Calling C code from Go is mostly trivial for simple cases and it is also simple to pass exported Go functions as callback functions to the C code. However, many C libraries expect a pointer that can be passed back to the callback function and it is not possible to directly pass non-trivial Go data structes to the callback function implemented in Go -- an attempt results in a "cgo argument has Go pointer to Go pointer" runtime panic.
The Pool type contained in this package implements a container for
such callback data. Its Put method stores any Go type and returns an
unsafe.Pointer which maps directly to void* in function calls
crossing the GO/C boundary. This pointer references a simple numeric
index which is stored in calloc-allocated memory that will not be
moved around by the GC, therefore the pointer will not suddenly point
to unrelated data. This numeric index is used by the Get method to
access the object from a []interface{} slice which may be moved by
the GC.
A previous implementation contained in
go-yara used a mutex-protected
map[uintptr]interface{} to store callback data. If the uintptr
values were simply cast to unsafe.Pointer, go vet would complain
about "possible misuse of unsafe.Pointer". Passing a reference to this
uintptr value instead would not work in all use cases because I
ended up storing pointers to Go data in memory not accessible by the
GC, breaking the CGO pointer rules and leading to crashes.
BSD 2-clause, see LICENSE file in the source distribution.
Hilko Bengen bengen@hilluzination.de