Spaces inserted by the C preprocessor - c

Question

Suppose we are given this input C code:
#define Y 20
#define A(x) (10+x+Y)
A(A(40))
gcc -E outputs like that (10+(10+40 +20)+20).
gcc -E -traditional-cpp outputs like that (10+(10+40+20)+20).
Why the default cpp inserts the space after 40 ?
Where can I find the most detailed specification of the cpp that covers that logic ?

Answer 1

The C standard doesn't specify this behaviour, since the output of the preprocessing phase is simply a stream of tokens and whitespace. Serializing the stream of tokens back into a character string, which is what gcc -E does, is not required or even mentioned by the standard, and does not form part of the translation processs specified by the standard.
In phase 3, the program "is decomposed into preprocessing tokens and sequences of white-space characters." Aside from the result of the concatenation operator, which ignores whitespace, and the stringification operator, which preserves whitespace, tokens are then fixed and whitespace is no longer needed to separate them. However, the whitespace is needed in order to:
parse preprocessor directives
correctly process the stringification operator
The whitespace elements in the stream are not eliminated until phase 7, although they are no longer relevant after phase 4 concludes.
Gcc is capable of producing a variety of information useful to programmers, but not corresponding to anything in the standard. For example, the preprocessor phase of the translation can also produce dependency information useful for inserting into a Makefile, using one of the -M options. Alternatively, a human-readable version of the compiled code can be output using the -S option. And a compilable version of the preprocessed program, roughly corresponding to the token stream produced by phase 4, can be output using the -E option. None of these output formats are in any way controlled by the C standard, which is only concerned with actually executing the program.
In order to produce the -E output, gcc must serialize the stream of tokens and whitespace in a format which does not change the semantics of the program. There are cases in which two consecutive tokens in the stream would be incorrectly glued together into a single token if they are not separated from each other, so gcc must take some precautions. It cannot actually insert whitespace into the stream being processed, but nothing stops it from adding whitespace when it presents the stream in response to gcc -E.
For example, if macro invocation in your example were modified to
A(A(0x40E))
then naive output of the token stream would result in
(10+(10+0x40E+20)+20)
which could not be compiled because 0x40E+20 is a single pp-number token which cannot be converted into a numeric token. The space before the + prevents this from happening.
If you attempt to implement a preprocessor as some kind of string transformation, you will undoubtedly confront serious issues in the corner cases. The correct implementation strategy is to tokenize first, as indicated in the standard, and then perform phase 4 as a function on a stream of tokens and whitespace.
Stringification is a particularly interesting case where whitespace affects semantics, and it can be used to see what the actual token stream looks like. If you stringify the expansion of A(A(40)), you can see that no whitespace was actually inserted:
$ gcc -E -x c - <<<'
#define Y 20
#define A(x) (10+x+Y)
#define Q_(x) #x
#define Q(x) Q_(x)
Q(A(A(40)))'
"(10+(10+40+20)+20)"
The handling of whitespace in stringification is precisely specified by the standard: (§6.10.3.2, paragraph 2, many thanks to John Bollinger for finding the specification.)
Each occurrence of white space between the argument’s preprocessing tokens
becomes a single space character in the character string literal. White space before the first preprocessing token and after the last preprocessing token composing the argument is deleted.
Here is a more subtle example where additional whitespace is required in the gcc -E output, but is not actually inserted into the token stream (again shown by using stringification to produce the real token stream.) The I (identify) macro is used to allow two tokens to be inserted into the token stream without intervening whitespace; that's a useful trick if you want to use macros to compose the argument to the #include directive (not recommended, but it can be done).
Maybe this could be a useful test case for your preprocessor:
#define Q_(x) #x
#define Q(x) Q_(x)
#define I(x) x
#define C(x,...) x(__VA_ARGS__)
// Uncomment the following line to run the program
//#include <stdio.h>
char*quoted=Q(C(I(int)I(main),void){I(return)I(C(puts,quoted));});
C(I(int)I(main),void){I(return)I(C(puts,quoted));}
Here's the output of gcc -E (just the good stuff at the end):
$ gcc -E squish.c | tail -n2
char*quoted="intmain(void){returnputs(quoted);}";
int main(void){return puts(quoted);}
In the token stream which is passed out of phase 4, the tokens int and main are not separated by whitespace (and neither are return and puts). That's clearly shown by the stringification, in which no whitespace separates the token. However, the program compiles and executes fine, even if passed explicitly through gcc -E:
$ gcc -E squish.c | gcc -x c - && ./a.out
intmain(void){returnputs(quoted);}
and compiling the output of gcc -E.
Different compilers and different versions of the same compiler may produce different serializations of a preprocessed program. So I don't think you will find any algorithm which is testable with a character-by-character comparison with the -E output of a given compiler.
The simplest possible serialization algorithm would be to unconditionally output a space between two consecutive tokens. Obviously, that would output unnecessary spaces, but it would never syntactically alter the program.
I think the minimal space algorithm would be to record the DFA state at the end of the last character in a token so that you can later output a space between two consecutive tokens if there exists a transition from the state at the end of the first token on the first character of the following token. (Keeping the DFA state as part of the token is not intrinsically different from keeping the token type as part of the token, since you can derive the token type from a simple lookup from the DFA state.) That algorithm would not insert a space after 40 in your original test case, but it would insert a space after 0x40E. So it is not the algorithm being used by your version of gcc.
If you use the above algorithm, you will need to rescan tokens created by token concatenation. However, that is necessary anyway, because you need to flag an error if the result of the concatenation is not a valid preprocessing token.
If you don't want to record states (although, as I said, there is essentially no cost in doing so) and you don't want to regenerate the state by rescanning the token as you output it (which would also be quite cheap), you could precompute a two-dimensional boolean array keyed by token type and following character. The computation would essentially be the same as the above: for every accepting DFA state which returns a particular token type, enter a true value in the array for that token type and any character with a transition out of the DFA state. Then you can look up the token type of a token and the first character of the following token to see if a space may be necessary. This algorithm does not produce a minimally-spaced output: it would, for example, put a space after the 40 in your example, since 40 is a pp-number and it is possible for some pp-number to be extended with a + (even though you cannot extend 40 in that way). So it's possible that gcc uses some version of this algorithm.

Answer 2

Adding some historical context to rici's excellent answer.
If you can get your hands on a working copy of gcc 2.7.2.3, experiment with its preprocessor. At that time the preprocessor was a separate program from the compiler, and it used a very naive algorithm for text serialization, which tended to insert far more spaces than were necessary. When Neil Booth, Per Bothner and I implemented the integrated preprocessor (appearing in gcc 3.0 and since), we decided to make -E output a bit smarter at the same time, but without making the implementation too complicated. The core of this algorithm is the library function cpp_avoid_paste, defined at https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libcpp/lex.c#l2990 , and its caller is here: https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/c-family/c-ppoutput.c#l177 (look for "Subtle logic to output a space...").
In the case of your example
#define Y 20
#define A(x) (10+x+Y)
A(A(40))
cpp_avoid_paste will be called with a CPP_NUMBER token (what rici called a "pp-number") on the left, and a '+' token on the right. In this case it unconditionally says "yes, you need to insert a space to avoid pasting" rather than checking whether the last character of the number token is one of eEpP.
Compiler design often comes down to a trade-off between accuracy and implementation simplicity.