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Editorial Team
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Asked: 2026-05-14T14:59:24+00:00

If I tell the C preprocessor to #include a file and use CPPFLAGS to

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If I tell the C preprocessor to #include a file and use CPPFLAGS to help find the needed file, then the file is included already, right? What, if any, use is telling the C compiler about the same include directory with CFLAGS?

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  1. Editorial Team

    I don’t think there is any use.

    The implicit make rules indicates that CFLAGS is only used when compiling C programs (from .c to .o). The value of CPPFLAGS is also added to the compiler command line.

    CPPFLAGS is also used in the following rules:

    • Compiling C++ programs
    • Compiling Fortran and Ratfor programs
    • Preprocessing Fortran and Ratfor programs
    • Assembling and preprocessing assembler programs
    • Making Lint Libraries from C, Yacc, or Lex programs

    Since CPPFLAGS is used in every case where CFLAGS is used, there seems to be no point in adding -I directives to CFLAGS that are already in CPPFLAGS.

    Of course, if your Makefile has custom rules that pass CFLAGS to the compiler, but omit CPPFLAGS, it’s a different story.

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