[musl] Problem with strdupa in C++ code

[musl] Problem with strdupa in C++ code

[Wolf]

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Hello,

I'm trying to compile C++ project in Alpine (so with musl) and I'm
hitting following error:

    In file included from /usr/include/fortify/string.h:22,
                     from src/main/tools/linux-sandbox-pid1.cc:34:
    src/main/tools/linux-sandbox-pid1.cc: In function 'int CreateTarget(const char*, bool)':
    src/main/tools/linux-sandbox-pid1.cc:149:28: error: invalid conversion from 'void*' to 'char*' [-fpermissive]
      149 |   if (CreateTarget(dirname(strdupa(path)), true) < 0) {
          |                            ^
          |                            |
          |                            void*
    In file included from /usr/include/fortify/string.h:22,
                     from src/main/tools/linux-sandbox-pid1.cc:34:
    /usr/include/string.h:31:15: note:   initializing argument 1 of 'char* strcpy(char*, const char*)'
       31 | char *strcpy (char *__restrict, const char *__restrict);
          |               ^~~~~~~~~~~~~~~~
    
If my reading for musl's code is correct, this is actually a bug in the
header file, since the declarations are:

    void *alloca(size_t);
    #define	strdupa(x)	strcpy(alloca(strlen(x)+1),x)

As far as I know, there is no automatic conversion from void* to char*
in C++. Should this be fixed by turning the define into

    #define	strdupa(x)	strcpy((char*)alloca(strlen(x)+1),x)

? I think that should be both valid C and C++.

Best regards,
Tomas Volf

-- 
There are only two hard things in Computer Science:
cache invalidation, naming things and off-by-one errors.

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Re: [musl] Problem with strdupa in C++ code

[Markus Wichmann]

Hi all,

so I tried to find some authority on this, but per the manpage,
strdupa() is a GNU extension. So I looked up the implementation in
glibc, which is near enough the only authority on whether this is
supposed to work in C++, and they do add the casts necessary to make it
work. Although they massively overcomplicate the matter with a statement
expression for God knows what reason.

So I guess we should add the cast, too. I see nothing obviously wrong
with the suggested replacement.

Ciao,
Markus

Re: [musl] Problem with strdupa in C++ code

[Joakim Sindholt]

On Tue, 12 Apr 2022 18:15:59 +0200, Markus Wichmann <nullplan@gmx.net> wrote:
> Although they massively overcomplicate the matter with a statement
> expression for God knows what reason.

To avoid referencing the argument twice, so you can strdupa(s++).