C - create a bin file of a structs's array inside other struct's array with fwrite

Question

I have a struct's array inside other struct's array and I wanna create a binary file which contains this data (only elements no null).

My structs are:

struct viaje {
    char identificador[30+1];
    char ciudadDestino[30+1];
    char hotel[30+1];
    int numeroNoches;
    char tipoTransporte[30+1];
    float precioAlojamiento;
    float precioDesplazamiento;
};

struct cliente {
    char dni[30+1];
    char nombre[30+1];
    char apellidos[30+1];
    char direccion[30+1];
    struct viaje viajes[50];
    int totalViajes;
} clientes[20];

I am trying next:

// For create bin file
for (i = 0; i < totalClientes; i++) {
    fwrite(clientes[i], sizeof(struct cliente), 1, fp_guardarCargarEstado);
    for (j = 0; j < clientes[i].totalViajes; j++) {
        fwrite(clientes[i].viajes[j], sizeof(struct viaje), 1, fp_guardarCargarEstado);
    }
}


// For read bin file
for (i = 0; i < totalClientes; i++) {
    fread(clientes[i], sizeof(struct cliente), 1, fp_guardarCargarEstado);
    for (j = 0; j < clientes[i].totalViajes; j++) {
        fread(clientes[i].viajes[j], sizeof(struct viaje), 1, fp_guardarCargarEstado);
    }
}

I have not tryed fread yet due to I get two errors in fwrite: error: incompatible type for argument 1 of 'fwrite' and note: expected 'const void *' but argument is of type 'struct cliente'

Why could it be?

dbush · Accepted Answer · 2018-03-07 17:08:05Z

3

The fread and fwrite functions take a pointer to a memory location as their first parameter. You're instead passing an instance of a struct.

You need to pass in the address of this struct using the address-of operator &. Also, there's no need to write the struct viaje instances separately, since they are already contained within a struct cliente

// For create bin file
for (i = 0; i < totalClientes; i++) {
    fwrite(&clientes[i], sizeof(struct cliente), 1, fp_guardarCargarEstado);
}


// For read bin file
for (i = 0; i < totalClientes; i++) {
    fread(&clientes[i], sizeof(struct cliente), 1, fp_guardarCargarEstado);
}

answered Mar 7, 2018 at 17:08

dbush

233k27 gold badges261 silver badges334 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

JuMoGar Over a year ago

Hi! Yes, I have tryed your first code (anidated for) and It creates a extra large file. When I try read it, does not load data. I wanna do a bucle beause I do now want that it saves all data (empty items too), I wanna that only save items which have data

JuMoGar Over a year ago

I mean, an array has empty elements and others with content, I want to only store the elements that have content, not all the array elements

David C. Rankin Over a year ago

Note you can fwrite and fread array of structs on the same machine, but do not consider this a portable solution. It isn't. Different compilers handle padding and alignment in different ways. So if you create a binary file with fwrite containing an array of struct, there is no guarantee you can move that file to another computer, recompile your code, and fread the values back in.

Adam VZ · Accepted Answer · 2018-03-07 19:13:27Z

It looks like there a a few things going on here. There are some important things to note here.

Location of totalViajes in struct cliente
The number of bytes you want to write in fwrite()
Resetting the FILE* before reading from the file again.

Here is what I used to test what I think you're looking for.

struct viaje {
    char identificador[30+1];
    char ciudadDestino[30+1];
    char hotel[30+1];
    int numeroNoches;
    char tipoTransporte[30+1];
    float precioAlojamiento;
    float precioDesplazamiento;
};
struct cliente {
    int totalViajes;
    char dni[30+1];
    char nombre[30+1];
    char apellidos[30+1];
    char direccion[30+1];
    struct viaje viajes[50];
} clientes[20];

int main()
{
    clientes[0].totalViajes = 1;
    clientes[0].viajes[0].numeroNoches = 52;
    int totalClientes = 1;

    FILE* fp_guardarCargarEstado = fopen("myFile.bin", "wb");
    // For create bin file
    for (int i = 0; i < totalClientes; i++) {
        fwrite(&clientes[i], sizeof(struct cliente)-(sizeof(struct viaje)*50), 1, fp_guardarCargarEstado);
        for (int j = 0; j < clientes[i].totalViajes; j++) {
            fwrite(&clientes[i].viajes[j], sizeof(struct viaje), 1, fp_guardarCargarEstado);
        }
    }
    fclose(fp_guardarCargarEstado);

    // set variables to 0 so you can tell if the read actually does anything
    clientes[0].totalViajes = 0;
    clientes[0].viajes[0].numeroNoches = 0;

    fp_guardarCargarEstado = fopen( "myFile.bin", "rb" );
    // For read bin file
    for (int i = 0; i < totalClientes; i++) {
        fread(&clientes[i], sizeof(struct cliente)-(sizeof(struct viaje)*50), 1, fp_guardarCargarEstado);
        for (int j = 0; j < clientes[i].totalViajes; j++) {
            fread(&clientes[i].viajes[j], sizeof(struct viaje), 1, fp_guardarCargarEstado);
        }
    }

    fclose(fp_guardarCargarEstado);

    printf("%i\n%i", clientes[0].totalViajes, clientes[0].viajes[0].numeroNoches );

    return 0;
}

Thank you so much!! That is what I was looking for. I have tested the code and works fine, it is great.

Collectives™ on Stack Overflow

C - create a bin file of a structs's array inside other struct's array with fwrite

2 Answers 2

3 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related