Removing an element from a linked list

Question

Assuming I have this list below :

List of CDData
    1: Test1
    2: Test2
    3: Test3
    4: Test4
    5: Test5
    6: Test6

Now I want to delete the third one from the list using the linked list : which means free(removeFromDList(3)); This is my function :

TCD *removeFromDList(int res)
{
    int count = 0;
    TCD *CDData = First;
    CDData->Prev = First;

    if (First == NULL)
    {
        printf("Error\n");
        return NULL;
    }

    while (CDData)
    {
        count++;
        if (count == res)
        {
            if (count == 1)
            {
                if (CDData == Last)
                    Last = NULL;
                First = First->Next;
                return CDData;
            }
            else
            {
                while (CDData != NULL)
                {
                    CDData->Prev = CDData->Next;
                    if (CDData == Last)
                        Last = CDData->Prev;

                    // printf("%s",CDData->Title) I tested here whether my function is going to 
                    //  delete the third one or not with the printf() and it's actually printing the third one 
                    // Which means it's correct 
                   
                    return CDData;
                }
            }
        }

        else
            CDData->Prev = CDData;
        CDData = CDData->Next;
    }
}

By the way , this is the definition of TCD

typedef struct F
{
 char *Title;
 struct F *Next;
 struct F *Prev;
}TCD;

Now after re-printing my list it seems that all of CDData(The whole Data structure ) have been freed. Any ideas why ?

I get this as an output

List of CDData

 
 
  
   

Answer 1

I think in this structure definition

typedef struct F
{
 char *Titel;
 struct F *Next;
 struct F *Prev;
}TCD;

you mean that the name of the first data member is Title instead of Titel.

And you need to allocate dynamically memory for a string that will be pointed to by this data member.

Your function can invoke undefined behavior at least because initially the global variable First (by the way it is a bad idea when functions depend on global variables) can be equal to NULL. So in this code snippet

TCD *CDData = First;
CDData->Prev = First;

there is an attempt to use a null pointer (CDData->Prev) to access memory.

This inner while loop

            while (CDData != NULL)
            {
                CDData->Prev = CDData->Next;
                if (CDData == Last)
                    Last = CDData->Prev;

                // printf("%s",CDData->Title) I tested here whether my function is going to 
                //  delete the third one or not with the printf() and it's actually printing the third one 
                // Which means it's correct 
               
                return CDData;
            }

does not make sense.

Pay attention to that in C indices start from 0.

Using your approach the function can be written the following way as it is shown in the demonstrative program below.

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

typedef struct F
{
    char *Title;
    struct F *Next;
    struct F *Prev;
} TCD;

TCD *First, *Last;

TCD * removeFromDList( size_t n )
{
    TCD **Current = &First;
    
    while ( *Current && n--)
    {
        Current = &( *Current )->Next;
    }
    
    TCD *target = *Current;
    
    if ( *Current )
    {
        if ( ( *Current )->Next == NULL ) 
        {
            Last = ( *Current )->Prev;  
        }
        else
        {
            ( *Current )->Next->Prev = ( *Current )->Prev;
        }
        
        *Current = ( *Current )->Next;
    }
    
    return target;
}

int append( const char *s )
{
    TCD *Current = malloc( sizeof( TCD ) );
    int success = Current != NULL;
    
    if ( success )
    {
        Current->Title = malloc( strlen( s ) + 1 );
        success = Current->Title != NULL;
        
        if ( success )
        {
            strcpy( Current->Title, s );
            Current->Prev = Last;
            Current->Next = NULL;
        }
        else
        {
            free( Current );
        }
    }
    
    if ( success )
    {
        if ( Last )
        {
            Last = Last->Next = Current;
        }
        else
        {
            First = Last = Current;
        }
    }
    
    return success;
}

FILE * display( FILE *fp )
{
    for ( TCD *Current = First; Current; Current = Current->Next )
    {
        fprintf( fp, "%s -> ", Current->Title );
    }
    
    fputs( "NULL", fp );
    
    return fp;
}

int main(void) 
{
    
    for ( char s[] = "A"; s[0] != 'E'; ++*s )
    {
        append( s );
    }
    
    
    fputc( '\n', display( stdout ) );
    
    TCD *p = removeFromDList( 3 );
    free( p->Title );
    free( p );
    fputc( '\n', display( stdout ) );
    
    p = removeFromDList( 1 );
    free( p->Title );
    free( p );
    fputc( '\n', display( stdout ) );
    
    p = removeFromDList( 0 );
    free( p->Title );
    free( p );
    fputc( '\n', display( stdout ) );
    
    p = removeFromDList( 0 );
    free( p->Title );
    free( p );
    fputc( '\n', display( stdout ) );
    
    printf( "First == NULL : %d, Last == NULL : %d\n", First == NULL, Last == NULL );
    return 0;
}

The program output is

A -> B -> C -> D -> NULL
A -> B -> C -> NULL
A -> C -> NULL
C -> NULL
NULL
First == NULL : 1, Last == NULL : 1