为EFS提供C API

问题：

EFS是我最近学习Erlang的过程中写的一个mini分布式文件系统，仿Google File System。希望给它提供一套C API，以方便利用现有的C库，如与NFSv3，或Fuse的整合。用Erl_interface库可以很easy的做到这一点，这是否从C调用Erlang的唯一方法？Port或linked-in Port是Erlang调C实现的功能，Port是在Erlang一方启动外部程序的，反过来行吗？[b](由port所启动的外部程序作为server，提供c api，也是可行的，按面向对象的理解，进程作为对象，提供服务作为接口）。[/b]

本质上，不管Port，还是Erl_interface，不同语言都是通过IPC机制来实现通信的，类似socket通信，要处理的是通信协议，链接管理，数据的编码和解码等。Erl_interface能极大的简化这些任务。

[quote]
Ports provide the basic mechanism for communication with the external world, from Erlang's point of view. They provide a byte-oriented interface to an external program. When a port has been created, Erlang can communicate with it by sending and receiving lists of bytes (not Erlang terms). This means that the programmer may have to invent a suitable encoding and decoding scheme.

A C program which uses the Erl_Interface functions for setting up a connection to and communicating with a distributed Erlang node is called a C node, or a hidden node. The main advantage with a C node is that the communication from the Erlang programmer's point of view is extremely easy, since the C program behaves as a distributed Erlang node.
[/quote]

大概思路如下：

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <errno.h>

#include "erl_interface.h"
#include "ei.h"

#include "efs.h"

int erlang_fd = -1;

/* -------------------------------------------------- */
/* API                                                */
/* -------------------------------------------------- */
int efs_init()
{
        int ret, fd;

        erl_init(NULL, 0);

        if (erl_connect_init(1, "secretcookie", 0) == -1)
                erl_err_quit("erl_connect_init");

        if ((fd = erl_connect("e1@uss")) < 0)
                erl_err_quit("erl_connect");

        fprintf(stderr, "Connected to e1@uss\n\r");
        erlang_fd = fd;

        return 0;
}

int efs_destroy()
{
        return 0;
}

int efs_create(const char *path, int mode)
{
        int ret;
        ETERM *args;
        ETERM *reply;

        args = erl_format("[~s]", path);

        reply = erl_rpc(erlang_fd, "efs", "create", args);
        if (reply == NULL) {
                ret = EINVAL;
                fprintf(stderr, "error %d\n", ret);
                goto err_ret;
        }

        erl_free_term(reply);
        erl_free_term(args);

        return 0;
err_ret:
        erl_free_term(args);
        return ret;
}

int efs_close(int fd)
{
        return 0;
}

/**
 * test utilities.
 * create fcount files, every file size is fsize.
 */
int efs_shell_ww(int fcount, int fsize)
{
        int ret;
        ETERM *args;
        ETERM *reply;

        args = erl_format("[~i,~i]", fcount, fsize);

        reply = erl_rpc(erlang_fd, "efs_shell", "ww", args);
        if (reply == NULL) {
                ret = EINVAL;
                fprintf(stderr, "error %d\n", ret);
                goto err_ret;
        }

        erl_free_term(reply);
        erl_free_term(args);

        return 0;
err_ret:
        erl_free_term(args);
        return ret;
}

/* -------------------------------------------------- */
/* Entry point                                        */
/* -------------------------------------------------- */
int main(int argc, char **argv)
{
        int ret;

        efs_init();

        ret = efs_create("afile", 0644);

        ret = efs_shell_ww(100, 1024*1024);


        return 0;
}