[Dev-C++] Inline assembly-linux

hi to all..
I am new to assembly programming...
I can't figure out what this inline assembly code means...
Can anyone help me..
#define sys_test_and_set(lock)    \
({                                      \
   volatile long temp = SEMAPHORE_C_SET;  \
   __asm__("xchgl %0,(%2)"              \
           :"=r" (temp)                 \
           :"0" (temp),"r" (lock));     \
   temp;                                \
})

thanking in advance
regards
smitha
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