Question

Some standard C libraries that I want to access with Cython have a ton of flags. The Cython docs state that I must replicate the parts of the header I need. Which is fine when it comes to functions definitions. They are usually replicated everywhere, docs included. But what about all those magic numbers?

If I want to call mmap, I can always find the function definition and paste it into a .pxd file:

void *mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset)

But calling it needs a ton of flags like PROT_READ, MAP_ANONYMOUS and so on. I have at least two problems with this:

Firstly, it is annoying work to hunt down exactly where those numbers are defined. In fact I’d rather write a .c file and printf the values I need. Are there any better way of finding the value of a given flag such as PROT_READ?

Secondly, how stable are these numbers? Having extracted all the values I need and hardcoded them into my Cython source, what are the chances that compiling on a different platform has switched around, let’s say PROT_READ and PROT_EXEC?

Even if the answer is that there are no good or proper ways to do it, I’d like to hear it. I can always accept that something is cumbersome as long as I know I’m not missing something.

Answer 1

To use these constants from Cython, you don’t need to figure exactly where they came from or what they are any more than you do from C. For example, your .pxd file can look like

cdef extern from "foo.h":
    void *mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset)
    cdef int PROT_READ
    cdef int MAP_ANONYMOUS
    ...

As long as the definitions are (directly or indirectly) included from foo.h, this should work fine.