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Chapter 4. Preprocessor
The preprocessor built into the Cx51 compiler handles directives found in the source file. The Cx51 compiler supports all of the ANSI Standard C directives. This chapter gives a brief overview of the preprocessor.
Directives
Preprocessor directives must be the first non-whitespace text specified on a line. All directives are prefixed with the pound or number-sign character (‘#’). For example:
#pragma
#include <stdio.h> #define DEBUG 1
The following table lists the preprocessor directives and gives a brief description |
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of each. |
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Directive |
Description |
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define |
Defines a preprocessor macro or constant. |
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elif |
Initiates an alternative branch of the if condition, when the previous if, ifdef, ifndef, |
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or elif branch was not taken. |
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else |
Initiates an alternative branch when the previous if, ifdef, or ifndef branch was not |
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taken. |
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endif |
Ends an if, ifdef, ifndef, elif, or else block. |
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error |
Outputs an error message defined by the user. This directive instructs the |
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compiler to emit the specified error message. |
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ifdef |
Evaluates an expression for conditional compilation. The argument to be evaluated |
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is the name of a definition. |
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ifndef |
Same as ifdef but the evaluation succeeds if the definition is not defined. |
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if |
Evaluates an expression for conditional compilation. |
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include |
Reads source text from an external file. The notation sequence determines the |
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search sequence of the included files. Cx51 searches for include files specified |
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with less-than/greater-than symbols (‘<’ ‘>’) in the include file directory. Cx51 |
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searches for include files specified with double-quotes (“ “) in the current directory. |
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line |
Specifies a line number together with an optional filename. These specifications |
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are used in error messages to identify the error position. |
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pragma |
Allows you to specify directives that may be included on the C51 command line. |
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Pragmas may contain the same directives that are specified on the command line. |
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undef |
Deletes a preprocessor macro or constant definition. |
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136 Chapter 4. Preprocessor
Stringize Operator
The stringize or number-sign operator (‘#’), when used within a macro definition, converts a macro parameter into a string constant. This operator may be used only in a macro that has a specified argument or parameter list.
When the stringize operator immediately precedes the name of one of the macro parameters, the parameter passed to the macro is enclosed within quotation marks and is treated as a string literal. For example:
#define stringer(x) printf (#x "\n")
stringer (text)
This example results in the following actual output from the preprocessor:
printf ("text\n")
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The expansion shows that the parameter is converted literally as if it were a |
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string. When the preprocessor stringizes the x parameter, the resulting line is: |
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printf ("text" "\n") |
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Because strings separated by whitespace are concatenated at compile time, these |
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two strings are combined into "text\n". |
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If the string passed as a parameter contains characters that should normally be |
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literalized or escaped (for example, " and \), the required \ character is |
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automatically added. |
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Token-pasting operator
The token-pasting operator (##) within a macro definition combines two arguments. It permits two separate tokens in the macro definition to be joined into a single token.
If the name of a macro parameter used in the macro definition is immediately preceded or followed by the token-pasting operator, the macro parameter and the token-pasting operator are replaced by the value of the passed parameter. Text that is adjacent to the token-pasting operator that is not the name of a macro parameter is not affected. For example:
#define paster(n) printf ("token" #n " = %d", token##n)
paster (9);
This example results in the following actual output from the preprocessor:
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printf ("token9 = %d", token9); |
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This example shows the concatenation of token##n into token9. Both the stringize and the token-pasting operators are used in this example.
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Chapter 4. Preprocessor |
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Predefined Macro Constants
The Cx51 compiler provides you with predefined constants to use in preprocessor directives and C code for more portable programs. The following table lists and describes each one.
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Constant |
Description |
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_ _C51_ _ |
Version number of the C51 compiler (for example, 610 for version 6.10). |
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_ _CX51_ _ |
Version number of the CX51 compiler (for example, 610 for version 6.10). |
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_ _DATE_ _ |
Date when the compilation was started in ANSI format (month dd yyyy). |
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_ _DATE2_ _ |
Date when the compilation in short form (mm/dd/yy). |
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_ _FILE_ _ |
Name of the file being compiled. |
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_ _LINE_ _ |
Current line number in the file being compiled. |
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_ _MODEL_ _ |
Memory model selected (0 for SMALL, 1 for COMPACT, 2 for LARGE). |
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_ _TIME_ _ |
Time when the compilation was started. |
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__STDC_ _ |
Defined to 1 to indicate full conformance with the ANSI C Standard. |
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