C语言预处理命令

1.#error Directive (C/C++)

The #error directive emits a user-specified error message at compile
time and then terminates the compilation.

#error    token-string

The error message that this directive emits includes the token-string
parameter. The token-string parameter is not subject to macro expansion.
This directive is most useful during preprocessing for notifying the
developer of a program inconsistency or the violation of a constraint.
The following example demonstrates error processing during
preprocessing:

#if    !defined(__cplusplus)
#error     C++ compiler required.
#endif

2.#line Directive (C/C++)

The #line directive tells the preprocessor to change the compiler’s
internally stored line number and filename to a given line number and
filename.

#line    digit-sequence ["filename"]

The compiler uses the line number and optional filename to refer to
errors that it finds during compilation. The line number usually refers
to the current input line, and the filename refers to the current input
file. The line number is incremented after each line is processed.

The digit-sequence value can be any integer constant. Macro
replacement can be performed on the preprocessing tokens, but the result
must evaluate to the correct syntax. The filename can be any
combination of characters and must be enclosed in double quotation marks
(” “). Iffilename is omitted, the previous filename remains
unchanged.

You can alter the source line number and filename by writing a #line
directive. The translator uses the line number and filename to determine
the values of the predefined macros __FILE__ and
__LINE__. You can use these macros to insert self-descriptive
error messages into the program text. For more information on these
predefined macros, see Predefined Macros.

The __FILE__ macro expands to a string whose contents are the
filename, surrounded by double quotation marks (” “).

If you change the line number and filename, the compiler ignores the
previous values and continues processing with the new values. The
#linedirective is typically used by program generators to cause error
messages to refer to the original source file instead of to the
generated program.

The following examples illustrate #line and the __LINE__ and
__FILE__ macros.

In this statement, the internally stored line number is set to 151 and
the filename is changed to copy.c.

#line    151 "copy.c"

In this example, the macro ASSERT uses the predefined macros
__LINE__ and __FILE__ to print an error message about
the source file if a given “assertion” is not true.

#define ASSERT(cond)
if( !(cond) )\
{printf( "assertion error line %d, file(%s)\n", \
__LINE__, __FILE__ );}

3.#undef Directive (C/C++)

Removes (undefines) a name previously created with #define.

#undef    identifier

The #undef directive removes the current definition of identifier.
Consequently, subsequent occurrences of identifier are ignored by the
preprocessor. To remove a macro definition using #undef, give only the
macro identifier ; do not give a parameter list.

You can also apply the #undef directive to an identifier that has no
previous definition. This ensures that the identifier is undefined.
Macro replacement is not performed within #undef statements.

The #undef directive is typically paired with a #define directive to
create a region in a source program in which an identifier has a special
meaning. For example, a specific function of the source program can use
manifest constants to define environment-specific values that do not
affect the rest of the program. The #undef directive also works with
the #if directive to control conditional compilation of the source
program.See The #if, #elif, #else, and #endif Directives for more
information.

In the following example, the #undef directive removes definitions of a
symbolic constant and a macro. Note that only the identifier of the
macro is given.

#define WIDTH 80
#define ADD( X, Y ) ((X) + (Y))
.
.
.
#undef WIDTH
#undef ADD

Microsoft Specific

Macros can be undefined from the command line using the /U option,
followed by the macro names to be undefined. The effect of issuing this
command is equivalent to a sequence of #undef macro-name statements
at the beginning of the file.

END Microsoft Specific

Directive (C/C++) The #error directive
emits a user-specified error message at compile time and then terminates
the compilation. #error token-string The error message that…

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