1# 2# @(#)README 1.2 08/30/82 3# 4August 30, 1982 5Fixed by Kurt Shoens, UCB 6If the "#line n name" occurs, then all future references 7to the current file are generated in terms of "name", instead 8of the name of file given to cpp in its command argument 9 10August 25, 1978 11 12Files in this directory form the C preprocessor, which handles '#include' 13files and macro definition and expansion for the C compiler. 14This new version was written by John F. Reiser and is from 5 to 12 15times faster (on UNIX systems) than the old. 16 17To create the executable file 'cpp' in the current directory: 18 make 19 20To install the preprocessor 'cpp' so it will be used by the C compiler: 21 : safety first: backup the existing version 22 cp /lib/cpp /lib/ocpp 23 : install the new version 24 cp cpp /lib/cpp 25 26Invocation 27 cpp [-CEPR] [-Dname] ... [-Dname=def] ... [-Idirectory] ... 28 [-Uname] ... [<infile>] [<outfile>] 29 30 If there are two non-flag arguments then the first is the name of the 31 input file and the second is the name of the output file. If there is 32 one non-flag argument then it is the name of the input file and the 33 output is written on the standard output. If there are no non-flag 34 arguments then the input is taken from the standard input and the output 35 is written on the standard output. Flag arguments are: 36 37 -C retain comments in output 38 -Dname define name as "1" 39 -Dname=def define name as def 40 -E ignored 41 -Idirectory add directory to search list for #include files 42 -P don't insert lines "# 12 \"foo.c\"" into output 43 -R allow recursive macros 44 -Uname undefine name 45 46Documentation clarifications: 47 Symbols defined on the command line by "-Dfoo" are defined as "1", 48 i.e., as if they had been defined by "#define foo 1" or "-Dfoo=1". 49 The directory search order for #include files is 50 1) the directory of the file which contains the #include request 51 (e.g. #include is relative to the file being scanned when 52 the request is made) 53 2) the directories specified by -I, in left-to-right order 54 3) the standard directory(s) (which for UNIX is /usr/include) 55 An unescaped linefeed (the single character "\n") terminates a 56 character constant or quoted string. 57 An escaped linefeed (the two-character sequence "\\\n") may be 58 used in the body of a '#define' statement to continue 59 the definition onto the next line. The escaped linefeed is 60 not included in the macro body. 61 Comments are uniformly removed (except if the argument -C is specified). 62 They are also ignored, except that a comment terminates a token. 63 Thus "foo/* la di da */bar" may expand 'foo' and 'bar' but 64 will never expand 'foobar'. If neither 'foo' nor 'bar' is a 65 macro then the output is "foobar", even if 'foobar' 66 is defined as something else. The file 67 #define foo(a,b)b/**/a 68 foo(1,2) 69 produces "21" because the comment causes a break which enables 70 the recognition of 'b' and 'a' as formals in the string "b/**/a". 71 Macro formal parameters are recognized in '#define' bodies even inside 72 character constants and quoted strings. The output from 73 #define foo(a) '\a' 74 foo(bar) 75 is the seven characters " '\\bar'". Macro names are not recognized 76 inside character constants or quoted strings during the regular scan. 77 Thus 78 #define foo bar 79 printf("foo"); 80 does not expand 'foo' in the second line, because it is inside 81 a quoted string which is not part of a '#define' macro definition. 82 Macros are not expanded while processing a '#define' or '#undef'. 83 Thus 84 #define foo bletch 85 #define bar foo 86 #undef foo 87 bar 88 produces "foo". The token appearing immediately after a 89 '#ifdef' or '#ifndef' is not expanded (of course!). 90 Macros are not expanded during the scan which determines the actual 91 parameters to another macro call. Thus 92 #define foo(a,b)b a 93 #define bar hi 94 foo(bar, 95 #define bar bye 96 ) 97 produces " bye" (and warns about the redefinition of 'bar'). 98 99There are some differences between the new and the old preprocessor. 100Bugs fixed: 101 "1.e4" is recognized as a floating-point number, rather than as an 102 opportunity to expand the possible macro name "e4". 103 Any kind and amount of white space (space, tab, linefeed, vertical tab, 104 formfeed, carriage return) is allowed between a macro name and 105 the left parenthesis which introduces its actual parameters. 106 The comma operator is legal in preprocessor '#if' statements. 107 Macros with parameters are legal in preprocessor '#if' statements. 108 Single-character character constants are legal in preprocessor '#if' statements. 109 Linefeeds are put out in the proper place when a multiline comment 110 is not passed through to the output. 111 The following example expands to "# # #" : 112 #define foo # 113 foo foo foo 114 If the -R flag is not specified then the invocation of some recursive 115 macros is trapped and the recursion forcibly terminated with an 116 error message. The recursions that are trapped are the ones 117 in which the nesting level is non-decreasing from some point on. 118 In particular, 119 #define a a 120 a 121 will be detected. (Use "#undef a" if that is what you want.) 122 The recursion 123 #define a c b 124 #define b c a 125 #define c foo 126 a 127 will not be detected because the nesting level decreases after 128 each expansion of "c". 129 The -R flag specifically allows recursive macros and recursion will 130 be strictly obeyed (to the extent that space is available). 131 Assuming that -R is specified: 132 #define a a 133 a 134 causes an infinite loop with very little output. The tail recursion 135 #define a <b 136 #define b >a 137 a 138 causes the string "<>" to be output infinitely many times. The 139 non-tail recursion 140 #define a b> 141 #define b a< 142 a 143 complains "too much pushback", dumps the pushback, and continues 144 (again, infinitely). 145 146Stylistic choice: 147 Nothing (not even linefeeds) is output while a false '#if', '#ifdef', 148 or '#ifndef' is in effect. Thus when all conditions become true 149 a line of the form "# 12345 \"foo.c\"" is output (unless -P). 150 Error and warning messages always appear on standard error (file 151 descriptor 2). 152 Mismatch between the number of formals and actuals in a macro call 153 produces only a warning, and not an error. Excess actuals 154 are ignored; missing actuals are turned into null strings. 155 156Incompatibility: 157 The virgule '/' in "a=/*b" is interpreted as the first character of 158 the pair "/*" which introduces a comment, rather than as the 159 second character of the divide-and-replace operator "=/". 160 This incompatibility reflects the recent change in the C language 161 which made "a/=*b" the legal way to write such a statement 162 if the meaning "a=a/ *b" is intended. 163