c - C 中的硬链接(hard link)表

Question

我无法理解 AddToList 的工作原理，如果 gHeadPtr 始终指向第一个(最小)评级结构我理解它，但 gHeadPtr 没有指向它，或者我错了？或者有人可以告诉我 AddToList 是如何工作的？我不知道最后一个字符串是什么意思，为什么我们需要双指针以及 gHeadPtr 的结构点以及 gHeadPtr 何时指向第一个(最小)评级结构，何时添加到我们刚刚添加的结构(具有最大评级)

struct DVDInfo
{
     char           rating;
     char           title[ kMaxTitleLength ];
     char           comment[ kMaxCommentLength ];
     struct DVDInfo  *prev;
     struct DVDInfo *next;
};

char            GetCommand( void );
struct DVDInfo  *ReadStruct( void );
void            AddToList( struct DVDInfo *curPtr );
void            ListDVDs( bool forward );
char            *TrimLine( char *line );

struct DVDInfo *gHeadPtr, *gTailPtr;

int main (int argc, const char * argv[])
{
     char command;   
     while ( (command = GetCommand() ) != 'q' ) 
     {
          switch( command ) 
          {
               case 'n':
                    AddToList( ReadStruct() );
                    break;
               case 'l':
               case 'r':
                    ListDVDs( command=='l' );
                    break;
          }
          printf( "\n----------\n" );
     }  
     printf( "Goodbye...\n" );  
     return 0;
}

char GetCommand( void )
{
     char buffer[ 100+1 ];
     printf( "Enter command (q=quit, n=new, l=list, r=reverse list):  " );
     fgets( buffer, sizeof(buffer), stdin );
     return *TrimLine( buffer );
}

struct DVDInfo *ReadStruct( void )
{
     struct DVDInfo *infoPtr;   
     infoPtr = malloc( sizeof( struct DVDInfo ) );  
     if ( infoPtr == NULL ) 
     {
          printf( "Out of memory!!!  Goodbye!\n" );
          exit( 1 );
     }
     char buffer[ 500+1 ];    
     printf( "Enter DVD Title:  " );
     fgets( buffer, sizeof(buffer), stdin );
     strlcpy( infoPtr->title, TrimLine( buffer ), sizeof(infoPtr->title) ); 
     printf( "Enter DVD Comment:  " );
     fgets( buffer, sizeof(buffer), stdin );
     strlcpy( infoPtr->comment, TrimLine( buffer ), sizeof(infoPtr->comment) ); 
     int num;
     do 
     {
          printf( "Enter DVD Rating (1-10):  " );
          fgets( buffer, sizeof(buffer), stdin );
          num = atoi( TrimLine( buffer ) );
     }
     while ( ( num < 1 ) || ( num > 10 ) );
     infoPtr->rating = num; 
     return( infoPtr );
}

void AddToList( struct DVDInfo *curPtr )
{
     struct DVDInfo **nextPtrPtr = &gHeadPtr;
     struct DVDInfo *prevPtr = NULL;    
     while ( *nextPtrPtr != NULL && curPtr->rating > (*nextPtrPtr)->rating ) 
     {
          prevPtr = *nextPtrPtr;
          nextPtrPtr = &(prevPtr->next);
     }
     curPtr->prev = prevPtr;                // link to previous struct
     curPtr->next = *nextPtrPtr;              // link to next struct
     if ( curPtr->next != NULL )
          curPtr->next->prev = curPtr;       // link prev of next struct to curPtr
     else
          gTailPtr = curPtr;                  // no next struct: curPtr is now the tail
     *nextPtrPtr = curPtr;                      // link next or previous struct (or head) to curPtr
} //когда функция передах структкру, а потом получает новую, указатели сохраняются?

void ListDVDs( bool forward )
{
     struct DVDInfo *curPtr = ( forward ? gHeadPtr : gTailPtr );
     bool separator = false;    
     if ( curPtr == NULL ) 
     {
          printf( "No DVDs have been entered yet...\n" );
     } 
     else 
     {
          while ( curPtr != NULL ) 
          {
               if ( separator )
                    printf( "--------\n" );
               printf( "Title:   %s\n", curPtr->title );
               printf( "Comment: %s\n", curPtr->comment );
               printf( "Rating:  %d\n", curPtr->rating );            
               curPtr = ( forward ? curPtr->next : curPtr->prev );
               separator = true;
          }
     }
}

char *TrimLine( char *line )
{
     size_t length = strlen( line );
     while ( length > 0 && isspace( line[length-1] )) 
     {
          line[length-1] = '\0';
          length--;       
     }
     return line + strspn( line, " \t" );
}

Answer 1

struct DVDInfo **nextPtrPtr = &gHeadPtr;
struct DVDInfo *prevPtr = NULL;

nextPtrPtr 是必需的，因为程序员不想弄乱全局头指针(gHeadPtr)。我们只是使用此指针来迭代列表，因为使用指针到指针比使用头指针本身进行迭代更好。

while ( *nextPtrPtr != NULL && curPtr->rating > (*nextPtrPtr)->rating ) 
    {
    prevPtr = *nextPtrPtr;
    nextPtrPtr = &(prevPtr->next);
    }

上面来自 AddToList 的代码处理查找新节点应在列表中进入的位置(即电影的降序评级)。

curPtr->prev = prevPtr;                 // link to previous struct
curPtr->next = *nextPtrPtr;               // link to next struct

上面的行用于插入链接列表。

if ( curPtr->next != NULL )
    curPtr->next->prev = curPtr;     // link prev of next struct to curPtr
else
    gTailPtr = curPtr; 

如果新节点 (curPtr) 是链表中的第一个节点或最后一个节点，则使用上面的这些行。