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Bug 779

Summary: Const issues with SDL_putenv() ...
Product: SDL Reporter: Ryan C. Gordon <icculus>
Component: *don't know*Assignee: Ryan C. Gordon <icculus>
Status: RESOLVED FIXED QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P1    
Version: HG 2.0   
Hardware: x86   
OS: Linux   

Description Ryan C. Gordon 2009-08-11 23:53:41 UTC 
Man page for putenv() on Linux says this:
   "SUSv2 removes the const from the prototype, and so does glibc 2.1.3."

Namely: it saves the string pointer, and allows you to overwrite it! (That is, apparently getenv() will return _that same pointer_ and it's legal to write directly to it to change the environment table's data.

Yeah, that's nasty.

But when SDL_putenv() isn't #define'd to use putenv() directly, it expects a const char * for the argument.

So on Ubuntu 9.10, for example, this causes a compiler warning:

   SDL_putenv("SDL_VIDEO_CENTERED=1");

"warning: deprecated conversion from string constant to ‘char*’"

Not sure what the best approach is here. I'm sure other Unixes want a const char * for putenv(), even though the standard apparently says that's wrong.

--ryan.
Comment 1 Sam Lantinga 2009-09-26 03:44:52 UTC 
Yuck.

Does putenv("FOO=1") generate a warning as well?  Or does the compiler put that string into the writable data segment?
Comment 2 Ryan C. Gordon 2009-12-14 23:18:44 UTC 
For the program...

#include <stdlib.h>

int main(void)
{
    char buf[10];
    char *char_ptr = buf;
    const char *const_char_ptr = char_ptr;
    putenv(buf);
    putenv(char_ptr);
    putenv(const_char_ptr);
    putenv("STRING_LITERAL=1");
    return 0;
}

... the only line it complains about is...

    putenv(const_char_ptr);

testenv.c: In function main:
testenv.c:10: warning: passing argument 1 of putenv discards qualifiers from pointer target type
/usr/include/stdlib.h:578: note: expected char * but argument is of type const char *

The string literal didn't trigger a warning, even with -Wall. Probably way too much legacy code that passes literals to things that want (char*). It DOES report the same warning with -Wwrite-strings. The manpage says this isn't part of -Wall because it's a nuisance for code with mismatched const qualifiers.

If compiled as C++...

testenv.cpp:10: error: invalid conversion from const char* to char*
testenv.cpp:10: error:   initializing argument 1 of int putenv(char*)
testenv.cpp:11: warning: deprecated conversion from string constant to char*

The string literal is a warning by default, the const_char_ptr is an error.

--ryan.
Comment 3 Ryan C. Gordon 2009-12-14 23:45:06 UTC 
After heated debate via Google Talk, I'm taking this bug.   :)

--ryan.
Comment 4 Ryan C. Gordon 2009-12-15 10:01:31 UTC 
After heated debate with myself, I'm leaving this alone.

Of several options, the "best" option was to just leave a warning comment and move on.

"Fixed" in svn revision #5402.

--ryan.
Comment 5 Sam Lantinga 2009-12-15 11:52:35 UTC 
Let's do setenv in 1.3 and throw putenv in SDL_compat.c
Comment 6 Ryan C. Gordon 2009-12-15 22:33:04 UTC 
Bumping priority on a set of bugs.

--ryan.
Comment 7 Ryan C. Gordon 2009-12-16 03:01:37 UTC 
Fixed in svn revision #5416.

Sam, can you look over this patch? It turned out to be bigger than I expected for a 6am quicky-fix, and I don't have a Windows install for testing that common case at the moment in any case.

--ryan.