What do gcc preprocessed output lines of form hash sign + digit "# 1" mean?
c++, c-preprocessor, gcc
Solution
It looks like line markers. As you might have noted, there's no trivial relation between line numbers in the original and preprocessed file. `# 7` in the preprocessed input indicates that the source of next line was line7 in the original (which was named test.cpp).
`3 4` are flags, meaning "macro expansion from system header" and `extern "C"`
GCC documentation
Problem
While doing a bug report for CUDA's compiler, I ended up finding this strange behavior with gcc's preprocessing step. I currently use gcc 4.8.2. Test file: test.cpp ``` #include <assert.h> int main() { int a = 1; assert (a >= 0); assert (a >= 0); } ``` Command gcc -E -x c++ -m64 -g -o "test.cpp4.ii" "test.cpp" Result file: test.cpp4.ii ``` # 2 "test.cpp" 2 int main() { int a = 1; ((a >= 0) ? static_cast<void> (0) : __assert_fail ("a >= 0", "test.cpp", 6, __PRETTY_FUNCTION__)); ((a >= 0) ? static_cast<void> (0) : __assert_fail ("a >= 0", "test.cpp" # 7 "test.cpp" 3 4 , 8 # 7 "test.cpp" 3 4 , __PRETTY_FUNCTION__)) ; ``` The multiline assert seems to be processed differently, leading to these `# 7 "test.cpp" 3 4` lines. What does that mean exactly? Update Apparently, gcc 4.7 gives `# 7 "test.cpp"` (without the last 2 numbers).