What is the meaning of lines starting with a hash sign and number like '# 1 "a.c"' in the gcc preprocessor output?

c gcc c-preprocessor
user607722 picture user607722 · Mar 20, 2011 · Viewed 40.8k times · Source

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?

Answer

jdehaan picture jdehaan · Mar 20, 2011

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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