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Re: [Discuss-gnuradio] Why are malloc_<type> in gr::fft?

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] Why are malloc_<type> in gr::fft?
Date: Thu, 23 Apr 2015 10:17:12 -0400

On Thu, Apr 23, 2015 at 4:53 AM, Marcus Müller <address@hidden> wrote:
Hi Jeon,

you just `malloc(...)` when you need to; why would you, in general, need GNU Radio to wrap that simple libc call, especially since there's calloc?

Or, is it not just a simple memory allocated array,
but a special array that something happens when items are put into the array?
It's special.
gr::fft needs these fftw_mallocs because FFTW (which is the FFT library underneath) expects specially aligned memory, and therefore offers it's own fftwf_malloc; thus, we need to wrap this up. It's an FFTW-speciality, and shouldn't be used for different things than memory that FFTW works on.
When you work with fftw (which is a C library) in C++, you'll notice that C++ is a bit more strict than C when it comes to pointer types, and you have to cast around the pointer type just to get it working. Since that is a bit ugly to do in-line, there's these malloc abstractions that you've mentioned.

Generally, as we try to write beautiful C++, we don't malloc much -- new[] might be your friend if you actually want to work with arrays.
However, GNU Radio is a bit doomed when it comes to having beautiful, type-safe code, because the core concept of gr_buffer basically is that buffers between blocks are just memory regions, not data of a specific type -- hence all the "ugly" pointer casting that happens in about every work().

There's a lot of places where `malloc` is really handy in our code, for example in the DVB decoders, where structures for decoding need to dynamically be allocated, or in the GUIs, where you often just want to have a bit of memory representing your framebuffer. But as a rule of thumb, you should try to avoid dynamically allocating memory manually (using malloc) as much as possible -- it's just one of the most popular causes for hard-to-debug bugs.

It's often practical to use `std::vector<type> myvector(number_of_elements)` instead -- it does the allocation for you, allows for resizing, and you can, just like with malloc, get the address of the first element ( `&(myvector[0])` ) . As a bonus, `myvector[x]` always actually has the right type, and you don't need to first cast the return value of malloc to a pointer of whatever you want to have.
If you don't need the resizing of `std::vector`, just the dynamic allocation, `std::array` is also awesome; both being STL containers offer a great deal of functionality that you'd have to implement yourself if you just go a C-style array in some malloc'ed memory.

Best regards,

I'll point people to the use of volk_malloc instead of malloc or new. See the bottom of this page:


This provides us with control over the alignment of the allocated memory, which sets us up nicely for VOLK calls.

For those "but actually's" in the works, I know there are various other methods out there and growing support in the compilers and C/C++ standards for this type of thing. The implementation of volk_malloc makes use of those when possible but provides the abstracted interface to make sure that the right thing(TM) is done for the given platform.

Still, we don't use this dynamically as Marcus said. If we need to allocate some memory in our blocks, we try to set this up in the constructor so that it's allocated once at startup (and then volk_free is called in the dtor).


On 04/23/2015 10:18 AM, Jeon wrote:
It's not a big deal, but I'm just curious.

Why are malloc functions, malloc_complex, malloc_double, malloc_float in gr::fft ?
I think memory allocating functions can be used in various purposes, not only in FFT.

Is it just a workaround that
there are no more proper places (gr::analog, gr::digital, gr::blocks...) for malloc?

Or, is it not just a simple memory allocated array,
but a special array that something happens when items are put into the array?


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