/usr/include/stdc-predef.h is automatically included when the preprocessor is invoked: $ gcc -E -x c /dev/null # 1 "/dev/null" # 1 "<built-in>" # 1 "<command-line>" # 31 "<command-line>" # 1 "/usr/include/stdc-predef.h" 1 3 4 # 32 "<command-line>" 2 # 1 "/dev/null"