I am writing a C program with the nDPI library, available here. (Coding on a Ubuntu machine, GCC compiler, nDPI version 3.2) nDPI is used to inspect network traffic. The code uses a lot of different structs to represent network stuff, like network flows, network protocols, network hosts, and so on.
So I think that if you want to create these structs, you must use the library's custom malloc() and free() functions, which makes sense. But I'm having a hard time understanding the function prototypes. Here's a few relevant lines of code from the API header file:
/* Utility functions to set ndpi malloc/free/print wrappers */
void set_ndpi_flow_malloc(void* (*__ndpi_flow_malloc)(size_t size));
void set_ndpi_flow_free(void (*__ndpi_flow_free)(void *ptr));
Consider the set_ndpi_flow_malloc() function. Elsewhere in the source code, there is a struct nDPI_flow_info defined, and I see that struct used everywhere in the code. So I assume set_ndpi_flow_malloc() is to allocate one of these structs on the heap, populate it with some information maybe, and return the pointer.
But I don't understand the arguments into this function. If I had to guess, I'd say that set_ndpi_flow_malloc() accepts a pointer to another function, called __ndpi_flow_malloc(), and that function takes a size_t integer as an argument. Turns out, there's this function prototype elsewhere in the API header file:
void * ndpi_flow_malloc(size_t size);
That's kind of the function I need: I need __ndpi_flow_malloc() (two underline characters on the front of the name) What do those double underlines mean?
Another question... if set_ndpi_flow_malloc() is a custom malloc() function, shouldn't it return a pointer to the allocated memory?
Another question... How would I actually write this in my code? Like this...?
struct nDPI_flow_info* myFlow;
set_ndpi_flow_malloc( (void*) &ndpi_flow_malloc( sizeof( struct nDPI_flow_info )) );
// ...use the struct...
set_ndpi_flow_free( &ndpi_flow_free* myFlow ) );
That can't be right. I don't understand how the second line returns the pointer and assigns it to variable myFlow.
Any advice or criticism is wildly appreciated. Thanks!
FULL DISCLOSURE :: I have also posted this question here.
The function set_ndpi_flow_malloc doesn't actually do the allocation but allows you to set the function that does. It's argument is a pointer to a function that takes a size_t and returns a void *, and the name of the argument is __ndpi_flow_malloc.
The same goes for set_ndpi_flow_free. It tells the library which function to use as its custom free function.
Most likely, ndpi_flow_malloc is the default custom allocator. So if this is the one you want to use you would do the following to set it as the custom allocation function:
set_ndpi_flow_malloc(ndpi_flow_malloc);
Then assuming there is a similar default free function called ndpi_flow_free, you would do this to set it as the custom free function:
set_ndpi_flow_free(ndpi_flow_free);