assembly problem - no "lock" command?

[rattusdatorum]

Hi!

I am trying to implement a semaphore, well the problem is the inline assembly of gcc doesn't recognize the "lock" command, which is needed prior to cmpxchg to make cmpxchg atomic :(
I am not quite sure why it doesn't works, could it be that it doesn't works because ubuntu uses a 386 architecture and the "lock" command was first used on the
486 architecture?

btw. as (gnu assembly compiler) has the same problem:
cmpxchg.s:6: Error: suffix or operands invalid for `cmpxchg'

if my assumptions is correct is there a way to get "lock" work and how?
if I am wrong, whats the real cause for my problem?

thx in advance

[rattusdatorum]

bump

[LordHunter317]

Post your assembly, but you're not going to get a truly atomic lock without kernel assistance, unless the lock is meant to be completely internal to your process only. Why are you trying to revent the wheel anyway?

[rattusdatorum]

oops, well I figured out, I don't need the lock anyway, at least if I stay with single cpu's.

Hmm I don't think need kernel support, because the cmpxchg instruction is only one machine instruction and prevents a race condition, because everything which is needed to prevent/allow entering a critical section is done by cmpxchg


why to reinvent the wheel? Well for better understanding!

asm code: (in Intel syntax!)

loop:
movl eax,1
cmpxchg 0,1
jz eax,loop ! I don't know if the syntax is correct (jump to loop if eax == zero)
criticial section:
!doSomethingHere




c code with inline asm: (AT/T syntax!)

void p(int s[1])
{
while (s[0] == 0) {}; //busy-wait
int dummy = 0;

asm("movl $1, %%eax\n\t" // eax = 1;
"cmpxchg %1, %0" // dummy, semaphore (src, dest); AT/T syntax != Intel Syntax
:"=r"(s[0]) //output %0
:"r"(dummy)//input %1
:"%eax"); //IMPORTANT, don't change eax in the asm command
}

void v(int s[1])
{
s[0] = 1;
}