This code is derived from software copyrighted by the Free Software
Foundation.
Modified 1991 by Donn Seeley at UUNET Technologies, Inc.
@(#)cc.1 6.5 (Berkeley) 5/9/91
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 pervasive use of "function prototypes" . 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 declarations is that user code that inadvertently redefines a C library function may break; for example it is no longer possible to write an abort function that takes different parameters or returns a different value from the standard abort , 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 Variable argument lists are handled differently; the old varargs (3) package is obsolete, replaced by stdarg (3), which unfortunately is not completely compatible.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 sizeof operator, which now yields an unsigned type rather than a signed type. One common problem is due to a change in scoping: external declarations are now scoped to the block they occur in, so a declaration for (say) errno inside one block will no longer declare it in all subsequent blocks. The syntax for braces in structure initializations is now a bit stricter, and it is sometimes necessary to add braces to please the compiler. Two very subtle and sometimes very annoying features apply to constant strings and to the longjmp (3) 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 mktemp function, which in the past often altered a constant string argument. It is now necessary to copy a constant string (for example, with strdup (3)) before it may be altered. The longjmp function may now destroy any register or stack variable in the function that made the corresponding call to the setjmp function; to protect a local variable, the new ANSI volatile modifier must be used. This often leads to confusing situations upon `return' from setjmp . The compiler has extended warning flags for dealing with read-only strings and setjmp , but these are not very effective. If your code has problems with any of these ANSI features, you will probably want to use -traditional .
Even with -traditional , 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 cc 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 type ''; this cc treats such forms as syntax errors. The old C dialect also permitted formal parameters to have the same names as typedef types; the current dialect does not.
Some questionable or illegal practices that were supported in the old C dialect are not supported by -traditional : non-comment text at the end of a ``#include'' preprocessor control line is an error, not ignored; compound assignment operators must not contain white space, e.g. ``*\0='' is not the same as ``*=''; the last member declaration in a structure or union must be terminated by a semicolon; 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 declaring 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 -traditional 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).
The GNU C compiler uses a command syntax much like the Unix C compiler. The cc program accepts options and file names as operands. Multiple single-letter options may not be grouped: -dr is very different from "-d -r" .
When you invoke GNU CC, it normally does preprocessing, compilation, assembly and linking. File names which end in .c are taken as C source to be preprocessed and compiled; file names ending in .i are taken as preprocessor output to be compiled; compiler output files plus any input files with names ending in .s are assembled; then the resulting object files, plus any other input files, are linked together to produce an executable.
Command options allow you to stop this process at an intermediate stage. For example, the -c 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 itself. Yet other options control the assembler and linker; these are not documented here, but you rarely need to use any of them.
-o " file" Place output in file file . This applies regardless to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code. If -o is not specified, the default is to put an executable file in a.out , the object file source .c in source .o, an assembler file in source .s, and preprocessed C on standard output.
-c Compile or assemble the source files, but do not link. Produce object files with names made by replacing .c or .s with .o at the end of the input file names. Do nothing at all for object files specified as input.
-S Compile into assembler code but do not assemble. The assembler output file name is made by replacing .c with .s at the end of the input file name. Do nothing at all for assembler source files or object files specified as input.
-E Run only the C preprocessor. Preprocess all the C source files specified and output the results to standard output.
-v Compiler 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.
-pipe 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 assembler has no trouble.
-B prefix Compiler driver program tries prefix as a prefix for each program it tries to run. These programs are cpp , cc1 , as and ld . For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B was not specified, the driver tries a standard prefix, which currently is /usr/libexec/ . If this does not result in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable. You can get a similar result from the environment variable GCC_EXEC_PREFIX ; if it is defined, its value is used as a prefix in the same way. If both the -B option and the GCC_EXEC_PREFIX variable are present, the -B option is used first and the environment variable value second.
-b prefix The argument prefix is used as a second prefix for the compiler executables and libraries. This prefix is optional: the compiler tries each file first with it, then without it. This prefix follows the prefix specified with -B or the default prefixes. Thus, -bvax- -Bcc/ in the presence of environment variable GCC_EXEC_PREFIX with definition /u/foo/ causes GNU CC to try the following file names for the preprocessor executable: cc/vax-cpp
These options control the details of C compilation itself.
-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. The alternate keywords __asm__ , __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 . 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.
-traditional Attempt to support some aspects of traditional C compilers. Specifically: P All extern declarations take effect globally even if they are written inside of a function definition. This includes implicit declarations of functions. P The keywords typeof , inline , signed , const and volatile are not recognized. P Comparisons between pointers and integers are always allowed. P Integer types "unsigned short" and "unsigned char" promote to "unsigned int" . P Out-of-range floating point literals are not an error. P All automatic variables not declared register are preserved by longjmp (3C). Ordinarily, GNU C follows ANSI C: automatic variables not declared volatile may be clobbered. P In the preprocessor, comments convert to nothing at all, rather than to a space. This allows traditional token concatenation. P 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. P 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.
-O Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function. Without -O , 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 variable 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 -O , only variables declared register are allocated in registers. The resulting compiled code is a little worse than produced by PCC without -O . With -O , the compiler tries to reduce code size and execution time. Some of the -f options described below turn specific kinds of optimization on or off.
-g Produce debugging information in the operating 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 -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.
-w Inhibit all warning messages.
-W Print extra warning messages for these events: P 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 as volatile all the functions you use that never return. P A nonvolatile automatic variable might be changed by a call to longjmp (3C). These warnings as well are possible only in optimizing compilation. The compiler sees only the calls to setjmp (3C). It cannot know where longjmp (3C) 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 (3C) cannot in fact be called at the place which would cause a problem. P 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 abort (3C) and longjmp (3C)) will never return. P An expression-statement contains no side effects. In the future, other useful warnings may also be enabled by this option.
-Wimplicit Warn whenever a function 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, and whenever a function is declared static but never defined.
-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).
-Wall All of the above -W 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 -W ... options below are not implied by -Wall because certain kinds of useful macros are almost impossible to write without causing those warnings.
-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 \(**" .
-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.
-p Generate extra code to write profile information suitable for the analysis program prof (1).
-pg Generate extra code to write profile information suitable for the analysis program gprof (1).
-a Generate extra code to write profile information for basic blocks, suitable for the analysis program tcov (1). Eventually GNU gprof (1) should be extended to process this data.
-l library Search a standard list of directories for a library named library , which is actually a file named liblibrary.a . The linker uses this file as if it had been specified precisely by name. The directories searched include several standard system directories plus any that you specify with -L . Normally the files found this way are library files--archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols 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 -l option and specifying a file name is that -l searches several directories.
-L dir Add directory dir to the list of directories to be searched for -l .
-nostdlib Don't use the standard system libraries and startup files when linking. Only the files you specify (plus gnulib ) will be passed to the linker.
-m machinespec Machine-dependent option specifying something about the type of target machine. These options are defined by the macro TARGET_SWITCHES in the machine description. The default for the options is also defined by that macro, which enables you to change the defaults. These are the -m options defined in the 68000 machine description: -m68020
-mc68020 Generate output for a 68020 (rather than a 68000). This is the default if you use the unmodified sources. -m68000
-mc68000 Generate output for a 68000 (rather than a 68020). -m68881 Generate output containing 68881 instructions for floating point. This is the default if you use the unmodified sources. -mfpa Generate output containing Sun FPA instructions for floating point. -msoft-float Generate output containing library calls for floating point. -mshort Consider type int to be 16 bits wide, like "short int" . -mnobitfield Do not use the bit-field instructions. -m68000 implies -mnobitfield . -mbitfield Do use the bit-field instructions. -m68020 implies -mbitfield . This is the default if you use the unmodified sources. -mrtd Use a different function-calling convention, in which functions that take a fixed number of arguments return with the rtd 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 normally 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 printf (3S)); otherwise incorrect code will be generated for calls to those functions. In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) The rtd instruction is supported by the 68010 and 68020 processors, but not by the 68000. These -m options are defined in the Vax machine description: -munix Do not output certain jump instructions ( aobleq and so on) that the Unix assembler for the Vax cannot handle across long ranges. -mgnu Do output those jump instructions, on the assumption that you will assemble with the GNU assembler. -mg Output code for g-format floating point numbers instead of d-format. These -m switches are supported on the Sparc: -mfpu Generate output containing floating point instructions. This is the default if you use the unmodified sources. -msoft-float Generate output containing library calls for floating point. -mno-epilogue Generate separate return instructions for return statements. This has both advantages and disadvantages; I don't recall what they are. These -m options are defined in the Convex machine description: -mc1 Generate output for a C1. This is the default when the compiler is configured for a C1. -mc2 Generate output for a C2. This is the default when the compiler is configured for a C2. -margcount 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.) -mnoargcount Omit the argument count word. This is the default if you use the unmodified sources.
-f flag Specify machine-independent flags. Most flags have both positive and negative forms; the negative form of -ffoo would be -fno-foo . 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 no- or adding it.
-fpcc-struct-return Use the same convention for returning struct and union 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.
-ffloat-store 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 precision than a double 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 -ffloat-store for such programs.
-fno-asm Do not recognize asm , inline or typeof as a keyword. These words may then be used as identifiers. You can use __asm__ , __inline__ and __typeof__ instead.
-fno-defer-pop 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.
-fstrength-reduce Perform the optimizations of loop strength reduction and elimination of iteration variables.
-fcombine-regs Allow the combine pass to combine an instruction that copies one register into another. This might or might not produce better code when used in addition to -O . I am interested in hearing about the difference this makes.
-fforce-mem 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 subexpressions. When they are not common subexpressions, instruction combination should eliminate the separate register-load. I am interested in hearing about the difference this makes.
-fforce-addr Force memory address constants to be copied into registers before doing arithmetic on them. This may produce better code just as -fforce-mem may. I am interested in hearing about the difference this makes.
-fomit-frame-pointer 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 restore frame pointers; it also makes an extra register available in many functions. "It also makes debugging impossible." On some machines, such as the Vax, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag.
-finline-functions Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be worth integrating in this way. If all calls to a given function are integrated, and the function is declared static , then the function is normally not output as assembler code in its own right.
-fcaller-saves Enable values to be allocated in registers 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.
-fkeep-inline-functions Even if all calls to a given function are integrated, and the function is declared static , nevertheless output a separate run-time callable version of the function.
-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. Writing into string constants is a very bad idea; constants should be constant.
-fcond-mismatch Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void.
-fno-function-cse Do not put function addresses 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 optimizations performed when this option is not used.
-fvolatile Consider all memory references through pointers to be volatile.
-fshared-data Requests that the data and non- const 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.
-funsigned-char Let the type char be the 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. (Actually, at present, the default is always signed.) The type char is always a distinct type from either "signed char" or "unsigned char" , even though its behavior is always just like one of those two. Note that this is equivalent to -fno-signed-char , which is the negative form of -fsigned-char .
-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 .
-fdelayed-branch If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.
-ffixed- reg Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role). reg must be the name of a register. The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file. This flag does not have a negative form, because it specifies a three-way choice.
-fcall-used- reg Treat the register named reg as an allocatable register 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 register 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.
-fcall-saved- reg Treat the register named reg as an allocatable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way will save and restore the register reg 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.
-d letters Says to make debugging dumps at times specified by letters . Here are the possible letters: r Dump after RTL generation. j Dump after first jump optimization. J Dump after last jump optimization. s Dump after CSE (including the jump optimization that sometimes follows CSE). L Dump after loop optimization. f Dump after flow analysis. c Dump after instruction combination. l Dump after local register allocation. g Dump after global register allocation. d Dump after delayed branch scheduling. m Print statistics on memory usage, at the end of the run.
-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. There is no reason to use this option; it exists only to satisfy pedants. -pedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __ .
-static On Suns running version 4, this prevents linking with the shared libraries. ( -g has the same effect.)
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.
-C Tell the preprocessor not to discard comments. Used with the -E option.
-I dir Search directory dir for include files.
-I- Any directories specified with -I options before the -I- option are searched only for the case of #include "file"; they are not searched for "#include <file>" . If additional directories are specified with -I options after the -I- , these directories are searched for all #include directives. (Ordinarily all -I directories are used this way.) In addition, the -I- option inhibits the use of the current directory as the first search directory for #include "file". Therefore, the current directory is searched only if it is requested explicitly with -I. . Specifying both -I- and -I. allows you to control precisely which directories are searched before the current one and which are searched after.
-nostdinc Do not search the standard system directories for header files. Only the directories you have specified with -I options (and the current directory, if appropriate) are searched. Between -nostdinc and -I- , you can eliminate all directories from the search path except those you specify.
-M Tell the preprocessor to output a rule suitable for make (1) describing the dependencies of each source file. For each source file, the preprocessor outputs one make -rule whose target is the object file name for that source file and whose dependencies are all the files #include d in it. This rule may be a single line or may be continued with \\\-newline if it is long. -M implies -E .
-MM Like -M but the output mentions only the user-header files included with #include "file". System header files included with "#include <file>" are omitted. -MM implies -E .
-D macro Define macro macro with the empty string as its definition.
-D macro=defn Define macro macro as defn .
-U macro Undefine macro macro .
-trigraphs Support ANSI C trigraphs. You don't want to know about this brain-damage. The -ansi option also has this effect.
file.s assembly language file
file.o object file
a.out link edited output
/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 intro (3)
/usr/include standard directory for #include files
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