Hi!
I have a question about "global" variable accessing when gcc compile some code. See example below:
//----------start-----------
#include <stdio.h>
int aiArray[10];
int iCurArrayIdx;
int GetNewValue( int iValue )
{
//do something...
//i change the global variable intentionally.
iCurArrayIdx = 1;
//assume the return is 1
return 1;
}
int main( int argc, char* argv[] )
{
aiArray[iCurArrayIdx] = GetNewValue( aiArray[iCurArrayIdx] );
printf( "aiArray[iCurArrayIdx] = %d\n", aiArray[iCurArrayIdx] );
return 0;
}
//----------end-------------
I know the coding is very bad. It's simple for demo.
I compile these code under VC6 and GCC-4.2.4/linux and run,
then I get the differ result.
VC6 printf: aiArray[iCurArrayIdx] = 1
GCC printf: aiArray[iCurArrayIdx] = 0
I think VC6 got the correct executable content.
So I use objdump to get the assemble code of GCC output:
08048454 <_Z11GetNewValuei>:
8048454: 55 push %ebp
8048455: 89 e5 mov %esp,%ebp
8048457: c7 05 a8 97 04 08 01 movl $0x1,0x80497a8
804845e: 00 00 00
8048461: b8 01 00 00 00 mov $0x1,%eax
8048466: 5d pop %ebp
8048467: c3 ret
08048468 <main>:
8048468: 8d 4c 24 04 lea 0x4(%esp),%ecx
804846c: 83 e4 f0 and $0xfffffff0,%esp
804846f: ff 71 fc pushl -0x4(%ecx)
8048472: 55 push %ebp
8048473: 89 e5 mov %esp,%ebp
8048475: 53 push %ebx
8048476: 51 push %ecx
8048477: 83 ec 10 sub $0x10,%esp
804847a: 8b 1d a8 97 04 08 mov 0x80497a8,%ebx
8048480: a1 a8 97 04 08 mov 0x80497a8,%eax
8048485: 8b 04 85 80 97 04 08 mov 0x8049780(,%eax,4),%eax
804848c: 89 04 24 mov %eax,(%esp)
804848f: e8 c0 ff ff ff call 8048454 <_Z11GetNewValuei>
8048494: 89 04 9d 80 97 04 08 mov %eax,0x8049780(,%ebx,4)
804849b: a1 a8 97 04 08 mov 0x80497a8,%eax
80484a0: 8b 04 85 80 97 04 08 mov 0x8049780(,%eax,4),%eax
80484a7: 89 44 24 04 mov %eax,0x4(%esp)
80484ab: c7 04 24 90 85 04 08 movl $0x8048590,(%esp)
80484b2: e8 f5 fe ff ff call 80483ac <printf@plt>
80484b7: b8 00 00 00 00 mov $0x0,%eax
80484bc: 83 c4 10 add $0x10,%esp
80484bf: 59 pop %ecx
80484c0: 5b pop %ebx
80484c1: 5d pop %ebp
80484c2: 8d 61 fc lea -0x4(%ecx),%esp
80484c5: c3 ret
80484c6: 90 nop
80484c7: 90 nop
80484c8: 90 nop
80484c9: 90 nop
Then I found the reason of run differ result:
Address of 804847a, iCurArrayIdx has load in EBX
Address of 8048494, use the old iCurArrayIdx keep in EBX and not update
Puzzle! Is this GCC's bug, or feature, or standard ambiguity? Why be that?
--horde
----------------------------------------------------------------------
Ian reply:
The order of evaluation of the left hand side and the right hand side of
the assigment operator is unspecified in C. Therefore, the behaviour of
this program is unspecified, because iCurArrayIdx could be retrieved
before or after GetNewValue is called, and different compilers are free
to implement this program differently.
----------------------------------------------------------------------
Thanks Ian!
Now can any people tell me how to ensure the order of evaluation to get the expectant result alike VC6. BTW:I wouldn't like to modify the expression:
aiArray[iCurArrayIdx] = GetNewValue( aiArray[iCurArrayIdx] );
--horde
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