main function already defined in ocaml asmrun library

Question

I'm trying to call some ocaml code from a C program. I've been following some of the documentation here. The c program is called hello.c, and it is attempting to use Ocaml functions defined in callme.ml.

As in the link, I'm doing this in two steps: first compile the ml file to an object file:

ocamlopt -output-obj -o callme2.o callme.ml

And then attempt to link it to my 'main' binary with this code:

gcc  -Wall -I`ocamlopt -where` -L`ocamlopt -where` -lasmrun -lm -ldl  -o hello hello.c callme2.o -lasmrun

But I run into the following problem: main is already defined in libasmrun.a, and so it conflicts with the main in my own hello.c:

/tmp/ccANhYNH.o: In function `main':
hello.c:(.text+0x58): multiple definition of `main'
/home/orm/.opam/4.02.0/lib/ocaml/libasmrun.a(main.o):main.c:(.text+0x0): first defined here

How can I work around this? (As the library path suggests, I'm using ocaml version 4.02)

update: this problem has more to do with proper usage of C linker flags, rather than ocaml. Using the flags in the following order fixes the problem:

gcc  -Wall -I`ocamlopt -where` -L`ocamlopt -where` -o hello hello.c -lasmrun callme2.o -lm -ldl -lasmrun

This is interesting, because I thought it was illegal to have the same function name defined twice in the same program. Maybe this is one of the exceptions in that document.

Answer 1

Your command line is a little strange, as -lasmrun shows up twice.

Here's something that has worked for me:

$ W=`ocamlopt -where`
$ gcc -I $W -L $W -o hello hello.c callme.o -lasmrun -lm -ldl

You can see a working example in my pseudo-blog: Further OCaml GC Disharmony.

(As I learned the hard way, make sure you follow the rules of GC harmony :-)