The meaning of lines starting with a hash sign and number like ‘# 1 “a.c”‘ in the gcc preprocessor output

cc-preprocessorgcc

I print out the output of C preprocessor by using

gcc -E a.c

The output contains many lines like

# 1 "a.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "a.c"
# 1 "c:\\mingw\\bin\\../lib/gcc/mingw32/4.5.0/../../../../include/stdio.h" 1 3
# 19 "c:\\mingw\\bin\\../lib/gcc/mingw32/4.5.0/../../../../include/stdio.h" 3
# 1 "c:\\mingw\\bin\\../lib/gcc/mingw32/4.5.0/../../../../include/_mingw.h" 1 3
# 31 "c:\\mingw\\bin\\../lib/gcc/mingw32/4.5.0/../../../../include/_mingw.h" 3
       
# 32 "c:\\mingw\\bin\\../lib/gcc/mingw32/4.5.0/../../../../include/_mingw.h" 3
# 20 "c:\\mingw\\bin\\../lib/gcc/mingw32/4.5.0/../../../../include/stdio.h" 2 3

I've never seen this kind of syntax in C. Can someone explain what this is doing?

Best Answer

These lines are hints for debugging (where the code following the line actually came from)

# line-number "source-file" [flags]

Meaning of flags (space separated):

  • 1 - Start of a new file
  • 2 - Returning to previous file
  • 3 - Following text comes from a system header file (#include <> vs #include "")
  • 4 - Following text should be treated as being wrapped in an implicit extern "C" block.
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