Linux的ldconfig命令的代码原理：ldconfig如何从指定目录中查找动态库文件信息？

/* Add library to list.  将动态库信息添加到链表*/
      struct dlib_entry *dlib_ptr;
      for (dlib_ptr = dlibs; dlib_ptr != NULL; dlib_ptr = dlib_ptr->next)
 {
   /* Is soname already in list?  检查SONAME记录在链表中是否已存在，避免同一个库重复添加多次*/
   if (strcmp (dlib_ptr->soname, soname) == 0)
     {
       /* Prefer a file to a link, otherwise check which one
   is newer.  
         SONAME的判定，以常规文件为优先来源，而不优先以链接文件作为SONAME的来源。
         */
       if ((!is_link && dlib_ptr->is_link)
    || (is_link == dlib_ptr->is_link
        && _dl_cache_libcmp (dlib_ptr->name, direntry->d_name) < 0))
  {
    /* It s newer - add it.  */
    /* Flag should be the same - sanity check.  */
    if (dlib_ptr->flag != flag)
      {
        if (dlib_ptr->flag == FLAG_ELF
     && (flag == FLAG_ELF_LIBC5 || flag == FLAG_ELF_LIBC6))
   dlib_ptr->flag = flag;
        else if ((dlib_ptr->flag == FLAG_ELF_LIBC5
    || dlib_ptr->flag == FLAG_ELF_LIBC6)
          && flag == FLAG_ELF)
   dlib_ptr->flag = flag;
        else
   error (0, 0, _("libraries %s and %s in directory %s have same soname but different type."),
          dlib_ptr->name, direntry->d_name,
          entry->path);
      }
    free (dlib_ptr->name);
    dlib_ptr->name = xstrdup (direntry->d_name);
    dlib_ptr->is_link = is_link;
    dlib_ptr->osversion = osversion;
    dlib_ptr->isa_level = isa_level;
  }
       /* Don t add this library, abort loop.  */
       /* Also free soname, since it s dynamically allocated.  */
       free (soname);
       break;
     }
 }
      /* Add the library if it s not already in.  
        如果该动态库没在缓存列表中，则添加到链表记录中。
        这里看到，动态库的信息分为name、SONAME、flag、是否为链接、os版本、isa_level这几个方面
      */
      if (dlib_ptr == NULL)
 {
   dlib_ptr = (struct dlib_entry *)xmalloc (sizeof (struct dlib_entry));
   dlib_ptr->name = xstrdup (direntry->d_name);
   dlib_ptr->soname = soname;
   dlib_ptr->flag = flag;
   dlib_ptr->is_link = is_link;
   dlib_ptr->osversion = osversion;
   dlib_ptr->isa_level = isa_level;
   /* Add at head of list.  */
   dlib_ptr->next = dlibs;
   dlibs = dlib_ptr;
 }
    }
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/* Now dlibs contains a list of all libs - add those to the cache
     and created all symbolic links.  
     dlibs 包含了指定目录下查找到的所有动态库信息。将用于添加到缓存，并创建软链接（SONAME 文件）。
     */
  struct dlib_entry *dlib_ptr;
  for (dlib_ptr = dlibs; dlib_ptr != NULL; dlib_ptr = dlib_ptr->next)
    {
      /* The cached file name is the soname for non-glibc-hwcaps
  subdirectories (relying on symbolic links; this helps with
  library updates that change the file name), and the actual
  file for glibc-hwcaps subdirectories.  
     /etc/ld.so.cache 文件的
     */
      const char *filename;
      if (entry->hwcaps == NULL)
 {
   /* Don t create links to links.  
      不创建指向链接文件的链接。
      */
   if (dlib_ptr->is_link == 0)
     create_links (dir_name, entry->path, dlib_ptr->name,
     dlib_ptr->soname);
   filename = dlib_ptr->soname;
 }
      else
 {
   /* Do not create links in glibc-hwcaps subdirectories, but
      still log the cache addition.  
         glibc 的 hwcaps 是2.33 版本开始引入的功能，
         支持hwcaps意味着除了常规编译的库之外，现在还可以使用来自最新 CPU 
         的扩展 cpu 指令集来安装库，glibc 将自动选择针对当前使用的
         cpu 优化的版本。ldconfig对于glibc-hwcaps子目录下的动态库，
         不会创建对应的SONAME，但仍会记录记录到/etc/ld.so.cache文件内。
         以被Linux在运行时按需使用。
         */
   if (opt_verbose)
     printf ("\t%s -> %s\n", dlib_ptr->soname, dlib_ptr->name);
   filename = dlib_ptr->name;
 }
      if (opt_build_cache)
 add_to_cache (entry->path, filename, dlib_ptr->soname,
        dlib_ptr->flag, dlib_ptr->osversion,
        dlib_ptr->isa_level, hwcap, entry->hwcaps);
    }
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add_to_cache (entry->path, filename, dlib_ptr->soname,
        dlib_ptr->flag, dlib_ptr->osversion,
        dlib_ptr->isa_level, hwcap, entry->hwcaps);
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main() {……
    search_dirs (void)//获取所有待查找的目录列表
    {
      struct dir_entry *entry;
      for (entry = dir_entries; entry != NULL; entry = entry->next)
        search_dir (entry); // 进入每个目录，查找动态库，并记录到链表中
      } 
    }
     if (opt_build_cache)//若命令行参数要求生成缓存文件
    {
      save_cache (cache_file); //保存最终的缓存结果数据到 指定的缓存文件
                               //不指定文件路径则默认是/etc/ld.so.cache 文件。
      if (aux_cache_file)// aux_cache_file是辅助缓存文件。
     save_aux_cache (aux_cache_file);
      }
}
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