dlopen from memory?

Question

I\'m looking for a way to load generated object code directly from memory.

I understand that if I write it to a file, I can call dlopen to dynamically load its symbo

Answer 1

You don't need to load the code generated in memory, since it is already in memory!

However, you can -in a non portable way- generate machine code in memory (provided it is in a memory segment mmap-ed with PROT_EXEC flag).

^{(in that case, no "linking" or relocation step is required, since you generate machine code with definitive absolute or relative addresses, in particular to call external functions)}

Some libraries exist which do that: On GNU/Linux under x86 or x86-64, I know of GNU Lightning (which generates quickly machine code which runs slowly), DotGNU LibJIT (which generates medium quality code), and LLVM & GCCJIT (which is able to generate quite optimized code in memory, but takes time to emit it). And LuaJit has some similar facility too. Since 2015 GCC 5 has a gccjit library.

And of course, you can still generate C code in a file, fork a compiler to compile it into a shared object, and dlopen that shared object file. I'm doing that in GCC MELT , a domain specific language to extend GCC. It does work quite well in practice.

addenda

If performance of writing the generated C file is a concern (it should not be, since compiling a C file is much slower than writing it) consider using some tmpfs file system for that (perhaps in /tmp/ which is often a tmpfs filesystem on Linux)