android开发----4.parser分析

android源码版本-->4.4.2
linux源码版本-->3.4
一. 总体分析
init_parse_config_file("/init.rc");

二.代码分析
在system/core/init/init.c中

int init_parse_config_file(const char *fn)
{
    char *data;
//将"/init.rc"的内容全部读取到以data为首指针的buffer中,函数会调用malloc(sz+2)
//多分配的两个字体是结束标志,会在文件最后添加结束标志\n\0
    data = read_file(fn, 0);
    if (!data) return -1;

    parse_config(fn, data);
    DUMP();
    return 0;
}

在system/core/init/init_parser.c中

static void parse_config(const char *fn, char *s)
{
    struct parse_state state;
    struct listnode import_list;
    struct listnode *node;
    char *args[INIT_PARSER_MAXARGS];
    int nargs;

    nargs = 0;
    state.filename = fn;
    state.line = 0;
    state.ptr = s;
    state.nexttoken = 0;
    state.parse_line = parse_line_no_op;     //这个函数的内容是空

    list_init(&import_list);
    state.priv = &import_list;

    for (;;) {
        switch (next_token(&state)) {
        case T_EOF:
            state.parse_line(&state, 0, 0);  //这个函数的内容是空
            goto parser_done;
        case T_NEWLINE:
            state.line++;
            if (nargs) {
                int kw = lookup_keyword(args[0]);
                if (kw_is(kw, SECTION)) {            //对service on import这三类进行处理
                    state.parse_line(&state, 0, 0);  //这个函数的内容是空? 这么想就错了
                    parse_new_section(&state, kw, nargs, args);
                } else {
                    state.parse_line(&state, nargs, args); //这个函数的内容是空? 这么想就错了
                }
                nargs = 0;
            }
            break;
        case T_TEXT:
            if (nargs < INIT_PARSER_MAXARGS) {
                args[nargs++] = state.text;   //如果是text则放到字符串数组中,直到section结束
                INFO("text=%s", state.text);  //做为section的参数
            }
            break;
        }
    }

parser_done:
    list_for_each(node, &import_list) {
         struct import *import = node_to_item(node, struct import, list);
         int ret;

         INFO("importing '%s'", import->filename);
         ret = init_parse_config_file(import->filename);
         if (ret)
             ERROR("could not import file '%s' from '%s'\n",
                   import->filename, fn);
    }
}

下面以为例说明一下对rc脚本的解析

# virtual sdcard daemon running as media_rw (1023)
service sdcard /system/bin/sdcard -u 1023 -g 1023 -l /data/media /mnt/shell/emulated
class late_start

第1步: next_token会顺序搜索service这一行,并按空格依次分隔每个字段

args[0]=service
args[1]=sdcard
args[2]=/system/bin/sdcard
args[3]=-u
args[4]=1023
args[5]=-g
args[6]=1023
args[7]=-l
args[8]=/data/media
args[9]=/mnt/shell/emulated
最后nargs=9,完成之后遇到\n换行符就到了case

执行完后nargs=9,此行解析完成后遇到\n换行符,case T_NEWLINE
第2步:进入parse_new_section

void parse_new_section(struct parse_state *state, int kw,
                       int nargs, char **args)
{
    switch(kw) {
    case K_service:   //因为本实例的关键字是service,所以进入parse_service
        state->context = parse_service(state, nargs, args);
        if (state->context) {
            state->parse_line = parse_line_service;   //改函数指针
            return;
        }
        break;
    case K_on:
        state->context = parse_action(state, nargs, args);
        if (state->context) {
            state->parse_line = parse_line_action;
            return;
        }
        break;
    case K_import:
        parse_import(state, nargs, args);
        break;
    }
    state->parse_line = parse_line_no_op;
}

第3步:进入parse_service-->构造struct service并放到链表中

static void *parse_service(struct parse_state *state, int nargs, char **args)
{
    struct service *svc;
    //检查参数的个数: 最小的例子:service sdcard /system/bin/sdcard,所以最小是3个
    if (nargs < 3) {
        parse_error(state, "services must have a name and a program\n");
        return 0;
    }
    //检查service_name是否合法:长度不能大于16,不能含有除字母 数字 _ - 之外的字符
    if (!valid_name(args[1])) {
        parse_error(state, "invalid service name '%s'\n", args[1]);
        return 0;
    }
    //如果在service链表中找到了,说明有重名的service,这也不合法
    svc = service_find_by_name(args[1]);
    if (svc) {
        parse_error(state, "ignored duplicate definition of service '%s'\n", args[1]);
        return 0;
    }
    //最后构造struct service,放到链表中
    nargs -= 2;
    svc = calloc(1, sizeof(*svc) + sizeof(char*) * nargs);
    if (!svc) {
        parse_error(state, "out of memory\n");
        return 0;
    }
    svc->name = args[1];
    svc->classname = "default";
    memcpy(svc->args, args + 2, sizeof(char*) * nargs);  //复制参数
    svc->args[nargs] = 0;
    svc->nargs = nargs;
    svc->onrestart.name = "onrestart";
    list_init(&svc->onrestart.commands);
    list_add_tail(&service_list, &svc->slist);
    return svc;
}

第4步: 设置flags

static void parse_line_service(struct parse_state *state, int nargs, char **args)
{
    svc->ioprio_class = IoSchedClass_NONE;
    kw = lookup_keyword(args[0]);
    switch (kw) {
    case K_capability:
        break;
    case K_class:
        if (nargs != 2) {
            parse_error(state, "class option requires a classname\n");
        } else {
            svc->classname = args[1];
        }
        break;
    ......
    case K_oneshot:
        svc->flags |= SVC_ONESHOT;     //只是改了标志位而己
        break;
    ....
    default:
        parse_error(state, "invalid option '%s'\n", args[0]);
    }
}