A problem with pointer initialization

**zyklon** · 01-17-2009

Can anybody help me with this? I am writing a program for my assignment in college. We are working with lists. I have a problem with pointer initialization. The pointer is pointing at the first element of the list. I have a structure

Code:

typedef struct cvor
                    {
                        int broj;
                        struct cvor *sledeci;
                    }Tcvor;

and a pointer type to the structure declared like this

Code:

typedef Tcvor  *Pcvor;

This is the declaration of the pointer to the first element of the list.

Code:

Pcvor glava;

I encounter a problem with printing and deleting a number from the list (and I can't know if the numbers are in the list at all). When I choose the option to run the selected function windows prints a message that the program has encountered an problem and needs to be shut down. After some debugging I realised that the pointer " *glava " was uninitialised even though the function for initialization did run.

Code:

void Inicijalizacija(Pcvor *glava)
{
    *glava=NULL;
}

My friend wrote a program with same declarations and initialization function and it works. He worked in Ubuntu.

The enviroment that I am working in is Code Blocks 8.02 for windows.

Code:

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

    typedef struct cvor
                        {
                            int broj;
                            struct cvor *sledeci;
                        }Tcvor;
    typedef Tcvor  *Pcvor;

void Inicijalizacija(Pcvor *glava);
void Osnovnimeni(char *izbor);
void Unos(Pcvor *glava);
void Brisanje(Pcvor *glava);
void Listanje(Pcvor *glava);
void Oslobodimemoriju(Pcvor *glava);

main()
{
    Pcvor glava;
    char izbor;

    Inicijalizacija(&glava);
    do
        {
            Osnovnimeni(&izbor);
            switch(izbor)
            {
                case '1':   Unos(&glava);
                            break;
                case '2':   Brisanje(&izbor);
                            break;
                case '3':   Listanje(&izbor);
                            break;
                case '4':   ;
            }
        }
    while(izbor!='4');
    Oslobodimemoriju(&glava);

}

void Inicijalizacija(Pcvor *glava)
{
    *glava=NULL;
}

void Osnovnimeni(char *izbor)
{
    char znak;
    printf("\n\n\t1) Dodaj\n\t2) Brisi\n\t3) Listaj\n\t4) Kraj?");
    do
    {
        printf("\n\t");
        *izbor = getchar();

    }while((*izbor > '4') || (*izbor < '1'));
}

void Unos(Pcvor *glava)
{
    Pcvor tek,pret,novi;
    int br;

    novi = (Tcvor *)malloc(sizeof(Tcvor));
    printf("\n\n\tUnesite broj: ");
    scanf("%d", &br);
    novi->broj=br;
    novi->sledeci=NULL;

    if(*glava == NULL)
    {
        *glava = novi;
        return;
    }
    tek=*glava;
    pret=*glava;
    while(tek != NULL)
    {
        pret = tek;
        tek = tek->sledeci;
    }
    pret->sledeci=novi;
}

void Brisanje(Pcvor *glava)
{
    Pcvor tek,pret;
    int br;
    printf("\n\n\tUnesite broj koji zelite da obrisete: ");
    scanf("%d", &br);
    if(*glava == NULL)
    {
        printf("\n\n\tLista je prazna!!!");
        return;
    }

    tek=*glava;
    pret=*glava;
    while(tek!=NULL && (tek->broj!=br))
    {
        pret=tek;
        tek=tek->sledeci;
    }

    if(tek == NULL)
        printf("\n\n\tZnaka nema u listi!!");
    else
    {
        if(tek==*glava)
        {
            *glava=tek->sledeci;
            tek->sledeci=NULL;
        }
        else
        {
            pret->sledeci=tek->sledeci;
            tek->sledeci=NULL;
        }
        free(tek);
    }
}

void Listanje(Pcvor *glava)
{
    Pcvor tek;
    if(*glava == NULL)
    {
        printf("\n\n\tLista je prazna!!!");
        return;
    }
    printf("\n\n\tPrikaz liste:");
    tek = *glava;
    while(tek != NULL)
    {
        printf("%d\n",tek->broj);
        tek=tek->sledeci;
    }
}

void Oslobodimemoriju(Pcvor *glava)
{
    Pcvor tek;
    while(*glava != NULL)
    {
        tek=*glava;
        *glava=tek->sledeci;
        free(tek);
    }
}

Here's the entire code. It maybe has some errors but I can'c correct them without correcting the pointer problem.

**MK27** · 01-17-2009

This is wrong:

Code:

void Inicijalizacija(Pcvor *glava)
{
    *glava=NULL;
}

You are dereferencing the pointer with that asterisk. The function would be a silly one even if it worked (keep reading).
This:

Code:

typedef Tcvor  *Pcvor;

seems questionable to me. If you want a pointer to your new datatype Tcvor, just use this, and set to NULL:

Code:

Tcvor *ptr=NULL;

So now the first 22 lines looks like this:

Code:

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

    typedef struct /* no need for a name */
                        {
                            int broj;
                            struct cvor *sledeci;
                        }Tcvor;

void Osnovnimeni(char *izbor);
void Unos(Pcvor *glava);
void Brisanje(Pcvor *glava);
void Listanje(Pcvor *glava);
void Oslobodimemoriju(Pcvor *glava);

main()
{
    Tcvor *glava=NULL;
    char izbor;

    do
        {

Notice I removed that silly function. Also, realize that char izbor is a single byte and cannot hold more than one character. Also note that glava is just a pointer and does not have any memory allocated to it.

**zyklon** · 01-17-2009

Corrected it and still reports the same error. :/

**laserlight** · 01-17-2009

What is your updated code?

**zyklon** · 01-17-2009

Code:

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

    typedef struct cvor
                        {
                            int broj;
                            struct cvor *sledeci;
                        }Tcvor;
//    typedef Tcvor  *Pcvor;

void Inicijalizacija(Pcvor *glava);
void Osnovnimeni(char *izbor);
void Unos(Pcvor *glava);
void Brisanje(Pcvor *glava);
void Listanje(Pcvor *glava);
void Oslobodimemoriju(Pcvor *glava);

main()
{
    Tcvor *glava=NULL;
    char izbor;

    Inicijalizacija(&glava);
    do
        {
            Osnovnimeni(&izbor);
            switch(izbor)
            {
                case '1':   Unos(&glava);
                            break;
                case '2':   Brisanje(&izbor);
                            break;
                case '3':   Listanje(&izbor);
                            break;
                case '4':   ;
            }
        }
    while(izbor!='4');
    Oslobodimemoriju(&glava);

}

/*void Inicijalizacija(Pcvor *glava)
{
    *glava=NULL;
}
*/

void Osnovnimeni(char *izbor)
{
    char znak;
    printf("\n\n\t1) Dodaj\n\t2) Brisi\n\t3) Listaj\n\t4) Kraj?");
    do
    {
        printf("\n\t");
        *izbor = getchar();

    }while((*izbor > '4') || (*izbor < '1'));
}

void Unos(Tcvor **glava)
{
    Tcvor *tek,*pret,*novi;
    int br;

    novi = (Tcvor *)malloc(sizeof(Tcvor));
    printf("\n\n\tUnesite broj: ");
    scanf("%d", &br);
    novi->broj=br;
    novi->sledeci=NULL;

    if(*glava == NULL)
    {
        *glava = novi;
        return;
    }
    tek=*glava;
    pret=*glava;
    while(tek != NULL)
    {
        pret = tek;
        tek = tek->sledeci;
    }
    pret->sledeci=novi;
}

void Brisanje(Tcvor **glava)
{
    Tcvor *tek, *pret;
    int br;
    printf("\n\n\tUnesite broj koji zelite da obrisete: ");
    scanf("%d", &br);
    if(*glava == NULL)
    {
        printf("\n\n\tLista je prazna!!!");
        return;
    }

    tek=*glava;
    pret=*glava;
    while(tek!=NULL && (tek->broj!=br))
    {
        pret=tek;
        tek=tek->sledeci;
    }

    if(tek == NULL)
        printf("\n\n\tZnaka nema u listi!!");
    else
    {
        if(tek==*glava)
        {
            *glava=tek->sledeci;
            tek->sledeci=NULL;
        }
        else
        {
            pret->sledeci=tek->sledeci;
            tek->sledeci=NULL;
        }
        free(tek);
    }
}

void Listanje(Tcvor **glava)
{
    Tcvor *tek;
    if(*glava == NULL)
    {
        printf("\n\n\tLista je prazna!!!");
        return;
    }
    printf("\n\n\tPrikaz liste:");
    tek = *glava;
    while(tek != NULL)
    {
        printf("%d\n",tek->broj);
        tek=tek->sledeci;
    }
}

void Oslobodimemoriju(Tcvor **glava)
{
    Tcvor *tek;
    while(*glava != NULL)
    {
        tek=*glava;
        *glava=tek->sledeci;
        free(tek);
    }
}

(if names of some functions are unclear)
Inicijalizacija - Initialization
Osnovnimeni - menu
Unos - adding
Brisanje - erasing
Listanje - printing
Oslobodimemoriju - freememory

**Elysia** · 01-17-2009

IF you would kindly post code that actually compiles next time.
One of the problems is this. These functions takes a Tcvor** as argument, but you pass it a char*.
Unos(&glava);
Brisanje(&izbor);
Listanje(&izbor);
I still do not understand why you think that would work properly.
Further, main returns int.

Also, avoid typedefs of pointers.
Your
// typedef Tcvor *Pcvor;
Only serves to complicate the code. Instead write it out using the real type and one or more *.

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