为什么malloc使用这种方式？

Question

我正在研究我要求研究团队提供的代码.我试图理解代码,然而,他们以一种奇怪的方式使用了malloc.这里;

在头文件中;

 #define ERROR_OUTPUT stderr
 #define FAILIF(b) {   \
   if (b) {  \
       fprintf(ERROR_OUTPUT, "FAILIF triggered on line %d, file %s. Memory allocated: %lld\n",  \
       __LINE__, __FILE__, totalAllocatedMemory); exit(1); \
   } \
  }
 #define MALLOC(amount) \ 
   ( (amount > 0) ? totalAllocatedMemory += amount, malloc(amount) : NULL)

在cpp文件中;

 double *memRatiosForNNStructs = NULL;
 double *listOfRadii = NULL;
 nRadii = 1;
 FAILIF(NULL == (memRatiosForNNStructs = (double*)MALLOC(nRadii * sizeof(double))));

根据我的理解,他们定义的MALLOC意味着以下内容;

if(amount>0) // which is true; at least in this case
{
   totalAllocatedMemory = totalAllocatedMemory + amount; // sounds reasonable
   malloc(amount)  // What?? This will leak memory...
}
else
{
   NULL; // Do nothing? I guess that's fine
}

这里有什么我想念的吗？或者他们只是犯了一个(天真的)错误？

Answer 1

你拥有的第三个代码片段并不等同.注意使用逗号运算符:

#define MALLOC(amount) \
    ( (amount > 0) ? totalAllocatedMemory += amount, malloc(amount) : NULL)  
                                                   ^
                                                  N.B.!

逗号运算符接受两个参数,计算并丢弃第一个表达式,然后计算并返回第二个表达式.

在三元条件操作中使用这种方式

a = ((b)?(c:d))

相当于此

if(b) {
    a = c;
}
else {
    a = d;
}

逗号运算符使用这种方式

e = f, g;

相当于此

f;
e = g;

所以,如果你有

a = ((b)?(f,g:d))

那相当于

if(b) {
    f;
    a = g;
}
else {
    a = d;
}

在原始帖子中提供的代码中,MALLOC宏的使用方式如下:

memRatiosForNNStructs = (double*)MALLOC(nRadii * sizeof(double));

这相当于:

   // First operand of ternary conditional
if(nRadii * sizeof(double) > 0)
{
    // Second  operand of ternary conditional

    // First expression of the comma operator
    totalAllocatedMemory += nRadii * sizeof(double));
    // Second expression of the comma operator
    memRatiosForNNStructs = (double*)malloc(nRadii * sizeof(double));
}
else
{
    // Third operand of ternary conditional
    memRatiosForNNStructs = (double*)NULL;
}

老实说,这可以作为C中的函数实现,而不失一般性:

void* my_malloc(unsigned int amount)
{
    if(amount > 0) {
        // I assume this is a global variable
        totalAllocatedMemory = totalAllocatedMemory + amount;
        return  malloc(amount);
    }
    else {
        return NULL;
    } 
}

memRatiosForNNStructs = (double*)my_malloc(nRadii * sizeof(double));

所以我不确定将它作为一个难以阅读的宏实现的重点是什么.