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#include 
   
    #include 
    
     #include 
     
      #include 
      
       long CAS(volatile unsigned long **addr,  long old,         long *new){
       /*       https://www.felixcloutier.com/x86/cmpxchg:       (* Accumulator = AL, AX, EAX, or RAX depending on whether a byte, word,       doubleword, or quadword comparison is being performed *)       TEMP ← DEST       IF accumulator = TEMP （比较）       THEN         ZF ← 1;         DEST ← SRC;       ELSE         ZF ← 0;         accumulator ← TEMP;         DEST ← TEMP;       FI;       Compares the value in the AL, AX, EAX, or RAX register with the first          operand (destination operand).       If the two values are equal, the second operand (source operand) is          loaded into the destination operand.       Otherwise, the destination operand is loaded into the AL, AX, EAX or RAX          register.       RAX register is available only in 64-bit mode.     */    long ret = 0;    __asm__ volatile (" lock; cmpxchg %4, %2"                      : "=a" (ret), "=m" (*addr)                      : "m" (*addr), "0" (old), "r" (new)                      : "cc"                      );    return ret == old;}static void *single = 0x0;void* getInstance(void * arg){
       if (single != NULL)    {
           return single;    }    long *p = calloc(1, sizeof(long));    *p = 8888;    do{
           if (single != NULL)            break;        if (CAS((volatile unsigned long **)&single, 0x0, p))            break;    }while(1);    return single;}
      
     
    
   

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