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This is Info file cpp.info, produced by Makeinfo-1.55 from the input file cpp.texi. This file documents the GNU C Preprocessor. Copyright 1987, 1989, 1991, 1992, 1993, 1994, 1995 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 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. File: cpp.info, Node: Misnesting, Next: Macro Parentheses, Prev: Macro Pitfalls, Up: Macro Pitfalls Improperly Nested Constructs ............................ Recall that when a macro is called with arguments, the arguments are substituted into the macro body and the result is checked, together with the rest of the input file, for more macro calls. It is possible to piece together a macro call coming partially from the macro body and partially from the actual arguments. For example, #define double(x) (2*(x)) #define call_with_1(x) x(1) would expand `call_with_1 (double)' into `(2*(1))'. Macro definitions do not have to have balanced parentheses. By writing an unbalanced open parenthesis in a macro body, it is possible to create a macro call that begins inside the macro body but ends outside of it. For example, #define strange(file) fprintf (file, "%s %d", ... strange(stderr) p, 35) This bizarre example expands to `fprintf (stderr, "%s %d", p, 35)'! File: cpp.info, Node: Macro Parentheses, Next: Swallow Semicolon, Prev: Misnesting, Up: Macro Pitfalls Unintended Grouping of Arithmetic ................................. You may have noticed that in most of the macro definition examples shown above, each occurrence of a macro argument name had parentheses around it. In addition, another pair of parentheses usually surround the entire macro definition. Here is why it is best to write macros that way. Suppose you define a macro as follows, #define ceil_div(x, y) (x + y - 1) / y whose purpose is to divide, rounding up. (One use for this operation is to compute how many `int' objects are needed to hold a certain number of `char' objects.) Then suppose it is used as follows: a = ceil_div (b & c, sizeof (int)); This expands into a = (b & c + sizeof (int) - 1) / sizeof (int); which does not do what is intended. The operator-precedence rules of C make it equivalent to this: a = (b & (c + sizeof (int) - 1)) / sizeof (int); But what we want is this: a = ((b & c) + sizeof (int) - 1)) / sizeof (int); Defining the macro as #define ceil_div(x, y) ((x) + (y) - 1) / (y) provides the desired result. However, unintended grouping can result in another way. Consider `sizeof ceil_div(1, 2)'. That has the appearance of a C expression that would compute the size of the type of `ceil_div (1, 2)', but in fact it means something very different. Here is what it expands to: sizeof ((1) + (2) - 1) / (2) This would take the size of an integer and divide it by two. The precedence rules have put the division outside the `sizeof' when it was intended to be inside. Parentheses around the entire macro definition can prevent such problems. Here, then, is the recommended way to define `ceil_div': #define ceil_div(x, y) (((x) + (y) - 1) / (y)) File: cpp.info, Node: Swallow Semicolon, Next: Side Effects, Prev: Macro Parentheses, Up: Macro Pitfalls Swallowing the Semicolon ........................ Often it is desirable to define a macro that expands into a compound statement. Consider, for example, the following macro, that advances a pointer (the argument `p' says where to find it) across whitespace characters: #define SKIP_SPACES (p, limit) \ { register char *lim = (limit); \ while (p != lim) { \ if (*p++ != ' ') { \ p--; break; }}} Here Backslash-Newline is used to split the macro definition, which must be a single line, so that it resembles the way such C code would be laid out if not part of a macro definition. A call to this macro might be `SKIP_SPACES (p, lim)'. Strictly speaking, the call expands to a compound statement, which is a complete statement with no need for a semicolon to end it. But it looks like a function call. So it minimizes confusion if you can use it like a function call, writing a semicolon afterward, as in `SKIP_SPACES (p, lim);' But this can cause trouble before `else' statements, because the semicolon is actually a null statement. Suppose you write if (*p != 0) SKIP_SPACES (p, lim); else ... The presence of two statements--the compound statement and a null statement--in between the `if' condition and the `else' makes invalid C code. The definition of the macro `SKIP_SPACES' can be altered to solve this problem, using a `do ... while' statement. Here is how: #define SKIP_SPACES (p, limit) \ do { register char *lim = (limit); \ while (p != lim) { \ if (*p++ != ' ') { \ p--; break; }}} \ while (0) Now `SKIP_SPACES (p, lim);' expands into do {...} while (0); which is one statement. File: cpp.info, Node: Side Effects, Next: Self-Reference, Prev: Swallow Semicolon, Up: Macro Pitfalls Duplication of Side Effects ........................... Many C programs define a macro `min', for "minimum", like this: #define min(X, Y) ((X) < (Y) ? (X) : (Y)) When you use this macro with an argument containing a side effect, as shown here, next = min (x + y, foo (z)); it expands as follows: next = ((x + y) < (foo (z)) ? (x + y) : (foo (z))); where `x + y' has been substituted for `X' and `foo (z)' for `Y'. The function `foo' is used only once in the statement as it appears in the program, but the expression `foo (z)' has been substituted twice into the macro expansion. As a result, `foo' might be called two times when the statement is executed. If it has side effects or if it takes a long time to compute, the results might not be what you intended. We say that `min' is an "unsafe" macro. The best solution to this problem is to define `min' in a way that computes the value of `foo (z)' only once. The C language offers no standard way to do this, but it can be done with GNU C extensions as follows: #define min(X, Y) \ ({ typeof (X) __x = (X), __y = (Y); \ (__x < __y) ? __x : __y; }) If you do not wish to use GNU C extensions, the only solution is to be careful when *using* the macro `min'. For example, you can calculate the value of `foo (z)', save it in a variable, and use that variable in `min': #define min(X, Y) ((X) < (Y) ? (X) : (Y)) ... { int tem = foo (z); next = min (x + y, tem); } (where we assume that `foo' returns type `int'). File: cpp.info, Node: Self-Reference, Next: Argument Prescan, Prev: Side Effects, Up: Macro Pitfalls Self-Referential Macros ....................... A "self-referential" macro is one whose name appears in its definition. A special feature of ANSI Standard C is that the self-reference is not considered a macro call. It is passed into the preprocessor output unchanged. Let's consider an example: #define foo (4 + foo) where `foo' is also a variable in your program. Following the ordinary rules, each reference to `foo' will expand into `(4 + foo)'; then this will be rescanned and will expand into `(4 + (4 + foo))'; and so on until it causes a fatal error (memory full) in the preprocessor. However, the special rule about self-reference cuts this process short after one step, at `(4 + foo)'. Therefore, this macro definition has the possibly useful effect of causing the program to add 4 to the value of `foo' wherever `foo' is referred to. In most cases, it is a bad idea to take advantage of this feature. A person reading the program who sees that `foo' is a variable will not expect that it is a macro as well. The reader will come across the identifier `foo' in the program and think its value should be that of the variable `foo', whereas in fact the value is four greater. The special rule for self-reference applies also to "indirect" self-reference. This is the case where a macro X expands to use a macro `y', and the expansion of `y' refers to the macro `x'. The resulting reference to `x' comes indirectly from the expansion of `x', so it is a self-reference and is not further expanded. Thus, after #define x (4 + y) #define y (2 * x) `x' would expand into `(4 + (2 * x))'. Clear? But suppose `y' is used elsewhere, not from the definition of `x'. Then the use of `x' in the expansion of `y' is not a self-reference because `x' is not "in progress". So it does expand. However, the expansion of `x' contains a reference to `y', and that is an indirect self-reference now because `y' is "in progress". The result is that `y' expands to `(2 * (4 + y))'. It is not clear that this behavior would ever be useful, but it is specified by the ANSI C standard, so you may need to understand it. File: cpp.info, Node: Argument Prescan, Next: Cascaded Macros, Prev: Self-Reference, Up: Macro Pitfalls Separate Expansion of Macro Arguments ..................................... We have explained that the expansion of a macro, including the substituted actual arguments, is scanned over again for macro calls to be expanded. What really happens is more subtle: first each actual argument text is scanned separately for macro calls. Then the results of this are substituted into the macro body to produce the macro expansion, and the macro expansion is scanned again for macros to expand. The result is that the actual arguments are scanned *twice* to expand macro calls in them. Most of the time, this has no effect. If the actual argument contained any macro calls, they are expanded during the first scan. The result therefore contains no macro calls, so the second scan does not change it. If the actual argument were substituted as given, with no prescan, the single remaining scan would find the same macro calls and produce the same results. You might expect the double scan to change the results when a self-referential macro is used in an actual argument of another macro (*note Self-Reference::.): the self-referential macro would be expanded once in the first scan, and a second time in the second scan. But this is not what happens. The self-references that do not expand in the first scan are marked so that they will not expand in the second scan either. The prescan is not done when an argument is stringified or concatenated. Thus, #define str(s) #s #define foo 4 str (foo) expands to `"foo"'. Once more, prescan has been prevented from having any noticeable effect. More precisely, stringification and concatenation use the argument as written, in un-prescanned form. The same actual argument would be used in prescanned form if it is substituted elsewhere without stringification or concatenation. #define str(s) #s lose(s) #define foo 4 str (foo) expands to `"foo" lose(4)'. You might now ask, "Why mention the prescan, if it makes no difference? And why not skip it and make the preprocessor faster?" The answer is that the prescan does make a difference in three special cases: * Nested calls to a macro. * Macros that call other macros that stringify or concatenate. * Macros whose expansions contain unshielded commas. We say that "nested" calls to a macro occur when a macro's actual argument contains a call to that very macro. For example, if `f' is a macro that expects one argument, `f (f (1))' is a nested pair of calls to `f'. The desired expansion is made by expanding `f (1)' and substituting that into the definition of `f'. The prescan causes the expected result to happen. Without the prescan, `f (1)' itself would be substituted as an actual argument, and the inner use of `f' would appear during the main scan as an indirect self-reference and would not be expanded. Here, the prescan cancels an undesirable side effect (in the medical, not computational, sense of the term) of the special rule for self-referential macros. But prescan causes trouble in certain other cases of nested macro calls. Here is an example: #define foo a,b #define bar(x) lose(x) #define lose(x) (1 + (x)) bar(foo) We would like `bar(foo)' to turn into `(1 + (foo))', which would then turn into `(1 + (a,b))'. But instead, `bar(foo)' expands into `lose(a,b)', and you get an error because `lose' requires a single argument. In this case, the problem is easily solved by the same parentheses that ought to be used to prevent misnesting of arithmetic operations: #define foo (a,b) #define bar(x) lose((x)) The problem is more serious when the operands of the macro are not expressions; for example, when they are statements. Then parentheses are unacceptable because they would make for invalid C code: #define foo { int a, b; ... } In GNU C you can shield the commas using the `({...})' construct which turns a compound statement into an expression: #define foo ({ int a, b; ... }) Or you can rewrite the macro definition to avoid such commas: #define foo { int a; int b; ... } There is also one case where prescan is useful. It is possible to use prescan to expand an argument and then stringify it--if you use two levels of macros. Let's add a new macro `xstr' to the example shown above: #define xstr(s) str(s) #define str(s) #s #define foo 4 xstr (foo) This expands into `"4"', not `"foo"'. The reason for the difference is that the argument of `xstr' is expanded at prescan (because `xstr' does not specify stringification or concatenation of the argument). The result of prescan then forms the actual argument for `str'. `str' uses its argument without prescan because it performs stringification; but it cannot prevent or undo the prescanning already done by `xstr'. File: cpp.info, Node: Cascaded Macros, Next: Newlines in Args, Prev: Argument Prescan, Up: Macro Pitfalls Cascaded Use of Macros ...................... A "cascade" of macros is when one macro's body contains a reference to another macro. This is very common practice. For example, #define BUFSIZE 1020 #define TABLESIZE BUFSIZE This is not at all the same as defining `TABLESIZE' to be `1020'. The `#define' for `TABLESIZE' uses exactly the body you specify--in this case, `BUFSIZE'--and does not check to see whether it too is the name of a macro. It's only when you *use* `TABLESIZE' that the result of its expansion is checked for more macro names. This makes a difference if you change the definition of `BUFSIZE' at some point in the source file. `TABLESIZE', defined as shown, will always expand using the definition of `BUFSIZE' that is currently in effect: #define BUFSIZE 1020 #define TABLESIZE BUFSIZE #undef BUFSIZE #define BUFSIZE 37 Now `TABLESIZE' expands (in two stages) to `37'. (The `#undef' is to prevent any warning about the nontrivial redefinition of `BUFSIZE'.) File: cpp.info, Node: Newlines in Args, Prev: Cascaded Macros, Up: Macro Pitfalls Newlines in Macro Arguments --------------------------- Traditional macro processing carries forward all newlines in macro arguments into the expansion of the macro. This means that, if some of the arguments are substituted more than once, or not at all, or out of order, newlines can be duplicated, lost, or moved around within the expansion. If the expansion consists of multiple statements, then the effect is to distort the line numbers of some of these statements. The result can be incorrect line numbers, in error messages or displayed in a debugger. The GNU C preprocessor operating in ANSI C mode adjusts appropriately for multiple use of an argument--the first use expands all the newlines, and subsequent uses of the same argument produce no newlines. But even in this mode, it can produce incorrect line numbering if arguments are used out of order, or not used at all. Here is an example illustrating this problem: #define ignore_second_arg(a,b,c) a; c ignore_second_arg (foo (), ignored (), syntax error); The syntax error triggered by the tokens `syntax error' results in an error message citing line four, even though the statement text comes from line five. File: cpp.info, Node: Conditionals, Next: Combining Sources, Prev: Macros, Up: Top Conditionals ============ In a macro processor, a "conditional" is a directive that allows a part of the program to be ignored during compilation, on some conditions. In the C preprocessor, a conditional can test either an arithmetic expression or whether a name is defined as a macro. A conditional in the C preprocessor resembles in some ways an `if' statement in C, but it is important to understand the difference between them. The condition in an `if' statement is tested during the execution of your program. Its purpose is to allow your program to behave differently from run to run, depending on the data it is operating on. The condition in a preprocessing conditional directive is tested when your program is compiled. Its purpose is to allow different code to be included in the program depending on the situation at the time of compilation. * Menu: * Uses: Conditional Uses. What conditionals are for. * Syntax: Conditional Syntax. How conditionals are written. * Deletion: Deleted Code. Making code into a comment. * Macros: Conditionals-Macros. Why conditionals are used with macros. * Assertions:: How and why to use assertions. * Errors: #error Directive. Detecting inconsistent compilation parameters. File: cpp.info, Node: Conditional Uses, Next: Conditional Syntax, Up: Conditionals Why Conditionals are Used ------------------------- Generally there are three kinds of reason to use a conditional. * A program may need to use different code depending on the machine or operating system it is to run on. In some cases the code for one operating system may be erroneous on another operating system; for example, it might refer to library routines that do not exist on the other system. When this happens, it is not enough to avoid executing the invalid code: merely having it in the program makes it impossible to link the program and run it. With a preprocessing conditional, the offending code can be effectively excised from the program when it is not valid. * You may want to be able to compile the same source file into two different programs. Sometimes the difference between the programs is that one makes frequent time-consuming consistency checks on its intermediate data, or prints the values of those data for debugging, while the other does not. * A conditional whose condition is always false is a good way to exclude code from the program but keep it as a sort of comment for future reference. Most simple programs that are intended to run on only one machine will not need to use preprocessing conditionals. File: cpp.info, Node: Conditional Syntax, Next: Deleted Code, Prev: Conditional Uses, Up: Conditionals Syntax of Conditionals ---------------------- A conditional in the C preprocessor begins with a "conditional directive": `#if', `#ifdef' or `#ifndef'. *Note Conditionals-Macros::, for information on `#ifdef' and `#ifndef'; only `#if' is explained here. * Menu: * If: #if Directive. Basic conditionals using `#if' and `#endif'. * Else: #else Directive. Including some text if the condition fails. * Elif: #elif Directive. Testing several alternative possibilities. File: cpp.info, Node: #if Directive, Next: #else Directive, Up: Conditional Syntax The `#if' Directive ................... The `#if' directive in its simplest form consists of #if EXPRESSION CONTROLLED TEXT #endif /* EXPRESSION */ The comment following the `#endif' is not required, but it is a good practice because it helps people match the `#endif' to the corresponding `#if'. Such comments should always be used, except in short conditionals that are not nested. In fact, you can put anything at all after the `#endif' and it will be ignored by the GNU C preprocessor, but only comments are acceptable in ANSI Standard C. EXPRESSION is a C expression of integer type, subject to stringent restrictions. It may contain * Integer constants, which are all regarded as `long' or `unsigned long'. * Character constants, which are interpreted according to the character set and conventions of the machine and operating system on which the preprocessor is running. The GNU C preprocessor uses the C data type `char' for these character constants; therefore, whether some character codes are negative is determined by the C compiler used to compile the preprocessor. If it treats `char' as signed, then character codes large enough to set the sign bit will be considered negative; otherwise, no character code is considered negative. * Arithmetic operators for addition, subtraction, multiplication, division, bitwise operations, shifts, comparisons, and logical operations (`&&' and `||'). * Identifiers that are not macros, which are all treated as zero(!). * Macro calls. All macro calls in the expression are expanded before actual computation of the expression's value begins. Note that `sizeof' operators and `enum'-type values are not allowed. `enum'-type values, like all other identifiers that are not taken as macro calls and expanded, are treated as zero. The CONTROLLED TEXT inside of a conditional can include preprocessing directives. Then the directives inside the conditional are obeyed only if that branch of the conditional succeeds. The text can also contain other conditional groups. However, the `#if' and `#endif' directives must balance. File: cpp.info, Node: #else Directive, Next: #elif Directive, Prev: #if Directive, Up: Conditional Syntax The `#else' Directive ..................... The `#else' directive can be added to a conditional to provide alternative text to be used if the condition is false. This is what it looks like: #if EXPRESSION TEXT-IF-TRUE #else /* Not EXPRESSION */ TEXT-IF-FALSE #endif /* Not EXPRESSION */ If EXPRESSION is nonzero, and thus the TEXT-IF-TRUE is active, then `#else' acts like a failing conditional and the TEXT-IF-FALSE is ignored. Contrariwise, if the `#if' conditional fails, the TEXT-IF-FALSE is considered included. File: cpp.info, Node: #elif Directive, Prev: #else Directive, Up: Conditional Syntax The `#elif' Directive ..................... One common case of nested conditionals is used to check for more than two possible alternatives. For example, you might have #if X == 1 ... #else /* X != 1 */ #if X == 2 ... #else /* X != 2 */ ... #endif /* X != 2 */ #endif /* X != 1 */ Another conditional directive, `#elif', allows this to be abbreviated as follows: #if X == 1 ... #elif X == 2 ... #else /* X != 2 and X != 1*/ ... #endif /* X != 2 and X != 1*/ `#elif' stands for "else if". Like `#else', it goes in the middle of a `#if'-`#endif' pair and subdivides it; it does not require a matching `#endif' of its own. Like `#if', the `#elif' directive includes an expression to be tested. The text following the `#elif' is processed only if the original `#if'-condition failed and the `#elif' condition succeeds. More than one `#elif' can go in the same `#if'-`#endif' group. Then the text after each `#elif' is processed only if the `#elif' condition succeeds after the original `#if' and any previous `#elif' directives within it have failed. `#else' is equivalent to `#elif 1', and `#else' is allowed after any number of `#elif' directives, but `#elif' may not follow `#else'. File: cpp.info, Node: Deleted Code, Next: Conditionals-Macros, Prev: Conditional Syntax, Up: Conditionals Keeping Deleted Code for Future Reference ----------------------------------------- If you replace or delete a part of the program but want to keep the old code around as a comment for future reference, the easy way to do this is to put `#if 0' before it and `#endif' after it. This is better than using comment delimiters `/*' and `*/' since those won't work if the code already contains comments (C comments do not nest). This works even if the code being turned off contains conditionals, but they must be entire conditionals (balanced `#if' and `#endif'). Conversely, do not use `#if 0' for comments which are not C code. Use the comment delimiters `/*' and `*/' instead. The interior of `#if 0' must consist of complete tokens; in particular, singlequote characters must balance. But comments often contain unbalanced singlequote characters (known in English as apostrophes). These confuse `#if 0'. They do not confuse `/*'. File: cpp.info, Node: Conditionals-Macros, Next: Assertions, Prev: Deleted Code, Up: Conditionals Conditionals and Macros ----------------------- Conditionals are useful in connection with macros or assertions, because those are the only ways that an expression's value can vary from one compilation to another. A `#if' directive whose expression uses no macros or assertions is equivalent to `#if 1' or `#if 0'; you might as well determine which one, by computing the value of the expression yourself, and then simplify the program. For example, here is a conditional that tests the expression `BUFSIZE == 1020', where `BUFSIZE' must be a macro. #if BUFSIZE == 1020 printf ("Large buffers!\n"); #endif /* BUFSIZE is large */ (Programmers often wish they could test the size of a variable or data type in `#if', but this does not work. The preprocessor does not understand `sizeof', or typedef names, or even the type keywords such as `int'.) The special operator `defined' is used in `#if' expressions to test whether a certain name is defined as a macro. Either `defined NAME' or `defined (NAME)' is an expression whose value is 1 if NAME is defined as macro at the current point in the program, and 0 otherwise. For the `defined' operator it makes no difference what the definition of the macro is; all that matters is whether there is a definition. Thus, for example, #if defined (vax) || defined (ns16000) would succeed if either of the names `vax' and `ns16000' is defined as a macro. You can test the same condition using assertions (*note Assertions::.), like this: #if #cpu (vax) || #cpu (ns16000) If a macro is defined and later undefined with `#undef', subsequent use of the `defined' operator returns 0, because the name is no longer defined. If the macro is defined again with another `#define', `defined' will recommence returning 1. Conditionals that test whether just one name is defined are very common, so there are two special short conditional directives for this case. `#ifdef NAME' is equivalent to `#if defined (NAME)'. `#ifndef NAME' is equivalent to `#if ! defined (NAME)'. Macro definitions can vary between compilations for several reasons. * Some macros are predefined on each kind of machine. For example, on a Vax, the name `vax' is a predefined macro. On other machines, it would not be defined. * Many more macros are defined by system header files. Different systems and machines define different macros, or give them different values. It is useful to test these macros with conditionals to avoid using a system feature on a machine where it is not implemented. * Macros are a common way of allowing users to customize a program for different machines or applications. For example, the macro `BUFSIZE' might be defined in a configuration file for your program that is included as a header file in each source file. You would use `BUFSIZE' in a preprocessing conditional in order to generate different code depending on the chosen configuration. * Macros can be defined or undefined with `-D' and `-U' command options when you compile the program. You can arrange to compile the same source file into two different programs by choosing a macro name to specify which program you want, writing conditionals to test whether or how this macro is defined, and then controlling the state of the macro with compiler command options. *Note Invocation::. Assertions are usually predefined, but can be defined with preprocessor directives or command-line options. File: cpp.info, Node: Assertions, Next: #error Directive, Prev: Conditionals-Macros, Up: Conditionals Assertions ---------- "Assertions" are a more systematic alternative to macros in writing conditionals to test what sort of computer or system the compiled program will run on. Assertions are usually predefined, but you can define them with preprocessing directives or command-line options. The macros traditionally used to describe the type of target are not classified in any way according to which question they answer; they may indicate a hardware architecture, a particular hardware model, an operating system, a particular version of an operating system, or specific configuration options. These are jumbled together in a single namespace. In contrast, each assertion consists of a named question and an answer. The question is usually called the "predicate". An assertion looks like this: #PREDICATE (ANSWER) You must use a properly formed identifier for PREDICATE. The value of ANSWER can be any sequence of words; all characters are significant except for leading and trailing whitespace, and differences in internal whitespace sequences are ignored. Thus, `x + y' is different from `x+y' but equivalent to `x + y'. `)' is not allowed in an answer. Here is a conditional to test whether the answer ANSWER is asserted for the predicate PREDICATE: #if #PREDICATE (ANSWER) There may be more than one answer asserted for a given predicate. If you omit the answer, you can test whether *any* answer is asserted for PREDICATE: #if #PREDICATE Most of the time, the assertions you test will be predefined assertions. GNU C provides three predefined predicates: `system', `cpu', and `machine'. `system' is for assertions about the type of software, `cpu' describes the type of computer architecture, and `machine' gives more information about the computer. For example, on a GNU system, the following assertions would be true: #system (gnu) #system (mach) #system (mach 3) #system (mach 3.SUBVERSION) #system (hurd) #system (hurd VERSION) and perhaps others. The alternatives with more or less version information let you ask more or less detailed questions about the type of system software. On a Unix system, you would find `#system (unix)' and perhaps one of: `#system (aix)', `#system (bsd)', `#system (hpux)', `#system (lynx)', `#system (mach)', `#system (posix)', `#system (svr3)', `#system (svr4)', or `#system (xpg4)' with possible version numbers following. Other values for `system' are `#system (mvs)' and `#system (vms)'. *Portability note:* Many Unix C compilers provide only one answer for the `system' assertion: `#system (unix)', if they support assertions at all. This is less than useful. An assertion with a multi-word answer is completely different from several assertions with individual single-word answers. For example, the presence of `system (mach 3.0)' does not mean that `system (3.0)' is true. It also does not directly imply `system (mach)', but in GNU C, that last will normally be asserted as well. The current list of possible assertion values for `cpu' is: `#cpu (a29k)', `#cpu (alpha)', `#cpu (arm)', `#cpu (clipper)', `#cpu (convex)', `#cpu (elxsi)', `#cpu (tron)', `#cpu (h8300)', `#cpu (i370)', `#cpu (i386)', `#cpu (i860)', `#cpu (i960)', `#cpu (m68k)', `#cpu (m88k)', `#cpu (mips)', `#cpu (ns32k)', `#cpu (hppa)', `#cpu (pyr)', `#cpu (ibm032)', `#cpu (rs6000)', `#cpu (sh)', `#cpu (sparc)', `#cpu (spur)', `#cpu (tahoe)', `#cpu (vax)', `#cpu (we32000)'. You can create assertions within a C program using `#assert', like this: #assert PREDICATE (ANSWER) (Note the absence of a `#' before PREDICATE.) Each time you do this, you assert a new true answer for PREDICATE. Asserting one answer does not invalidate previously asserted answers; they all remain true. The only way to remove an assertion is with `#unassert'. `#unassert' has the same syntax as `#assert'. You can also remove all assertions about PREDICATE like this: #unassert PREDICATE You can also add or cancel assertions using command options when you run `gcc' or `cpp'. *Note Invocation::. File: cpp.info, Node: #error Directive, Prev: Assertions, Up: Conditionals The `#error' and `#warning' Directives -------------------------------------- The directive `#error' causes the preprocessor to report a fatal error. The rest of the line that follows `#error' is used as the error message. You would use `#error' inside of a conditional that detects a combination of parameters which you know the program does not properly support. For example, if you know that the program will not run properly on a Vax, you might write #ifdef __vax__ #error Won't work on Vaxen. See comments at get_last_object. #endif *Note Nonstandard Predefined::, for why this works. If you have several configuration parameters that must be set up by the installation in a consistent way, you can use conditionals to detect an inconsistency and report it with `#error'. For example, #if HASH_TABLE_SIZE % 2 == 0 || HASH_TABLE_SIZE % 3 == 0 \ || HASH_TABLE_SIZE % 5 == 0 #error HASH_TABLE_SIZE should not be divisible by a small prime #endif The directive `#warning' is like the directive `#error', but causes the preprocessor to issue a warning and continue preprocessing. The rest of the line that follows `#warning' is used as the warning message. You might use `#warning' in obsolete header files, with a message directing the user to the header file which should be used instead. File: cpp.info, Node: Combining Sources, Next: Other Directives, Prev: Conditionals, Up: Top Combining Source Files ====================== One of the jobs of the C preprocessor is to inform the C compiler of where each line of C code came from: which source file and which line number. C code can come from multiple source files if you use `#include'; both `#include' and the use of conditionals and macros can cause the line number of a line in the preprocessor output to be different from the line's number in the original source file. You will appreciate the value of making both the C compiler (in error messages) and symbolic debuggers such as GDB use the line numbers in your source file. The C preprocessor builds on this feature by offering a directive by which you can control the feature explicitly. This is useful when a file for input to the C preprocessor is the output from another program such as the `bison' parser generator, which operates on another file that is the true source file. Parts of the output from `bison' are generated from scratch, other parts come from a standard parser file. The rest are copied nearly verbatim from the source file, but their line numbers in the `bison' output are not the same as their original line numbers. Naturally you would like compiler error messages and symbolic debuggers to know the original source file and line number of each line in the `bison' input. `bison' arranges this by writing `#line' directives into the output file. `#line' is a directive that specifies the original line number and source file name for subsequent input in the current preprocessor input file. `#line' has three variants: `#line LINENUM' Here LINENUM is a decimal integer constant. This specifies that the line number of the following line of input, in its original source file, was LINENUM. `#line LINENUM FILENAME' Here LINENUM is a decimal integer constant and FILENAME is a string constant. This specifies that the following line of input came originally from source file FILENAME and its line number there was LINENUM. Keep in mind that FILENAME is not just a file name; it is surrounded by doublequote characters so that it looks like a string constant. `#line ANYTHING ELSE' ANYTHING ELSE is checked for macro calls, which are expanded. The result should be a decimal integer constant followed optionally by a string constant, as described above. `#line' directives alter the results of the `__FILE__' and `__LINE__' predefined macros from that point on. *Note Standard Predefined::. The output of the preprocessor (which is the input for the rest of the compiler) contains directives that look much like `#line' directives. They start with just `#' instead of `#line', but this is followed by a line number and file name as in `#line'. *Note Output::. File: cpp.info, Node: Other Directives, Next: Output, Prev: Combining Sources, Up: Top Miscellaneous Preprocessing Directives ====================================== This section describes three additional preprocessing directives. They are not very useful, but are mentioned for completeness. The "null directive" consists of a `#' followed by a Newline, with only whitespace (including comments) in between. A null directive is understood as a preprocessing directive but has no effect on the preprocessor output. The primary significance of the existence of the null directive is that an input line consisting of just a `#' will produce no output, rather than a line of output containing just a `#'. Supposedly some old C programs contain such lines. The ANSI standard specifies that the `#pragma' directive has an arbitrary, implementation-defined effect. In the GNU C preprocessor, `#pragma' directives are not used, except for `#pragma once' (*note Once-Only::.). However, they are left in the preprocessor output, so they are available to the compilation pass. The `#ident' directive is supported for compatibility with certain other systems. It is followed by a line of text. On some systems, the text is copied into a special place in the object file; on most systems, the text is ignored and this directive has no effect. Typically `#ident' is only used in header files supplied with those systems where it is meaningful. File: cpp.info, Node: Output, Next: Invocation, Prev: Other Directives, Up: Top C Preprocessor Output ===================== The output from the C preprocessor looks much like the input, except that all preprocessing directive lines have been replaced with blank lines and all comments with spaces. Whitespace within a line is not altered; however, a space is inserted after the expansions of most macro calls. Source file name and line number information is conveyed by lines of the form # LINENUM FILENAME FLAGS which are inserted as needed into the middle of the input (but never within a string or character constant). Such a line means that the following line originated in file FILENAME at line LINENUM. After the file name comes zero or more flags, which are `1', `2' or `3'. If there are multiple flags, spaces separate them. Here is what the flags mean: This indicates the start of a new file. This indicates returning to a file (after having included another file). This indicates that the following text comes from a system header file, so certain warnings should be suppressed.