Thread: Problem reading a database from a file using pointers

Hi, I hope someone can clarify me how can I read from a file using a pointer that I need to read all the pets that I am adding through a function. Before reading from a file, everything worked perfectly, I was able to print all the records I added previously. I am not able to do so with files. This is my code:

Structure:

Code:

typedef struct {
    unsigned int dia;
    unsigned int mes;
    unsigned int anio;
} fecha;


typedef struct mascota {
    char nombre_mascota[100];
    char tipo_mascota[100];
    char raza[100];
    char sexo[100];
    char observaciones[1024];
    char dolencia[1024];
    fecha nacimiento;
    struct mascota *siguiente;
} tmascota;


tmascota *primero=(tmascota*) NULL;
tmascota *ultimo=(tmascota*) NULL;

The function to print all the records added:

Code:

void listar_mascotas(tmascota *ptr) {


    FILE *file;
    int cont=0;
    ptr=primero;
    file=fopen("gestion_veterinaria.dat", "rb");
    int num_leidos=1;
    while (ptr!=NULL && num_leidos) {
        num_leidos=fread(ptr, sizeof(tmascota),1, file);
        if (num_leidos) {
            printf("\n\n%s \nNombre de la Mascota: %s \nFecha de Nacimiento: %d/%d/%d\nTipo: %s \n"
                   "Raza: %s \nSexo: %s \nObservaciones: %s \nDolencia: %s\n",
                   ptr->nombre_mascota, ptr->nombre_mascota, ptr->nacimiento.dia, ptr->nacimiento.mes, 
                   ptr->nacimiento.anio, ptr->tipo_mascota, ptr->raza, ptr->sexo, ptr->observaciones, ptr->dolencia);
        ptr=ptr->siguiente;
        cont++;
        }
    }
    fclose(file);
}

I have problems with: num_leidos=fread(ptr, sizeof(tmascota),1, file);

Can someone see what is wrong? As I said, before file management everything worked perfectly.

ok, I have an update I made this changes:

Code:

typedef struct {
    unsigned int dia;
    unsigned int mes;
    unsigned int anio;
} fecha;




typedef struct mascota {
    char nombre_mascota[100];
    char tipo_mascota[100];
    char raza[100];
    char sexo[100];
    char observaciones[1024];
    char dolencia[1024];
    fecha nacimiento;
    struct mascota *siguiente;
} tmascota;


tmascota var;
tmascota *primero=(tmascota*) NULL;
tmascota *ultimo=(tmascota*) NULL;

Code:

void listar_mascotas(tmascota *ptr) {


    FILE *file;
    int cont=0;
    ptr=primero;
    file=fopen("gestion_veterinaria.dat", "rb");
    int num_leidos=1;
    while (ptr!=NULL && num_leidos) {
        num_leidos=fread(var, sizeof(tmascota),1, file);
        if (num_leidos) {
            printf("\n\n%s \nNombre de la Mascota: %s \nFecha de Nacimiento: %d/%d/%d\nTipo: %s \n"
                   "Raza: %s \nSexo: %s \nObservaciones: %s \nDolencia: %s\n",
                   ptr->nombre_mascota, ptr->nombre_mascota, ptr->nacimiento.dia, ptr->nacimiento.mes, 
                   ptr->nacimiento.anio, ptr->tipo_mascota, ptr->raza, ptr->sexo, ptr->observaciones, ptr->dolencia);
        ptr=ptr->siguiente;
        cont++;
        }
    }
    fclose(file);
}

I added tmascota var; and in fwrite and fread: var. At least compiles but only display the first element added in the list. Why?

Never mind, solved by myself, intnum_leidos=1; num_leidos= removed and substituted by &primero. Now prints all the records. You can ignore this post.

Originally Posted by Salem

This makes no sense at all.

> num_leidos=fread(var, sizeof(tmascota),1, file);
You do realise that you're also reading the siguiente field (which is a pointer) directly from the file.

So what does this do?
ptr=ptr->siguiente;

TBH, the only chance this code has of working is if you populate your list, save it to a file and then immediately load it in again.

If you just run the code and load a previously saved file, then I'd expect it to just blow up in your face.

Yes, I removed num_leidos, took the idea of one of my classes. This is exactly what I am doing, saving and loading the file in every function. After a hard work I will have the chance to submit the project, I don't expect a 10/10 but at least a 5/10 to pass it.

This course will end the next week but I am still interested in learning C/C++. I will come back again soon to ask you advice to do so.