Matti Linsu <matti.linsu@xxxxxxxxx> writes: > I have been playing around with gcc Pre-Processor and I cannot explain one > behavior. > The code bellow demonstrate my question. > > Input file : > --------------------------------------- > #define START_WITH_A_NUMBER 0_start_with_a_number > #define START_WITH_A_LETTER a_start_with_a_letter > #define I_LOVE_SPACE I love space > START_WITH_A_NUMBER+0 > START_WITH_A_LETTER+0 > I_LOVE_SPACE > --------------------------------------- > > Pre-processed output (gcc -E -P myfile): > --------------------------------------- > 0_start_with_a_number +0 > a_start_with_a_letter+0 > I love space > --------------------------------------- > > 1) Why is there a space after '0_start_with_a_number' whereas no space is > present after 'a_start_with_a_letter'? > > 2) Why does gcc remove the spaces inside the replacement list of I_LOVE_SPACE? > 3) Does it serve a purpose? > 4) Is there a PP flag to force gcc to pre-process by the standard? To answer your last question first, the C99 standard describes a sequence of steps to compile a C program. gcc is fully standard compliant in that regard. The standard does not define the output of the preprocessor in a textual form. So there is no standard to which gcc should conform. > 1) Why is there a space after '0_start_with_a_number' whereas no space is > present after 'a_start_with_a_letter'? There is a space after 0_start_with_a_number because gcc emits a space whenever necessary to prevent tokens from accidentally pasting together. E.g., a space is required between '+' and '=' in preprocessor output. The test for whether a space is necessary is conservative. 0_start_with_a_number is not an identifier, so the conservative test says that a space is required. > 2) Why does gcc remove the spaces inside the replacement list of I_LOVE_SPACE? The spaces in the replacement list of I_LOVE_SPACE are removed because only one space is required to avoid accidental token pasting. The additional spaces serve no purpose and are discarded. > 3) Does it serve a purpose? I don't understand this question. In general the C/C++ preprocessor is not a general purpose text processing tool. It is specifically designed as one of the passes in a C/C++ compiler. Attempts to use it as a text processing tool are generally misguided. Ian
