Thread: Please Need help with a linked List

The code bellow creates structures and links them using pointers,
prints the structures or quits and frees the allocated memory
for the structures

what happens is the following
i add the first structure... prints fine
i add the second structure... prints fine
i add the third... press print and the program crashes

Code:

//*FSpointer is the first structure in the list
void printfunc(typeLS *FSpointer){
    
    typeLS *holder3 = FSpointer;
    
    while(holder3 != NULL){

        printf("name:%s float:%f\n",holder3->name,holder3->decimals);
        
        holder3 = holder3->forward;
        if(holder3 != NULL){
            printf("not NULL\n");
        }
    }
    printf("exited loop\n");

so far i know that the while loop can't stop if i add more than 2
structures, because for some reason structure 3 forward pointer
is not NULL even though there is no structure after it.

I can't grasp where the code logic is wrong, adding a second structure works fine, and adding a 3rd structure uses the same logic as adding the second structure but it doesn't work WTF?

Code:

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

typedef struct LSexample{
    
    char name[52];
    float decimals;
    struct LSexample *forward;
}typeLS;

typeLS *returnPointerToAllocatedMemory(typeLS *MainPointers){
    char UserInput5[16];
    fgets(UserInput5, 15,stdin);
    typeLS *PointertoAllocatedMemory = malloc(sizeof(typeLS));

    sscanf(UserInput5,"%s %f",PointertoAllocatedMemory->name,&PointertoAllocatedMemory->decimals );

    printf("Name:%s float:%f\n",PointertoAllocatedMemory->name,PointertoAllocatedMemory->decimals);
    if(MainPointers != NULL){//if there is somethign behind then set
                                //    whats behind pointer to point at the newly created structure 
        MainPointers->forward = PointertoAllocatedMemory;//mainpointers is the previous structure
                                                        // if there isn't one it will be NULL
    }
    return PointertoAllocatedMemory;
}
void cleanUp(typeLS *FirstStructure1){
    
    
    typeLS *freeThisAllocatedMemory = NULL;
    typeLS *HoldPointerForward = NULL;
    freeThisAllocatedMemory = FirstStructure1;
    
    if(freeThisAllocatedMemory != NULL){
        printf("Free name:%s float %f\n",freeThisAllocatedMemory->name,freeThisAllocatedMemory->decimals);
        HoldPointerForward = freeThisAllocatedMemory->forward;
        free(freeThisAllocatedMemory); 
        freeThisAllocatedMemory = HoldPointerForward;

    }
}
void printfunc(typeLS *FSpointer){
    
    typeLS *holder3 = FSpointer;
    
    while(holder3 != NULL){

        printf("name:%s float:%f\n",holder3->name,holder3->decimals);
        
        holder3 = holder3->forward;
        if(holder3 != NULL){
            printf("not NULL\n");
        }
    }
    printf("exited loop\n");
    
}


int main(){
    typeLS *Firststructure = NULL;//this pointer will point at the first structure in the list 
                                    
    typeLS *holderr = NULL; //this pointer will point at structures temporarily
    char UserInput[16];
    char translation[16];
    while(fgets(UserInput, 15,stdin)){
        sscanf(UserInput,"%s",translation);
        if(strncmp(translation, "add",3)==0){
            printf("Ready to add...\n");
            if(Firststructure == NULL){
                Firststructure = returnPointerToAllocatedMemory(NULL);
                holderr = Firststructure;
            }else{
                holderr = returnPointerToAllocatedMemory(holderr);
            }
        }else if(strncmp(translation, "print",5)==0){
            printf("Printing...\n");
            printfunc(Firststructure);
        }else if(strncmp(translation, "quit",4)==0){
            printf("Breaking...\n");
            break;
        }
    }
    cleanUp(Firststructure);
    return 0;
}

That code looks really messy.

I think this should be a working version of adding a node to a list though :

Code:

struct node
{
    int data;
    struct node *next;
};

// append node after current one
void insert( struct node *curr, int data )
{
    struct node *tmp = new_node( data );
    struct node *old = curr->next;

    curr->next = tmp
    tmp->next = old;

    return;
}

I think that should do it. This code isn't tested but the basic idea is, you take the previous "next" pointer of the current node and then assign it to the new node. The new node's "next" pointer inherits the old value.

Thanks mutantjohn but i already know how a singly linked list works
anyway it turns out there was no problem with any of my code logic
apart from the cleanUp() function but that's besides the point,
it turns out that the crash was caused by the name[52]
when i changed it to 12 the crashing stopped, really weird,
is there a limit to how big an array should be inside a structure?

That sounds super duper odd and like something is very, very wrong.

This code works perfectly.

Code:

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


struct node
{
    char data[ 52 ];
    struct node *next;
};


struct node* new_node( const char *str )
{
    struct node *tmp = malloc( sizeof( *tmp ) );


    strcpy( tmp->data, str );
    tmp->next = 0;


    return tmp;
}


// insert node after current one
// return pointer to newly created node
struct node* insert( struct node *curr, const char *str )
{
    struct node *tmp = new_node( str );


    struct node *old = curr->next;


    curr->next = tmp;
    tmp->next = old;


    return tmp;
}


void free_list( struct node *head )
{
    while ( head != 0 )
    {
        struct node *old = head;
        head = head->next;


        free( old );
    }
}


void print_list( struct node *head )
{
    while ( head != 0 )
    {
        printf( "%s\n", head->data );


        head = head->next;
    }
}


int main( void )
{
    struct node *head = new_node( "I" );


    struct node *tail = insert( head, "love" );
    tail = insert( tail, "myself!" );
    tail = insert( tail, "And" );
    tail = insert( tail, "when" );
    tail = insert( tail, "you" );
    tail = insert( tail, "lookin'" );
    tail = insert( tail, "at" );
    tail = insert( tail, "me," );
    tail = insert( tail, "tell" );
    tail = insert( tail, "me," );
    tail = insert( tail, "what" );
    tail = insert( tail, "do" );
    tail = insert( tail, "you" );
    tail = insert( tail, "see?" );


    print_list( head );


    free_list( head );


    return 0;
}

how long have you been coding?

sorry i just thought if it's a bug maybe i should warn everyone

I think the problem might be in your sscanf calls. Or rather, it's in how you're acquiring user input.

i was asking MutantJohn, and dude how you post 5k times in 3 months you must be skipping sleep and eating

Lol I've been coding for about 2.5 years. Posters like phantom have really helped me out.

yeah he is good, and he has good profile picture

Originally Posted by Anonymouss

yeah he is good, and he has good profile picture

Not better than mine!

You have a nice profile picture too.