Re: implicit declaration of function 'utime' in trailing slashes test

[Ryan Schmidt]

On Jan 22, 2021, at 16:47, Tim Rühsen wrote:

> On 21.01.21 01:34, Ryan Schmidt wrote:
>> Hi, I'm the maintainer of wget in MacPorts.
>> In the version of clang included with Xcode 12 and later, implicit 
>> declaration of functions is an error.
>> During configure, wget 1.12.1 prints this:
>> checking whether utime handles trailing slashes on files... no
>> config.log contains this:
>> configure:49368: checking whether utime handles trailing slashes on files
>> configure:49414: ccache /usr/bin/clang -o conftest -DNDEBUG -pipe -Os 
>> -Werror=implicit-function-declaration -arch x86_64 -I/opt/local/include 
>> -L/opt/local/lib -Wl,-headerpad_max_install_names -arch x86_64 conftest.c  
>> >&5
>> conftest.c:491:23: error: implicit declaration of function 'utime' is 
>> invalid in C99 [-Werror,-Wimplicit-function-declaration]
>>                  if (!utime ("conftest.tmp/", NULL))
>>                       ^
>> 1 error generated.
>> configure:49414: $? = 1
>> configure: program exited with status 1
>> So the results of this test don't represent what you want them to.
>> This configure test uses #include <time.h>. The problem goes away if I 
>> change that to #include <utime.h>. Then configure output is still:
>> checking whether utime handles trailing slashes on files... no
>> but config.log then contains:
>> configure:49368: checking whether utime handles trailing slashes on files
>> configure:49414: ccache /usr/bin/clang -o conftest -DNDEBUG -pipe -Os 
>> -Werror=implicit-function-declaration -arch x86_64 -I/opt/local/include 
>> -L/opt/local/lib -Wl,-headerpad_max_install_names -arch x86_64 conftest.c  
>> >&5
>> configure:49414: $? = 0
>> configure:49414: ./conftest
>> configure:49414: $? = 2
>> configure: program exited with status 2
>> Now the results of the test are accurate.
>> I included this patch in the MacPorts wget 1.12.1 port:
>> https://github.com/macports/macports-ports/blob/13fd7facb9e1ea9e70b79c8c0b429058b9bb8698/net/wget/files/implicit.patch
>> Of course you'll want to patch m4/utime.m4 instead of configure.
> 
> ./configure scripts are not made for using -Werror in CFLAGS.
> 
> This is documented somewhere but (sorry), I am just too tired to search for 
> the URL right now.

Unfortunately, you no longer have much of a choice. Treating implicit 
declarations of functions as an error is the *default* behavior of the compiler 
included with Xcode 12 and later.

In the output I showed above I have specified 
-Werror=implicit-function-declaration manually in CFLAGS because I am using an 
earlier version of Xcode but I want to simulate the experience of an Xcode 12 
user so that I can identify and fix this type of problem.

Apple intentionally changed the default behavior of the compiler in order to be 
able to ship Macintosh computers with ARM processors ("Apple Silicon"). ARM 
processors have different calling conventions for variadic and non-variadic 
functions, so the compiler must know before you call a function what type of 
function it is, and the way it knows that is by the function declaration.

The C99 standard requires that you declare functions before you use them. It 
has been this way for over 2 decades. wget should do so in all of its code, 
regardless of whether the code is in a configure test or elsewhere.

This one configure test that I mentioned is the only place where I noticed a 
problem, so as far as I know only the one-byte change I proposed above needs to 
be made.