pointer return value changes after function call

Question

Code snippets from two C source files:

A.c

Channel *testChannelGet()
{
    Channel *ch = channelGet (parser,parserCh);
    return ch;
}

B.c

Channel *channelGet(UINT8 parser, UINT16 parserCh)
{
    chnl.player = &solPlayer;
    return((Channel *)&chnl);
}

I compile both files and create a static and a shared library. Now I call testChannelGet from a sample program. When I link it against the static library, it works perfectly. But if I link it against the shared library, its SEGFAULTing. Debugging tells me that the pointer returned from channelGet is changing the moment it returns. GDB output below.

174         Channel *ch = channelGet (parser,parserCh);
(gdb) s
channelGet (parser=1 '\001', parserCh=1) at B.c:15174
15174           chnl.player = &solPlayer;
(gdb) n
15175           return((Channel *)&chnl);
(gdb) p ((Channel *)&chnl)
$1 = (Channel *) 0x7ffff7fed1a0
(gdb) n
15176   }
(gdb) n
testChannelGet at A.c:175
175         return ch;
(gdb) p ch
$2 = (Channel *) 0xfffffffff7fed1a0

It seems the address value points to a different offset now - 0xfffffffff7fed1a0 vs 0x7ffff7fed1a0 . The last bytes in both addresses are the same.

Any hints? I have tried the -fPIC option to no avail.

Answer 1

Is there a prototype in scope for channelGet() in A.c?

If not, the results you're seeing could be explained as follows:

• channelGet() is assumed to return int (due to lack of prototype), so the result is truncated to 0xf7fed1a0
• then it is cast to a 64-bit pointer, so gets sign-extended to 0xfffffffff7fed1a0

(You should get complaints about this if you compile with warnings enabled, of course...)

Answer 2

Run your program under valgrind. Find and fix any errors it reports.