Busybox 框架代码分析

对Busybox 1.1.2版的大体框架进行了抽取，以便于看清。

/*
Author: 
Framework of busybox

A simple demo to show how busybox works

complie:
gcc ***.c -o busybox

run:
./busybox bbtest

*/

#include <stdlib.h>
#include <stdarg.h>
#include <errno.h>
#include <string.h>
#include <stdio.h>

/*
包含名字和入口地址函数的结构体
*/
struct BB_applet
{
   const char* name;
   int (*main) (int argc, char** argv);
};

int bbtest_main(int argc, char* argv[]);
void bb_error_msg_and_die(const char *s, ...);
void run_applet_by_name(const char* name,int argc,char* argv[]);
static struct BB_applet* applet_using;
char* bb_applet_name = NULL;
/*
结构体数组
*/
const struct BB_applet applets[] =
{
#define APPLET(a,b) {#a,b}
   APPLET(bbtest, bbtest_main)
};

const size_t NUM_APPLETS = (sizeof (applets) / sizeof (struct BB_applet));

int main(int argc, char** argv)
{
   const char* s;

   bb_applet_name = argv[0];
   /*
   if (*bb_applet_name == '-') bb_applet_name++;
   */
/*
对类似于 “./busybox vi”
的命令进行解析
*/
   for (s = bb_applet_name; *s ;)
       if (*(s++) == '/') bb_applet_name = s;

   /* Set locale for everybody except `init' */
   /*
   if(ENABLE_LOCALE_SUPPORT && getpid() != 1)
       setlocale(LC_ALL, "");
       */

   printf("%s\n",bb_applet_name);
/*
按名字找到对应的组件程序，并执行
*/
   run_applet_by_name(bb_applet_name, argc, argv);
   /*
   bb_error_msg_and_die("applet not found");
   */
}


int busybox_main(int argc, char** argv)
{
   /*
    * This style of argument parsing doesn't scale well
    * in the event that busybox starts wanting more --options.
    * If someone has a cleaner approach, by all means implement it.
    */
   /*
   if (ENABLE_FEATURE_INSTALLER && argc > 1 && !strcmp(argv[1], "--install")) {
       int use_symbolic_links = 0;
       int rc = 0;
       char *busybox;
       */

   /* to use symlinks, or not to use symlinks... */
   /*
       if (argc > 2) {
           if ((strcmp(argv[2], "-s") == 0)) {
               use_symbolic_links = 1;
           }
       }
       */

   /* link */
   /*
       busybox = xreadlink("/proc/self/exe");
       if (busybox) {
           install_links(busybox, use_symbolic_links);
           free(busybox);
       } else {
           rc = 1;
       }
       return rc;
   }
   */

   /* Deal with --help.  (Also print help when called with no arguments) */

   if (argc == 1 || !strcmp(argv[1], "--help") )
   {
       if (argc > 2)
       {
           run_applet_by_name(bb_applet_name = argv[2], 2, argv);
       }
       else
       {
           const struct BB_applet* a;
           int col, output_width;

           /*
           if (ENABLE_FEATURE_AUTOWIDTH)
           {
           */
               /* Obtain the terminal width.  */
           /*
               get_terminal_width_height(0, &output_width, NULL);
               */
               /* leading tab and room to wrap */
           /*
               output_width -= 20;
           }
           else output_width = 60;
           */


           printf(/*"%s\n\n"*/
                  "Usage: busybox [function] [arguments]...\n"
                  "   or: [function] [arguments]...\n\n"
                  "\tBusyBox is a multi-call binary that combines many common Unix\n"
                  "\tutilities into a single executable.  Most people will create a\n"
                  "\tlink to busybox for each function they wish to use and BusyBox\n"
                  "\twill act like whatever it was invoked as!\n"
                  "\nCurrently defined functions:\n"/*, bb_msg_full_version*/);
           col = 0;
           for (a = applets; a->name;)
           {
               col += printf("%s%s", (col ? ", " : "\t"), (a++)->name);
               if (col > output_width && a->name)
               {
                   printf(",\n");
                   col = 0;
               }
           }

           printf("\n\n");
           exit(0);
       }
   }
   else run_applet_by_name(bb_applet_name = argv[1], argc - 1, argv + 1);

   bb_error_msg_and_die("applet not found");
}

struct BB_applet* find_applet_by_name (const char* name);
void run_applet_by_name (const char* name, int argc, char** argv)
{
   /*
   if(ENABLE_FEATURE_SUID_CONFIG) parse_config_file ();
   */
/*
如果命令是“./busybox vi”形式的
*/
   if (!strncmp(name, "busybox", 7)) busybox_main(argc, argv);
   /* Do a binary search to find the applet entry given the name. */
   applet_using = find_applet_by_name(name);
   if (applet_using)
   {
       bb_applet_name = applet_using->name;
       /*if (argc == 2 && !strcmp(argv[1], "--help")) bb_show_usage ();*/
       /*        if(ENABLE_FEATURE_SUID) check_suid (applet_using);*/
/*
跳转到对应的组件程序执行
*/
       exit ((*(applet_using->main)) (argc, argv));
   }
}

/*
比较组件程序名字的回调函数
*/
static int applet_name_compare (const void* x, const void* y)
{
   const char* name = x;
   const struct BB_applet* applet = y;

   return strcmp (name, applet->name);
}

/*
按名字进行二分查找
这也是applet.h 的head中一直强调要按字符顺序来添加组件入口的原因
后续版本的busybox中已经做了修改，会对组件结构体进行qsort排序
*/
struct BB_applet* find_applet_by_name (const char* name)
{
   return bsearch (name, applets, NUM_APPLETS, sizeof (struct BB_applet),
                   applet_name_compare);
}

/*
作为测试组件的main函数
*/
int bbtest_main(int argc, char* argv[])
{
   printf("/***********************************************************/\n"
          "                  Now in bbtest!\n");
   return 0;
}


void bb_verror_msg(const char *s, va_list p)
{
   fflush(stdout);
   fprintf(stderr, "%s: ", bb_applet_name);
   vfprintf(stderr, s, p);
}

int bb_default_error_retval=EXIT_FAILURE;
void bb_error_msg_and_die(const char *s, ...)
{
   va_list p;

   va_start(p, s);
   bb_verror_msg(s, p);
   va_end(p);
   putc('\n', stderr);
   exit(bb_default_error_retval);
}

阅读的时候遇到下面这段代码里的类似USE_TEST,USE_ADDGROUP之类的宏定义一直不理解。

这些宏定义只有在applets.h里面出现过一次，在所有的源文件中找不到其定义，按常理是不会通过编译的。

后来问了寝室里的大师兄才了解到，编译的时候会自动生成bb_config.h这个头文件。里面定义了这些宏定义，其实就是直接脱掉了外层的宏展开。

（编译时提示“procps.c:15:22: error: asm/page.h: No such find.”的解决方法）

这种在编译的最开始时才生成头文件的技术是第一次遇到，果然见识还是太短了。

  const struct BB_applet applets[] = {
# define APPLET(a,b,c,d) {#a,b,c,d},
# define APPLET_NOUSAGE(a,b,c,d) {#a,b,c,d},
# define APPLET_ODDNAME(a,b,c,d,e) {#a,b,c,d},
#endif

#ifdef CONFIG_INSTALL_NO_USR
# define _BB_DIR_USR_BIN _BB_DIR_BIN
# define _BB_DIR_USR_SBIN _BB_DIR_SBIN
#endif


USE_TEST(APPLET_NOUSAGE([, test_main, _BB_DIR_USR_BIN, _BB_SUID_NEVER))
USE_TEST(APPLET_NOUSAGE([[, test_main, _BB_DIR_USR_BIN, _BB_SUID_NEVER))
USE_ADDGROUP(APPLET(addgroup, addgroup_main, _BB_DIR_BIN, _BB_SUID_NEVER))
/*
.............
*/
};