mainAutoconf - Support: sr #110484, CFLAGS/CXXFLAGS and CPPFLAGS order...


sr #110484: CFLAGS/CXXFLAGS and CPPFLAGS order in c.m4 ac_compile is nonstandard

Submitter:  None
Submitted:  Wed 05 May 2021 03:10:42 PM UTC
Priority:  * 5 - Unprioritized Severity:  3 - Normal
Status:  Invalid Privacy:  Public
Assigned to:  None Originator Email:  -email is unavailable-
Open/Closed:  Closed Operating System:  None
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Fri 08 Dec 2023 05:46:53 PM UTC, comment #1: 

Autoconf is consistent with GNU make's implicit rules:

make: *** No targets.  Stop.

Therefore I think it is actually Automake, and the expectations of AX_CHECK_COMPILE_FLAG, that are incorrect here.

Please take this up with both the maintainers of Automake (see and the maintainers of GNU make (write to  If they both agree that the order ought to be CPPFLAGS first, and GNU make's implicit rules are changed, then I will change Autoconf to match.

Zack Weinberg <zackw>
Group administrator
Wed 05 May 2021 03:10:42 PM UTC, original submission:  

The definition of ac_compile in lib/autoconf/c.m4 contains $CFLAGS/$CXXFLAGS/$OBJCFLAGS before $CPPFLAGS, whereas the usual expected order (for example in the implicit rules of GNU automake) is the other way around. This prevents other macros from working as expected when flag shadowing is involved.

One such case is AX_CHECK_COMPILE_FLAG of autoconf-archive being unable to properly detect accepted compiler flags: it appends the tested flag to C/CXXFLAGS and tries to compile with AC_COMPILE_IFELSE, but a flag set in CPPFLAGS is able to take precedence, falsifying the result.

A minimal working example is attached, using the latest AX_CHECK_COMPILE_FLAG, where running

./configure CPPFLAGS=-march=native


checking whether C compiler accepts "-march=pentiumpro"... yes

whereas running without CPPFLAGS gives

checking whether C compiler accepts "-march=pentiumpro"... no

This leads to projects which use AX_CC_MAXOPT being unable to compile when CPPFLAG=-march=native is set.



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file #51384: added by None (2KiB - application/


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    Follow 4 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2023-12-08 zackw Open/ClosedOpen Closed
    2023-12-08 zackw Severity4 - Important 3 - Normal
        StatusNone Invalid
    2021-05-05 None Attached File- Added, #51384

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