How does GCC process quotes in macro? - c++

Question

I'm trying to test gcc preprocessor for its macro expansion.
I write following code: (just for testing)
#include <stdio.h>
#define QUOTE "
#define TMPL hello
int main(){
printf(QUOTE TMPL QUOTE);
return 0;
}
the compiling result is:
$ gcc main.c -o main
main.c:3:15: warning: missing terminating " character
main.c: In function ‘main’:
main.c:7: error: missing terminating " character
main.c:7: error: missing terminating " character
main.c:7: error: ‘hello’ undeclared (first use in this function)
main.c:7: error: (Each undeclared identifier is reported only once
main.c:7: error: for each function it appears in.)
main.c:7: warning: format not a string literal and no format arguments
main.c:7: warning: format not a string literal and no format arguments
$
Then I try to have a look at the preprocessed result
$ gcc -E main.c -o tmp.c
main.c:3:15: warning: missing terminating " character
$
Though giving a warning, it somehow produces correct preprocessed code in tmp.c
int main(){
printf(" hello ");
return 0;
}
And I compiler tmp.c, hello is correctly printed.
I'm wondering why gcc -E could produce correct code, while using gcc compiling directly failed. Is there difference between the two method of gcc preprocessor?
$ gcc --version
i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)

Answer 1

As I commented, a preprocessor macro should expand into a sequence of lexer tokens. Withing GCC source code, the libcpp (in charge of preprocessing and tokenizing) is producing a stream of tokens (not plain chars). A recent GCC 4.8, when run as gcc -Wall endyul.c -o endyl on your example, gives quite helpful diagnostics:
endyul.c:3:15: warning: missing terminating " character [enabled by default]
#define QUOTE "
^
endyul.c: In function 'main':
endyul.c:7:5: error: missing terminating " character
printf(QUOTE TMPL QUOTE);
^
endyul.c:7:5: error: missing terminating " character
endyul.c:4:14: error: 'hello' undeclared (first use in this function)
#define TMPL hello
^
endyul.c:7:18: note: in expansion of macro 'TMPL'
printf(QUOTE TMPL QUOTE);
^
endyul.c:4:14: note: each undeclared identifier is reported only once for each function it appears in
#define TMPL hello
^
endyul.c:7:18: note: in expansion of macro 'TMPL'
printf(QUOTE TMPL QUOTE);
^
Your GCC 4.2 is very old. You should consider upgrading it.
And clang (3.3) gives also a good diagnostic:
clang -Wall endyul.c -o endyul
endyul.c:3:15: warning: missing terminating '"' character [-Winvalid-pp-token]
#define QUOTE "
^
endyul.c:7:12: error: expected expression
printf(QUOTE TMPL QUOTE);
^
endyul.c:3:15: note: expanded from macro 'QUOTE'
1 warning and 1 error generated.
Read again the CPP manual of GCC, notably the chapters on Stringification and Concatenation.