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cc - GNU project C Compiler [ options ] files is a version of the GNU C compiler. It accepts a dialect of ANSI C with extensions; this dialect is different from the dialect used in 4.3 BSD and earlier distributions. The flag causes the compiler to accept a dialect of extended Classic C, much like the C of these earlier distributions. If you are not already familiar with ANSI C and its new features, you will want to build your software with Most older C compiler flags are supported by Three that are not are: to generate symbol tables for the unsupported debugger; for sin- gle precision floating point in expressions, which is now the de- fault; and for alternate compiler passes. The differences between ANSI C and Classic C dialects are too numerous to describe here in detail. The following quick summary is meant to make users aware of potential subtle problems when converting Classic C code to ANSI C. The most obvious change is the per- vasive use of Under the ANSI C dialect, the number and type of arguments to C library functions are checked by the compiler when standard header files are included; calls that fail to match will yield errors. A subtle consequence of adding prototype declara- tions is that user code that inadvertently redefines a C library function may break; for example it is no longer possible to write an function that takes different parameters or returns a dif- ferent value from the standard when including standard header files. Another issue with prototypes is that functions that take different parameter types no longer have the same type; function pointers now differ by parameter types as well as return types. Variable argument lists are handled differently; the old package is obsolete, replaced by which unfortunately is not completely compatible. A subtle change in type promotion can be confusing: small unsigned types are now widened into signed types rather than unsigned types. A similar problem can occur with the opera- tor, which now yields an unsigned type rather than a signed type. One common problem is due to a change in scoping: external de- clarations are now scoped to the block they occur in, so a de- claration for (say) inside one block will no longer declare it in all subsequent blocks. The syntax for braces in structure ini- tializations is now a bit stricter, and it is sometimes necessary to add braces to please the compiler. Two very subtle and some- times very annoying features apply to constant strings and to the function. Constant strings in the ANSI dialect are read-only; attempts to alter them cause protection violations. This ANSI feature permits the compiler to coalesce identical strings in the same source file, and saves space when multiple copies of a binary are running at the same time, since the read-only part of a binary is sharable. The most common difficulty with read-only strings lies with the use of the function, which in the past often altered a constant string argument. It is now necessary to copy a constant string (for example, with before it may be al- tered. The function may now destroy any register stack variable in the function that made the corresponding call to the function; to protect a local variable, the new ANSI modifier must be used. This often leads to confusing situations upon `return' from The compiler has extended warning flags for dealing with read-only strings and but these are not very effective. If your code has problems with any of these ANSI features, you will probably want to use Even with there are some differences between this dialect of Classic C and the dialect supported on older distributions. There are at least two differences that are a consequence of the fact that uses an ANSI C style grammar for both traditional and ANSI modes. The old C dialect permitted a typedef to replace a simple type in the idiom ``unsigned this treats such forms as syntax errors. The old C dialect also permitted formal parame- ters to have the same names as typedef types; the current dialect does not. Some questionable or illegal practices that were sup- ported in the old C dialect are not supported by non-comment text at the end of a ``#include'' preprocessor control line is an er- ror, not ignored; compound assignment operators must not contain white space, ``* ='' is not the same as ``*=''; the last member declaration in a structure or union must be terminated by a semi- colon; it is not possible to ``switch'' on function pointers; more than one occurrence of ``#else'' at the same level in a preprocessor ``#if'' clause is an error, not ignored. Some truly ancient C practices are no longer supported. The idiom of de- claring an anonymous structure and using its members to extract fields from other structures or even non-structures is illegal. Integers are not automatically converted to pointers when they are dereferenced. The dialect does not retain the so-called ``old-fashioned'' assignment operators (with the ``='' preceding rather than following the operator) or initializations (with no ``='' between initializer and initializee). This rest of man page is an extract of the documentation of the and is limited to the meaning of the options. If you want to be certain of the in- formation below, check it in the manual "Using and Porting GCC". Refer to the Info file or the DVI file which are made from the Texinfo source file The uses a command syntax much like the Unix C compiler. The program accepts options and file names as operands. Multiple single-letter options may be grouped: is very different from When you invoke GNU CC, it normally does prepro- cessing, compilation, assembly and linking. File names which end in are taken as C source to be preprocessed and compiled; file names ending in are taken as preprocessor output to be compiled; compiler output files plus any input files with names ending in are assembled; then the resulting object files, plus any other input files, are linked together to produce an executable. Com- mand options allow you to stop this process at an intermediate stage. For example, the option says not to run the linker. Then the output consists of object files output by the assembler. Other command options are passed on to one stage of processing. Some options control the preprocessor and others the compiler it- self. Yet other options control the assembler and linker; these are not documented here, but you rarely need to use any of them. Here are the options to control the overall compilation process, including those that say whether to link, whether to assemble, and so on. Place output in file This applies regardless to what- ever sort of output is being produced, whether it be an execut- able file, an object file, an assembler file or preprocessed C code. If is not specified, the default is to put an executable file in the object file in an assembler file in and preprocessed C on standard output. Compile or assemble the source files, but do not link. Produce object files with names made by replacing or with at the end of the input file names. Do nothing at all for object files specified as input. Compile into assembler code but do not assemble. The assembler output file name is made by re- placing with at the end of the input file name. Do nothing at all for assembler source files or object files specified as in- put. Run only the C preprocessor. Preprocess all the C source files specified and output the results to standard output. Com- piler driver program prints the commands it executes as it runs the preprocessor, compiler proper, assembler and linker. Some of these are directed to print their own version numbers. Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assem- bler has no trouble. Compiler driver program tries as a prefix for each program it tries to run. These programs are and For each subprogram to be run, the compiler driver first tries the prefix, if any. If that name is not found, or if was not specified, the driver tries a standard prefix, which currently is If this does not result in a file name that is found, the unmodi- fied program name is searched for using the directories specified in your environment variable. You can get a similar result from the environment variable if it is defined, its value is used as a prefix in the same way. If both the option and the variable are present, the option is used first and the environment variable value second. The argument is used as a second prefix for the compiler executables and li- braries. This prefix is optional: the compiler tries each file first with it, then without it. This prefix follows the prefix specified with or the default prefixes. Thus, in the presence of environment variable with definition causes GNU CC to try the following file names for the preproces- sor executable: cc/vax-cpp cc/cpp /u/foo/vax-cpp /u/foo/cpp /usr/libexec/vax-cpp /usr/libexec/cpp These options control the details of C compilation itself. Support all ANSI standard C programs. This turns off certain features of GNU C that are incompatible with ANSI C, such as the and keywords, and predefined macros such as and that identify the type of system you are using. It also enables the undesirable and rarely used ANSI trigraph feature. The alternate keywords and continue to work despite 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 compila- tions done with Alternate predefined macros such as and are also available, with or without The option does not cause non-ANSI programs to be rejected gra- tuitously. For that, is required in addition to The macro is predefined when the 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. Attempt to support some aspects of traditional C compilers. Specifically: * All declarations take effect globally even if they are written inside of a function definition. This includes implicit declara- tions of functions. * The keywords and are not recognized. * Comparisons between pointers and integers are always allowed. * Integer types and promote to * Out-of-range floating point literals are not an error. * All automatic variables not declared are preserved by Ordinari- ly, GNU C follows ANSI C: automatic variables not declared 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 ap- pear in such a context). The preprocessor always considers a string constant to end at a newline. * The predefined macro is not defined when you use but is (since the GNU extensions which indicates are not affected by If you need to write header files that work differently depending on whether 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. Optimize. Optimiz- ing compilation takes somewhat more time, and a lot more memory for a large function. Without the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any vari- able or change the program counter to any other statement in the function and get exactly the results you would expect from the source code. Without only variables declared are allocated in registers. The resulting compiled code is a little worse than produced by PCC without With the compiler tries to reduce code size and execution time. Some of the options described below turn specific kinds of optim- ization on or off. Produce debugging information in the operat- ing system's native format (for DBX or SDB). GDB also can work with this debugging information. Unlike most other C compilers, GNU CC allows you to use with 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 con- stant results or their values were already at hand; some state- ments 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 pro- grams that might have bugs. Inhibit all warning messages. Print extra warning messages for these events: * An automatic variable is used without first being initialized. These warnings are possible only in optimizing compilation, be- cause they require data flow information that is computed only when optimizing. If you don't specify 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 or whose address is taken, or whose size is oth- er than 1, 2, 4 or 8 bytes. Also, they do not occur for struc- tures, 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 is always 1, 2 or 3, then 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 is used only if it is set. Some spurious warnings can be avoided if you declare as all the functions you use that never return. * A nonvolatile automatic variable might be changed by a call to These warnings as well are possible only in optimizing compila- tion. The compiler sees only the calls to It cannot know where 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 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; } Spurious warnings can occur because GNU CC does not realize that certain functions (including and will never return. * An expression-statement contains no side effects. In the future, other useful warnings may also be enabled by this option. Warn whenever a function is implicitly declared. Warn whenever a function is defined with a return-type that defaults to Also warn about any statement with no return-value in a func- tion whose return-type is not Warn whenever a local variable is unused aside from its declaration, and whenever a function is de- clared static but never defined. Warn whenever a statement has an index of enumeral type and lacks a for one or more of the named codes of that enumeration. (The presence of a label prevents this warning.) labels outside the enumeration range also provoke warnings when this option is used. Warn whenever a comment-start sequence appears in a comment. Warn if any tri- graphs are encountered (assuming they are enabled). All of the above options combined. These are all the options which pertain to usage that we do not recommend and that we believe is always easy to avoid, even in conjunction with macros. The other options below are not implied by because certain kinds of useful macros are almost impossible to write without causing those warnings. Warn whenever a local variable shadows another local variable. Warn whenever two distinct identifiers match in the first characters. This may help you prepare a program that will compile with certain obsolete, brain-damaged compilers. Warn about anything that depends on the size of a function type or of GNU C assigns these types a size of 1, for convenience in calculations with pointers and pointers to functions. Warn when- ever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a is cast to an ordinary Give string constants the type so that copying the address of one into a pointer will get a warning. These warnings will help you find at compile time code that can try to write into a string con- stant, but only if you have been very careful about using in de- clarations and prototypes. Otherwise, it will just be a nui- sance; this is why we did not make request these warnings. Gen- erate extra code to write profile information suitable for the analysis program Generate extra code to write profile information suitable for the analysis program Generate extra code to write profile information for basic blocks, suitable for the analysis program Eventually GNU should be extended to process this data. Search a standard list of directories for a library named which is actually a file named The linker uses this file as if it had been specified precisely by name. The directories searched include several standard system direc- tories plus any that you specify with Normally the files found this way are library files--archive files whose members are object files. The linker handles an ar- chive file by scanning through it for members which define sym- bols that have so far been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in the usual fashion. The only difference between using an op- tion and specifying a file name is that searches several direc- tories. Add directory to the list of directories to be searched for Don't use the standard system libraries and startup files when linking. Only the files you specify (plus will be passed to the linker. Machine-dependent option specifying something about the type of target machine. These options are defined by the macro in the machine description. The default for the options is also defined by that macro, which enables you to change the de- faults. These are the options defined in the 68000 machine description: Generate output for a 68020 (rather than a 68000). This is the default if you use the unmodified sources. Generate output for a 68000 (rather than a 68020). Generate output containing 68881 instructions for floating point. This is the default if you use the unmodified sources. Generate output containing Sun FPA instructions for floating point. Generate output containing library calls for floating point. Consider type to be 16 bits wide, like Do not use the bit-field instructions. implies Do use the bit-field instructions. implies This is the de- fault if you use the unmodified sources. Use a different function-calling convention, in which func- tions that take a fixed number of arguments return with the instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there. This calling convention is incompatible with the one normal- ly used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. Also, you must provide function prototypes for all functions that take variable numbers of arguments (including otherwise incorrect code will be generated for calls to those func- tions. In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) The instruction is supported by the 68010 and 68020 proces- sors, but not by the 68000. These options are defined in the Vax machine description: Do not output certain jump instructions and so on) that the Unix assembler for the Vax cannot handle across long ranges. Do output those jump instructions, on the assumption that you will assemble with the GNU assembler. Output code for g-format floating point numbers instead of d-format. These switches are supported on the Sparc: Generate output containing floating point instructions. This is the default if you use the unmodified sources. Generate output containing library calls for floating point. Generate separate return instructions for statements. This has both advantages and disadvantages; I don't recall what they are. These options are defined in the Convex machine description: Generate output for a C1. This is the default when the com- piler is configured for a C1. Generate output for a C2. This is the default when the com- piler is configured for a C2. Generate code which puts an argument count in the word preceding each argument list. Some nonportable Convex and Vax programs need this word. (Debuggers don't; this info is in the symbol table.) Omit the argument count word. This is the default if you use the unmodified sources. Specify machine-independent flags. Most flags have both positive and negative forms; the negative form of would be In the table below, only one of the forms is listed--the one which is not the default. You can figure out the other form by either removing or adding it. Use the same convention for returning and values that is used by the usual C compiler on your system. This convention is less efficient for small structures, and on many machines it fails to be reentrant; but it has the advantage of allowing intercallability between GCC-compiled code and PCC-compiled code. Do not store floating-point variables in registers. This prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more pre- cision than a is supposed to have. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use for such programs. Do not recognize or as a keyword. These words may then be used as identifiers. You can use and instead. Always pop the arguments to each function call as soon as that function returns. Normally the compiler (when optimizing) lets arguments accumulate on the stack for several function calls and pops them all at once. Perform the optimizations of loop strength reduction and elimination of iteration variables. Allow the combine pass to combine an instruction that copies one regis- ter into another. This might or might not produce better code when used in addition to I am interested in hearing about the difference this makes. Force memory operands to be copied into registers before doing arithmetic on them. This may produce better code by making all memory references potential common su- bexpressions. When they are not common subexpressions, instruc- tion combination should eliminate the separate register-load. I am interested in hearing about the difference this makes. Force memory address constants to be copied into registers before doing arithmetic on them. This may produce better code just as may. I am interested in hearing about the difference this makes. Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and re- store frame pointers; it also makes an extra register available in many functions. On some machines, such as the Vax, this flag has no effect, be- cause the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't ex- ist. The machine-description macro controls whether a target machine supports this flag. Integrate all simple functions into their callers. The compiler heuristically decides which func- tions are simple enough to be worth integrating in this way. If all calls to a given function are integrated, and the function is declared then the function is normally not output as assembler code in its own right. Enable values to be allocated in regis- ters that will be clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation is done only when it seems to result in better code than would otherwise be produced. This option is enabled by default on certain machines, usually those which have no call-preserved registers to use instead. Even if all calls to a given function are integrated, and the function is declared nevertheless output a separate run-time callable version of the function. Store string constants in the writable data segment and don't uniquize them. This is for com- patibility with old programs which assume they can write into string constants. Writing into string constants is a very bad idea; constants should be constant. Allow conditional expres- sions with mismatched types in the second and third arguments. The value of such an expression is void. Do not put function ad- dresses in registers; make each instruction that calls a constant function contain the function's address explicitly. This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the op- timizations performed when this option is not used. Consider all memory references through pointers to be volatile. Requests that the data and variables of this compilation be shared data rather than private data. The distinction makes sense only on certain operating systems, where shared data is shared between processes running the same program, while private data exists in one copy per process. Let the type be the unsigned, like Each kind of machine has a default for what should be. It is ei- ther like by default or like by default. (Actually, at present, the default is always signed.) The type is always a distinct type from either or even though its behavior is always just like one of those two. Note that this is equivalent to which is the negative form of Let the type be signed, like Note that this is equivalent to which is the negative form of If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch in- structions. Treat the register named as a fixed register; gen- erated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role). must be the name of a register. The register names accepted are machine-specific and are defined in the macro in the machine description macro file. This flag does not have a negative form, because it specifies a three-way choice. Treat the register named as an allocatable re- gister that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way will not save and restore the regis- ter REG. Use of this flag for a register that has a fixed pervasive role in the machine's execution model, such as the stack pointer or frame pointer, will produce disastrous results. This flag does not have a negative form, because it specifies a three-way choice. Treat the register named as an allocatable re- gister saved by functions. It may be allocated even for tem- poraries or variables that live across a call. Functions com- piled this way will save and restore the register if they use it. Use of this flag for a register that has a fixed pervasive role in the machine's execution model, such as the stack pointer or frame pointer, will produce disastrous results. A different sort of disaster will result from the use of this flag for a register in which function values may be returned. This flag does not have a negative form, because it specifies a three-way choice. Says to make debugging dumps at times speci- fied by Here are the possible letters: Dump after RTL generation. Dump after first jump optimization. Dump after last jump optimization. Dump after CSE (including the jump optimization that some- times follows CSE). Dump after loop optimization. Dump after flow analysis. Dump after instruction combination. Dump after local register allocation. Dump after global register allocation. Dump after delayed branch scheduling. Print statistics on memory usage, at the end of the run. 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 However, without this option, certain GNU extensions and traditional C features are supported as well. With this option, they are re- jected. There is no reason to use this option; it exists only to satisfy pedants. does not cause warning messages for use of the alternate keywords whose names begin and end with On Suns running version 4, this prevents linking with the shared libraries. has the same ef- fect.) These options control the C preprocessor, which is run on each C source file before actual compilation. If you use the `- E' option, nothing is done except C preprocessing. Some of these options make sense only together with `-E' because they request preprocessor output that is not suitable for actual compilation. Tell the preprocessor not to discard comments. Used with the op- tion. Search directory for include files. Any directories specified with options before the option are searched only for the case of "file"; they are not searched for If additional directories are specified with options after the these directories are searched for all directives. (Ordinarily directories are used this way.) In addition, the option inhibits the use of the current directory as the first search directory for "file". Therefore, the current directory is searched only if it is requested explicitly with Specifying both and allows you to control precisely which direc- tories are searched before the current one and which are searched after. Do not search the standard system directories for header files. Only the directories you have specified with options (and the current directory, if appropriate) are searched. Between and you can eliminate all directories from the search path except those you specify. Tell the preprocessor to output a rule suitable for describing the dependencies of each source file. For each source file, the preprocessor outputs one whose target is the object file name for that source file and whose dependencies are all the files in it. This rule may be a single line or may be continued with if it is long. implies Like but the output mentions only the user-header files included with "file". System header files included with are om- itted. implies Define macro with the empty string as its definition. Define macro as Undefine macro Support ANSI C trigraphs. You don't want to know about this brain-damage. The option also has this effect. file.c C source file file.s assembly language file file.o object file a.out link edited output /tmp/cc* temporary files /usr/libexec/cpp preprocessor /usr/libexec/cc1 compiler /usr/lib/libgnulib.a library needed by GCC on some machines /usr/lib/crt0.o start-up routine /usr/lib/libc.a standard C library, see /usr/include standard directory for files as(1), ld(1), adb(1), dbx(1), gdb(1). Bugs should be reported to Bugs tend ac- tually to be fixed if they can be isolated, so it is in your in- terest to report them in such a way that they can be easily reproduced. Copyright (c) 1988 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission no- tice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the condi- tions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission no- tice 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 this permission notice may be included in translations approved by the Free Software Foundation instead of in the original En- glish. See the GNU CC Manual for the contributors to GNU CC.