSI standard meaning. `-trigraphs' Support ANSI C trigraphs. You don't want to know about this brain-damage. The `-ansi' option implies `-trigraphs'. `-traditional' Attempt to support some aspects of traditional C compilers. Specifically: * All `extern' declarations take effect globally even if they are written inside of a function definition. This includes implicit declarations of functions. * The newer keywords `typeof', `inline', `signed', `const' and `volatile' are not recognized. (You can still use the alternative keywords such as `__typeof__', `__inline__', and so on.) * Comparisons between pointers and integers are always allowed. * Integer types `unsigned short' and `unsigned char' promote to `unsigned int'. * Out-of-range floating point literals are not an error. * Certain constructs which ANSI regards as a single invalid preprocessing number, such as `0xe-0xd', are treated as expressions instead. * String "constants" are not necessarily constant; they are stored in writable space, and identical looking constants are allocated separately. (This is the same as the effect of `-fwritable-strings'.) * All automatic variables not declared `register' are preserved by `longjmp'. Ordinarily, GNU C follows ANSI C: automatic variables not declared `volatile' may be clobbered. * In the preprocessor, comments convert to nothing at all, rather than to a space. This allows traditional token concatenation. * In the preprocessor, macro arguments are recognized within string constants in a macro definition (and their values are stringified, though without additional quote marks, when they appear in such a context). The preprocessor always considers a string constant to end at a newline. * The predefined macro `__STDC__' is not defined when you use `-traditional', but `__GNUC__' is (since the GNU extensions which `__GNUC__' indicates are not affected by `-traditional'). If you need to write header files that work differently depending on whether `-traditional' is in use, by testing both of these predefined macros you can distinguish four situations: GNU C, traditional GNU C, other ANSI C compilers, and other old C compilers. *Note Standard Predefined Macros: (cpp.info)Standard Predefined, for more discussion of these and other predefined macros. * The preprocessor considers a string constant to end at a newline (unless the newline is escaped with `\'). (Without `-traditional', string constants can contain the newline character as typed.) * The character escape sequences `\x' and `\a' evaluate as the literal characters `x' and `a' respectively. Without `-traditional', `\x' is a prefix for the hexadecimal representation of a character, and `\a' produces a bell. * In C++ programs, assignment to `this' is permitted with `-traditional'. (The option `-fthis-is-variable' also has this effect.) You may wish to use `-fno-builtin' as well as `-traditional' if your program uses names that are normally GNU C builtin functions for other purposes of its own. `-traditional-cpp' Attempt to support some aspects of traditional C preprocessors. This includes the last three items in the table immediately above, but none of the other effects of `-traditional'. `-fcond-mismatch' Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. `-funsigned-char' Let the type `char' be unsigned, like `unsigned char'. Each kind of machine has a default for what `char' should be. It is either like `unsigned char' by default or like `signed char' by default. Ideally, a portable program should always use `signed char' or `unsigned char' when it depends on the signedness of an object. But many programs have been written to use plain `char' and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default. The type `char' is always a distinct type from each of `signed char' or `unsigned char', even though its behavior is always just like one of those two. `-fsigned-char' Let the type `char' be signed, like `signed char'. Note that this is equivalent to `-fno-unsigned-char', which is the negative form of `-funsigned-char'. Likewise, the option `-fno-signed-char' is equivalent to `-funsigned-char'. `-fsigned-bitfields' `-funsigned-bitfields' `-fno-signed-bitfields' `-fno-unsigned-bitfields' These options control whether a bitfield is signed or unsigned, when the declaration does not use either `signed' or `unsigned'. By default, such a bitfield is signed, because this is consistent: the basic integer types such as `int' are signed types. However, when `-traditional' is used, bitfields are all unsigned no matter what. `-fwritable-strings' Store string constants in the writable data segment and don't uniquize them. This is for compatibility with old programs which assume they can write into string constants. The option `-traditional' also has this effect. Writing into string constants is a very bad idea; "constants" should be constant. `-fallow-single-precision' Do not promote single precision math operations to double precision, even when compiling with `-traditional'. Traditional K&R C promotes all floating point operations to double precision, regardless of the sizes of the operands. On the architecture for which you are compiling, single precision may be faster than double precision. If you must use `-traditional', but want to use single precision operations when the operands are single precision, use this option. This option has no effect when compiling with ANSI or GNU C conventions (the default).  File: gcc.info, Node: C++ Dialect Options, Next: Warning Options, Prev: C Dialect Options, Up: Invoking GCC Options Controlling C++ Dialect =============================== This section describes the command-line options that are only meaningful for C++ programs; but you can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file `firstClass.C' like this: g++ -g -felide-constructors -O -c firstClass.C In this example, only `-felide-constructors' is an option meant only for C++ programs; you can use the other options with any language supported by GNU CC. Here is a list of options that are *only* for compiling C++ programs: `-fall-virtual' Treat all possible member functions as virtual, implicitly. All member functions (except for constructor functions and `new' or `delete' member operators) are treated as virtual functions of the class where they appear. This does not mean that all calls to these member functions will be made through the internal table of virtual functions. Under some circumstances, the compiler can determine that a call to a given virtual function can be made directly; in these cases the calls are direct in any case. `-fdollars-in-identifiers' Accept `$' in identifiers. You can also explicitly prohibit use of `$' with the option `-fno-dollars-in-identifiers'. (GNU C++ allows `$' by default on some target systems but not others.) Traditional C allowed the character `$' to form part of identifiers. However, ANSI C and C++ forbid `$' in identifiers. `-felide-constructors' Elide constructors when this seems plausible. With this option, GNU C++ initializes `y' directly from the call to `foo' without going through a temporary in the following code: A foo (); A y = foo (); Without this option, GNU C++ (1) initializes `y' by calling the appropriate constructor for type `A'; (2) assigns the result of `foo' to a temporary; and, finally, (3) replaces the initial value of `y' with the temporary. The default behavior (`-fno-elide-constructors') is specified by the draft ANSI C++ standard. If your program's constructors have side effects, `-felide-constructors' can change your program's behavior, since some constructor calls may be omitted. `-fenum-int-equiv' Permit implicit conversion of `int' to enumeration types. Normally GNU C++ allows conversion of `enum' to `int', but not the other way around. `-fexternal-templates' Produce smaller code for template declarations, by generating only a single copy of each template function where it is defined. To use this option successfully, you must also mark all files that use templates with either `#pragma implementation' (the definition) or `#pragma interface' (declarations). *Note Declarations and Definitions in One Header: C++ Interface, for more discussion of these pragmas. When your code is compiled with `-fexternal-templates', all template instantiations are external. You must arrange for all necessary instantiations to appear in the implementation file; you can do this with a `typedef' that references each instantiation needed. Conversely, when you compile using the default option `-fno-external-templates', all template instantiations are explicitly internal. You do not need to specify `-fexternal-templates' when compiling a file that does not define and instantiate templates used in other files, even if your file *uses* templates defined in other files that are compiled with `-fexternal-templates'. The only side effect is an increase in object size for each file that you compile without `-fexternal-templates'. `-fmemoize-lookups' `-fsave-memoized' Use heuristics to compile faster. These heuristics are not enabled by default, since they are only effective for certain input files. Other input files compile more slowly. The first time the compiler must build a call to a member function (or reference to a data member), it must (1) determine whether the class implements member functions of that name; (2) resolve which member function to call (which involves figuring out what sorts of type conversions need to be made); and (3) check the visibility of the member function to the caller. All of this adds up to slower compilation. Normally, the second time a call is made to that member function (or reference to that data member), it must go through the same lengthy process again. This means that code like this: cout << "This " << p << " has " << n << " legs.\n"; makes six passes through all three steps. By using a software cache, a "hit" significantly reduces this cost. Unfortunately, using the cache introduces another layer of mechanisms which must be implemented, and so incurs its own overhead. `-fmemoize-lookups' enables the software cache. Because access privileges (visibility) to members and member functions may differ from one function context to the next, G++ may need to flush the cache. With the `-fmemoize-lookups' flag, the cache is flushed after every function that is compiled. The `-fsave-memoized' flag enables the same software cache, but when the compiler determines that the context of the last function compiled would yield the same access privileges of the next function to compile, it preserves the cache. This is most helpful when defining many member functions for the same class: with the exception of member functions which are friends of other classes, each member function has exactly the same access privileges as every other, and the cache need not be flushed. `-fno-strict-prototype' Treat a function declaration with no arguments, such as `int foo ();', as C would treat it--as saying nothing about the number of arguments or their types. Normally, such a declaration in C++ means that the function `foo' takes no arguments. `-fnonnull-objects' Assume that objects reached through references are not null. Normally, GNU C++ makes conservative assumptions about objects reached through references. For example, the compiler must check that `a' is not null in code like the following: obj &a = g (); a.f (2); Checking that references of this sort have non-null values requires extra code, however, and it is unnecessary for many programs. You can use `-fnonnull-objects' to omit the checks for null, if your program doesn't require checking. `-fthis-is-variable' Permit assignment to `this'. The incorporation of user-defined free store management into C++ has made assignment to `this' an anachronism. Therefore, by default it is invalid to assign to `this' within a class member function; that is, GNU C++ treats the type of `this' in a member function of class `X' to be `X *const'. However, for backwards compatibility, you can make it valid with `-fthis-is-variable'. `-nostdinc++' Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building libg++.) `-traditional' For C++ programs (in addition to the effects that apply to both C and C++), this has the same effect as `-fthis-is-variable'. *Note Options Controlling C Dialect: C Dialect Options. In addition, these optimization, warning, and code generation options have meanings only for C++ programs: `-fno-default-inline' Do not assume `inline' for functions defined inside a class scope. *Note Options That Control Optimization: Optimize Options. `-Wenum-clash' `-Woverloaded-virtual' `-Wtemplate-debugging' Warnings that apply only to C++ programs. *Note Options to Request or Suppress Warnings: Warning Options. `+eN' Control how virtual function definitions are used, in a fashion compatible with `cfront' 1.x. *Note Options for Code Generation Conventions: Code Gen Options.  File: gcc.info, Node: Warning Options, Next: Debugging Options, Prev: C++ Dialect Options, Up: Invoking GCC Options to Request or Suppress Warnings ======================================= Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error. You can request many specific warnings with options beginning `-W', for example `-Wimplicit' to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning `-Wno-' to turn off warnings; for example, `-Wno-implicit'. This manual lists only one of the two forms, whichever is not the default. These options control the amount and kinds of warnings produced by GNU CC: `-fsyntax-only' Check the code for syntax errors, but don't do anything beyond that. `-w' Inhibit all warning messages. `-Wno-import' Inhibit warning messages about the use of `#import'. `-pedantic' Issue all the warnings demanded by strict ANSI standard C; reject all programs that use forbidden extensions. Valid ANSI standard C programs should compile properly with or without this option (though a rare few will require `-ansi'). However, without this option, certain GNU extensions and traditional C features are supported as well. With this option, they are rejected. `-pedantic' does not cause warning messages for use of the alternate keywords whose names begin and end with `__'. Pedantic warnings are also disabled in the expression that follows `__extension__'. However, only system header files should use these escape routes; application programs should avoid them. *Note Alternate Keywords::. This option is not intended to be useful; it exists only to satisfy pedants who would otherwise claim that GNU CC fails to support the ANSI standard. Some users try to use `-pedantic' to check programs for strict ANSI C conformance. They soon find that it does not do quite what they want: it finds some non-ANSI practices, but not all--only those for which ANSI C *requires* a diagnostic. A feature to report any failure to conform to ANSI C might be useful in some instances, but would require considerable additional work and would be quite different from `-pedantic'. We recommend, rather, that users take advantage of the extensions of GNU C and disregard the limitations of other compilers. Aside from certain supercomputers and obsolete small machines, there is less and less reason ever to use any other C compiler other than for bootstrapping GNU CC. `-pedantic-errors' Like `-pedantic', except that errors are produced rather than warnings. `-W' Print extra warning messages for these events: * A nonvolatile automatic variable might be changed by a call to `longjmp'. These warnings as well are possible only in optimizing compilation. The compiler sees only the calls to `setjmp'. It cannot know where `longjmp' will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because `longjmp' cannot in fact be called at the place which would cause a problem. * A function can return either with or without a value. (Falling off the end of the function body is considered returning without a value.) For example, this function would evoke such a warning: foo (a) { if (a > 0) return a; } * An expression-statement contains no side effects. * An unsigned value is compared against zero with `>' or `<='. * A comparison like `x<=y<=z' appears; this is equivalent to `(x<=y ? 1 : 0) <= z', which is a different interpretation from that of ordinary mathematical notation. * Storage-class specifiers like `static' are not the first things in a declaration. According to the C Standard, this usage is obsolescent. * An aggregate has a partly bracketed initializer. For example, the following code would evoke such a warning, because braces are missing around the initializer for `x.h': struct s { int f, g; }; struct t { struct s h; int i; }; struct t x = { 1, 2, 3 }; `-Wimplicit' Warn whenever a function or parameter is implicitly declared. `-Wreturn-type' Warn whenever a function is defined with a return-type that defaults to `int'. Also warn about any `return' statement with no return-value in a function whose return-type is not `void'. `-Wunused' Warn whenever a local variable is unused aside from its declaration, whenever a function is declared static but never defined, and whenever a statement computes a result that is explicitly not used. If you want to prevent a warning for a particular variable, you can use this macro: #define USE(var) \ static void * use_##var = (&use_##var, (void *) &var) USE (string); `-Wswitch' Warn whenever a `switch' statement has an index of enumeral type and lacks a `case' for one or more of the named codes of that enumeration. (The presence of a `default' label prevents this warning.) `case' labels outside the enumeration range also provoke warnings when this option is used. `-Wcomment' Warn whenever a comment-start sequence `/*' appears in a comment. `-Wtrigraphs' Warn if any trigraphs are encountered (assuming they are enabled). `-Wformat' Check calls to `printf' and `scanf', etc., to make sure that the arguments supplied have types appropriate to the format string specified. `-Wchar-subscripts' Warn if an array subscript has type `char'. This is a common cause of error, as programmers often forget that this type is signed on some machines. `-Wuninitialized' An automatic variable is used without first being initialized. These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing. If you don't specify `-O', you simply won't get these warnings. These warnings occur only for variables that are candidates for register allocation. Therefore, they do not occur for a variable that is declared `volatile', or whose address is taken, or whose size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for structures, unions or arrays, even when they are in registers. Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed. These warnings are made optional because GNU CC is not smart enough to see all the reasons why the code might be correct despite appearing to have an error. Here is one example of how this can happen: { int x; switch (y) { case 1: x = 1; break; case 2: x = 4; break; case 3: x = 5; } foo (x); } If the value of `y' is always 1, 2 or 3, then `x' is always initialized, but GNU CC doesn't know this. Here is another common case: { int save_y; if (change_y) save_y = y, y = new_y; ... if (change_y) y = save_y; } This has no bug because `save_y' is used only if it is set. Some spurious warnings can be avoided if you declare all the functions you use that never return as `volatile'. *Note Function Attributes::. `-Wparentheses' Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about. `-Wenum-clash' Warn about conversion between different enumeration types. (C++ only). `-Wtemplate-debugging' When using templates in a C++ program, warn if debugging is not yet fully available (C++ only). `-Wall' All of the above `-W' options combined. These are all the options which pertain to usage that we recommend avoiding and that we believe is easy to avoid, even in conjunction with macros. The remaining `-W...' options are not implied by `-Wall' because they warn about constructions that we consider reasonable to use, on occasion, in clean programs. `-Wtraditional' Warn about certain constructs that behave differently in traditional and ANSI C. * Macro arguments occurring within string constants in the macro body. These would substitute the argument in traditional C, but are part of the constant in ANSI C. * A function declared external in one block and then used after the end of the block. * A `switch' statement has an operand of type `long'. `-Wshadow' Warn whenever a local variable shadows another local variable. `-Wid-clash-LEN' Warn whenever two distinct identifiers match in the first LEN characters. This may help you prepare a program that will compile with certain obsolete, brain-damaged compilers. `-Wpointer-arith' Warn about anything that depends on the "size of" a function type or of `void'. GNU C assigns these types a size of 1, for convenience in calculations with `void *' pointers and pointers to functions. `-Wcast-qual' Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a `const char *' is cast to an ordinary `char *'. `-Wcast-align' Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a `char *' is cast to an `int *' on machines where integers can only be accessed at two- or four-byte boundaries. `-Wwrite-strings' Give string constants the type `const char[LENGTH]' so that copying the address of one into a non-`const' `char *' pointer will get a warning. These warnings will help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using `const' in declarations and prototypes. Otherwise, it will just be a nuisance; this is why we did not make `-Wall' request these warnings. `-Wconversion' Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed point argument except when the same as the default promotion. Also, warn if a negative integer constant expression is implicitly converted to an unsigned type. For example, warn about the assignment `x = -1' if `x' is unsigned. But do not warn about explicit casts like `(unsigned) -1'. `-Waggregate-return' Warn if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warning.) `-Wstrict-prototypes' Warn if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration which specifies the argument types.) `-Wmissing-prototypes' Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. The aim is to detect global functions that fail to be declared in header files. `-Wredundant-decls' Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing. `-Wnested-externs' Warn if an `extern' declaration is encountered within an function. `-Winline' Warn if a function can not be inlined, and either it was declared as inline, or else the `-finline-functions' option was given. `-Woverloaded-virtual' Warn when a derived class function declaration may be an error in defining a virtual function (C++ only). In a derived class, the definitions of virtual functions must match the type signature of a virtual function declared in the base class. With this option, the compiler warns when you define a function with the same name as a virtual function, but with a type signature that does not match any declarations from the base class. `-Werror' Make all warnings into errors.  File: gcc.info, Node: Debugging Options, Next: Optimize Options, Prev: Warning Options, Up: Invoking GCC Options for Debugging Your Program or GNU CC ============================================ GNU CC has various special options that are used for debugging either your program or GCC: `-g' Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information. On most systems that use stabs format, `-g' enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use `-gstabs+', `-gstabs', `-gxcoff+', `-gxcoff', `-gdwarf+', or `-gdwarf' (see below). Unlike most other C compilers, GNU CC allows you to use `-g' with `-O'. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops. Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs. The following options are useful when GNU CC is generated with the capability for more than one debugging format. `-ggdb' Produce debugging information in the native format (if that is supported), including GDB extensions if at all possible. `-gstabs' Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS and Alpha systems this option produces embedded stabs debugging output which is not understood by DBX. `-gstabs+' Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program. `-gcoff' Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4. `-gxcoff' Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems. `-gxcoff+' Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program. `-gdwarf' Produce debugging information in DWARF format (if that is supported). This is the format used by SDB on most System V Release 4 systems. `-gdwarf+' Produce debugging information in DWARF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program. `-gLEVEL' `-ggdbLEVEL' `-gstabsLEVEL' `-gcoffLEVEL' `-gxcoffLEVEL' `-gdwarfLEVEL' Request debugging information and also use LEVEL to specify how much information. The default level is 2. Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, but no information about local variables and no line numbers. Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use `-g3'. `-p' Generate extra code to write profile information suitable for the analysis program `prof'. You must use this option when compiling the source files you want data about, and you must also use it when linking. `-pg' Generate extra code to write profile information suitable for the analysis program `gprof'. You must use this option when compiling the source files you want data about, and you must also use it when linking. `-a' Generate extra code to write profile information for basic blocks, which will record the number of times each basic block is executed, the basic block start address, and the function name containing the basic block. If `-g' is used, the line number and filename of the start of the basic block will also be recorded. If not overridden by the machine description, the default action is to append to the text file `bb.out'. This data could be analyzed by a program like `tcov'. Note, however, that the format of the data is not what `tcov' expects. Eventually GNU `gprof' should be extended to process this data. `-dLETTERS' Says to make debugging dumps during compilation at times specified by LETTERS. This is used for debugging the compiler. The file names for most of the dumps are made by appending a word to the source file name (e.g. `foo.c.rtl' or `foo.c.jump'). Here are the possible letters for use in LETTERS, and their meanings: `M' Dump all macro definitions, at the end of preprocessing, and write no output. `N' Dump all macro names, at the end of preprocessing. `D' Dump all macro definitions, at the end of preprocessing, in addition to normal output. `y' Dump debugging information during parsing, to standard error. `r' Dump after RTL generation, to `FILE.rtl'. `x' Just generate RTL for a function instead of compiling it. Usually used with `r'. `j' Dump after first jump optimization, to `FILE.jump'. `s' Dump after CSE (including the jump optimization that sometimes follows CSE), to `FILE.cse'. `L' Dump after loop optimization, to `FILE.loop'. `t' Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to `FILE.cse2'. `f' Dump after flow analysis, to `FILE.flow'. `c' Dump after instruction combination, to the file `FILE.combine'. `S' Dump after the first instruction scheduling pass, to `FILE.sched'. `l' Dump after local register allocation, to `FILE.lreg'. `g' Dump after global register allocation, to `FILE.greg'. `R' Dump after the second instruction scheduling pass, to `FILE.sched2'. `J' Dump after last jump optimization, to `FILE.jump2'. `d' Dump after delayed branch scheduling, to `FILE.dbr'. `k' Dump after conversion from registers to stack, to `FILE.stack'. `a' Produce all the dumps listed above. `m' Print statistics on memory usage, at the end of the run, to standard error. `p' Annotate the assembler output with a comment indicating which pattern and alternative was used. `-fpretend-float' When running a cross-compiler, pretend that the target machine uses the same floating point format as the host machine. This causes incorrect output of the actual floating constants, but the actual instruction sequence will probably be the same as GNU CC would make when running on the target machine. `-save-temps' Store the usual "temporary" intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling `foo.c' with `-c -save-temps' would produce files `foo.i' and `foo.s', as well as `foo.o'. `-print-libgcc-file-name' Print the full absolute name of the library file `libgcc.a' that would be used when linking--and don't do anything else. With this option, GNU CC does not compile or link anything; it just prints the file name. This is useful when you use `-nostdlib' but you do want to link with `libgcc.a'. You can do gcc -nostdlib FILES... `gcc -print-libgcc-file-name`