1> erlang:function_exported(crypto,start,0). false 2> crypto:start(). ok 3> erlang:function_exported(crypto,start,0). true
-module(test). -compile(export_all). function_exported(Module, Function, Arity) -> case erlang:module_loaded(Module) of true -> next; _ -> code:load_file(Module) end, erlang:function_exported(Module, Function, Arity).另外，如果模块被改成bif也无法判断了。不过这倒不用太过担心，我们自己写的函数都不会是bif
BIF_RETTYPE function_exported_3(BIF_ALIST_3)
{
if (is_not_atom(BIF_ARG_1) || is_not_atom(BIF_ARG_2) || is_not_small(BIF_ARG_3)) {
BIF_ERROR(BIF_P, BADARG);
}
if (erts_find_function(BIF_ARG_1, BIF_ARG_2, signed_val(BIF_ARG_3),
erts_active_code_ix()) == NULL) {
BIF_RET(am_false);
}
BIF_RET(am_true);
}erts_find_function实现在erts\emulator\beam\export.c中：/*
* Find the export entry for a loaded function.
* Returns a NULL pointer if the given function is not loaded, or
* a pointer to the export entry.
*
* Note: This function never returns export entries for BIFs
* or functions which are not yet loaded. This makes it suitable
* for use by the erlang:function_exported/3 BIF or whenever you
* cannot depend on the error_handler.
*/
Export* erts_find_function(Eterm m, Eterm f, unsigned int a, ErtsCodeIndex code_ix)
{
struct export_templ templ;
struct export_entry* ee;
ee = hash_get(&export_tables[code_ix].htable, init_template(&templ, m, f, a));
if (ee == NULL || (ee->ep->addressv[code_ix] == ee->ep->code+3 &&
ee->ep->code[3] != (BeamInstr) BeamOp(op_i_generic_breakpoint))) {
return NULL;
}
return ee->ep;
}这里，export_tables是一个全局变量，还是在export.c，其中，code_ix被用来控制代码版本，有时间的话再讨论erlang热更新机制static IndexTable export_tables[ERTS_NUM_CODE_IX]; /* Active not locked */所以，erlang:function_exported/3只是去查找导出函数的哈希表，找到返回true，没有就false
原文：http://blog.csdn.net/mycwq/article/details/40663737