Question

Hello i tried to compile my code but i get acess violation error. Im trying to make an agenda that i can insert values using a list. What is the error in my code?

#include <stdio.h>
#include <iostream>

using namespace std;

typedef struct ap_agenda{
    char *name;
    char *telefone;
    struct ap_agenda *proximo;
};

void init(ap_agenda* lista){
    lista = NULL;
}

void insere(char *nome, char *telefone, ap_agenda* lista){
    ap_agenda *p;
    p = (ap_agenda*) malloc(sizeof(ap_agenda*));
    p->name = nome;
    p->telefone = telefone;

    if(lista == NULL){
        lista = p;
    }else{
        lista->proximo = p;
    }
}

void imprime(ap_agenda *lista){
    cout << lista[0].name << endl;
}

int main(){
    ap_agenda agenda;

    init(&agenda);
    insere("test","123456",&agenda);
    imprime(&agenda);

    system("pause");
}

Thanks !

Hello, thanks for the answers! I changed my code and now its “working” but when i try to print the list, its jump one line.

void insere(std::string nome, std::string telefone, ap_agenda* lista){
ap_agenda *p = new ap_agenda;

p->name = nome;
p->telefone = telefone;
p->proximo = NULL;

if(lista == NULL){
    lista = p;
}else{
    while(lista->proximo != NULL)
        lista = lista->proximo;

    lista->proximo =  p;
    }
}

void print(ap_agenda* lista){
    ap_agenda *p;
    for(p=lista; p!=NULL; p=p->proximo)
        cout << p->name.c_str() << endl;
}

The output is:
[blankline]
test1
test2

Answer 1

The access violation probably comes from your call to insere which isn’t working like you think it does.

int main(){
    ap_agenda agenda; //<-- local variable lives on the stack

    init(&agenda); //<-- passes the address of the local variable

When this is passed to init:

void init(ap_agenda* lista){ // lista is a temporary variable that contains a
                             // copy of the address
    lista = NULL; //<-- this overwrites the value in the temporary variable.
} // when this function returns, the temporary variable is destroyed.

At this point agenda had not been modified or initialized in any way. Now you pass the address of agenda off to insere.

    insere("test","123456",&agenda);

insere is defined

void insere(char *nome, char *telefone, ap_agenda* lista){
    ap_agenda *p;
    p = (ap_agenda*) malloc(sizeof(ap_agenda*)); // you allocate a new `ap_agenda`
                                                 // pointer. not enough for a struct
    p->name = nome; // initialize name (probably ok but not what you expect)
    p->telefone = telefone;  // initialize telefone (possible access violation)

    if(lista == NULL){ // since lista is the address of a stack variable it won't
                       // be NULL here
        lista = p;
    }else{
        lista->proximo = p; // this sets the allocated struct to the `proximo` member
                            // of the stack variable that was passed in
    }
}

Note that when this returns, nome and telefone of the stack variable agenda have not been initialized.

    imprime(&agenda);

When the address of the stack variable agenda is passed to imprime it tries to print the value of name which has not been initialized.

void imprime(ap_agenda *lista){
    cout << lista[0].name << endl; // possible access violation
}

If instead, you passed in the proximo member of agenda which was initialized in insere you would see the name value printed.

    imprime(agenda->proximo);

However, as others have pointed out, there are a lot of other issues in this code.