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Re: Issues with _GL_CXXALIAS_SYS (strndup)

From: Martin Storsjö
Subject: Re: Issues with _GL_CXXALIAS_SYS (strndup)
Date: Sun, 5 Jan 2020 00:22:36 +0200 (EET)
User-agent: Alpine 2.20 (DEB 67 2015-01-07)

Hi Bruno,

On Sat, 4 Jan 2020, Bruno Haible wrote:

Hi Martin,

Thank you for the careful analysis.

The root cause for this issue seems to be that clang has a builtin for the
strndup function, even if the target platform actually doesn't have it.

This manifests itself during configure like this:

configure:31903: checking whether strndup is declared
configure:31903: x86_64-w64-mingw32-gcc -c -g -O2  conftest.c >&5
conftest.c:267:10: warning: implicitly declaring library function
'strndup' with type 'char *(const char *, unsigned long long)'
   (void) strndup;
conftest.c:267:10: note: include the header <string.h> or explicitly
provide a declaration for 'strndup'
1 warning generated.
configure:31903: $? = 0
configure:31903: result: yes

This results in variables set up like this:


This is correct and expected: mingw does not have strndup.


This is wrong: it should be ac_cv_have_decl_strndup=no.


Which then leads to the errors quoted above.

There are two possible ways to fix this issue:
 * Modify AC_CHECK_DECL to disable the clang built-in declarations
   for strndup and many other functions.
 * Ignore the result of AC_CHECK_DECL when the compiler is clang,
   and use the AC_CHECK_FUNC result instead.

The second option is work in many places, in particular because in some
cases the AC_CHECK_FUNC needs to know which libraries to add to LIBS
for the test. I therefore prefer the first one.

With this patch, the configure output should be:

checking whether the compiler is clang... yes
checking for compiler option needed when checking for declarations... 
checking whether strndup is declared... no

Awesome, this patch makes a lot of sense to me, and seems to work fine.


// Martin

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