C++ C 编程：另一个函数中的 malloc()

我需要malloc()inside another function的帮助.

我从main()向函数传递了pointer和size，我想从调用的函数内部使用malloc()为该指针动态分配内存，但我看到的是.正在分配的内存是为我调用的函数中声明的指针分配的，而不是为main()内的指针分配的.

我应该如何将指针传递给函数并为传递的指针from inside the called function分配内存？

我已经编写了以下代码，并得到如下所示的输出.

SOURCE:

int main()
{
   unsigned char *input_image;
   unsigned int bmp_image_size = 262144;

   if(alloc_pixels(input_image, bmp_image_size)==NULL)
     printf("\nPoint2: Memory allocated: %d bytes",_msize(input_image));
   else
     printf("\nPoint3: Memory not allocated");     
   return 0;
}

signed char alloc_pixels(unsigned char *ptr, unsigned int size)
{
    signed char status = NO_ERROR;
    ptr = NULL;

    ptr = (unsigned char*)malloc(size);

    if(ptr== NULL)
    {
        status = ERROR;
        free(ptr);
        printf("\nERROR: Memory allocation did not complete successfully!");
    }

    printf("\nPoint1: Memory allocated: %d bytes",_msize(ptr));

    return status;
}

程序输出:

Point1: Memory allocated ptr: 262144 bytes
Point2: Memory allocated input_image: 0 bytes