Question

I need help with malloc() inside another function.

I’m passing a pointer and size to the function from my main() and I would like to allocate memory for that pointer dynamically using malloc() from inside that called function, but what I see is that…. the memory, which is getting allocated, is for the pointer declared within my called function and not for the pointer which is inside the main().

How should I pass a pointer to a function and allocate memory for the passed pointer from inside the called function?

---

I have written the following code and I get the output as shown below.

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;
}

PROGRAM OUTPUT:

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

Answer 1

You need to pass a pointer to a pointer as the parameter to your function.

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

   if(alloc_pixels(&input_image, bmp_image_size) == NO_ERROR)
     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);      /* this line is completely redundant */
        printf("\nERROR: Memory allocation did not complete successfully!"); 
    } 

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

    return status; 
}