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1.1 root 1:
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3: C preprocessor Overview C preprocessor
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8: Preprocessing encompasses all tasks that logically precede the
9: translation of a program. The preprocessor processes headers,
10: expands macros, and conditionally includes or excludes source
11: code.
12:
13: ***** Directives *****
14:
15: The C preprocessor recognizes the following directives:
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17:
18: #iiff Include code if a condition is true
19: #eelliiff Include code if directive is true
20: #eellssee Include code if preceding directives fail
21: #eennddiiff End of code to be included conditionally
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24:
25: #iiffddeeff Include code if a given macro is defined
26: #iiffnnddeeff Include code if a given macro is not defined
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30: #ddeeffiinnee Define a macro
31: #uunnddeeff Undefine a macro
32: #iinncclluuddee Read another file and include it
33: #lliinnee Reset current line number
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35:
36: A preprocessing directive is always introduced by the `#' charac-
37: ter. The `#' must be the first non-white space character on a
38: line, but it may be preceded by white space and it may be
39: separated from the directive name that follows it by one or more
40: white space characters.
41:
42: ***** Preprocessing Operators *****
43:
44: The Standard defines two operators that are recognized by the
45: preprocessor: the ``stringize'' operator #, and the ``token-
46: paste'' operator ##.
47:
48: The operator # indicates that the following argument is to be
49: replaced by a string literal; this literal names the preproces-
50: sing token that replaces the argument. For example, consider the
51: macro:
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53:
54: #define display(x) show((long)(x), #x)
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56:
57: When the preprocessor reads the line
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64: COHERENT Lexicon Page 1
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69: C preprocessor Overview C preprocessor
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73: display(abs(-5));
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75:
76: it replaces it with the following:
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79: show((long)(abs(-5)), "abs(-5)");
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81:
82: The ## operator performs ``token pasting'' -- that is, it joins
83: two tokens together, to create a single token. For example, con-
84: sider the macro:
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86:
87: #define printvar(x) printf("%d\n", variable ## x)
88:
89:
90: When the preprocessor reads the line
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92:
93: printvar(3);
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95:
96: it translates it into:
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99: printf("%d\n", variable3);
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101:
102: In the past, token pasting had been performed by inserting a
103: comment between the tokens to be pasted. This no longer works.
104:
105: ***** Predefined Macros *****
106:
107: The ANSI Standard describes the following macros that must be
108: recognized by the preprocessor:
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110:
111: _ _DDAATTEE_ _Date of translation
112: _ _FFIILLEE_ _Source-file name
113: _ _LLIINNEE_ _Current line within source file
114: _ _SSTTDDCC_ _Conforming translator and level
115: _ _TTIIMMEE_ _Time of translation
116:
117:
118: For more information on any one of these macros, see its entry.
119:
120: ***** Conditional Inclusion *****
121:
122: The preprocessor will conditionally include lines of code within
123: a program. The directives that include code conditionally are
124: defined in such a way that you can construct a chain of inclusion
125: directives to include exactly the material you want.
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129:
130: COHERENT Lexicon Page 2
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133:
134:
135: C preprocessor Overview C preprocessor
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137:
138:
139: ***** Macro Definition and Replacement *****
140:
141: The preprocessor performs simple types of macro replacement. To
142: define a macro, use the preprocessor directive #ddeeffiinnee _i_d_e_n_t_i_f_i_e_r
143: _v_a_l_u_e. The preprocessor scans the translation unit for
144: preprocessor tokens that match identifier; when one is found, the
145: preprocessor substitutes value for it.
146:
147: ***** cpp *****
148:
149: Under COHERENT, C preprocessing is done by the program cpp. The
150: cc command runs cpp as the first step in compiling a C program.
151: cpp can also be run by itself.
152:
153: cpp reads each input file; it processes directives, and writes
154: its product on ssttddoouutt.
155:
156: If the -E option is not used, cpp also writes into its output
157: statements of the form #lliinnee _n _f_i_l_e_n_a_m_e, so that the parser cc0
158: can connect its error messages and debugger output with the
159: original line numbers in your source files.
160:
161: See the Lexicon entry on cpp for more information.
162:
163: ***** See Also *****
164:
165: C language, cc, cpp
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196: COHERENT Lexicon Page 3
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