rewrite a struct in a binary file, in C

Question

I'm writting a program for a car rental system. One of the functionalities should be updating car information in the binary file, but it isn't rewriting it at all. Here's what the struct looks like:

typedef struct {
    int id;
    char placa[8];
    char modelo[10];
    float motor;
    int ar;
    char cor[6];
    int ano;
    int kilo;
    float diaria;
    int dispo;
} Carros;

This is the function that is supposed to update the file, after searching for a specific car (I basically tried to use the same logic from the function that finds specific cars and prints them out.):

void updateCarro() {
    FILE *fp = fopen("Carros.dat", "rb+");
    Carros carro;
    int id, size, cont, regs;
    
    scanf("%d", &id); //variable to find a specific car's ID

//read new information that'll rewrite the old.
    scanf(" %d", &carro.id);
    scanf(" %s", carro.placa);
    scanf(" %s", carro.modelo);
    scanf(" %f", &carro.motor);
    scanf(" %d", &carro.ar)
    scanf(" %s", carro.cor);
    scanf(" %d", &carro.ano);
    scanf(" %d", &carro.kilo);
    scanf(" %f", &carro.diaria);
    scanf(" %d", &carro.dispo);

    if(fseek(fp, 0, SEEK_END) == -1) {
        printf("Erro ao localizar o registro\n");
        return;
    }
    size = ftell(fp);
    if(size == -1) {
        printf("Não há registros\n");
        return;
    }
    if(fseek(fp, 0, SEEK_SET) == -1) {
        printf("Erro ao localizar o registro\n");
        return;
    }
    records = size / sizeof(Carros);
    for(cout = 0; cout < regs; count++) {
        fread(&carro, sizeof(Carros), 1, fp);
        if(carro.id == id) {
            fwrite(&carro, sizeof(Carros), 1, fp);
            break;
        }
    }
}

Here's the function that looks for a car's ID in the binary file and prints all the information. It seems to work, I tested multiple times, but maybe there's a flaw that's relevant. It's probably very easy to tell I'm not very experienced, so any help is welcome.

void mostraCarro() {
    Carros carro;
    FILE *fp = fopen("Carros.dat", "rb");
    int id;
    size_t count, records;
    long size;
    printf("entre com a indentificação do carro: \n");
    scanf("%d", &id);
    if(fseek(fp, 0, SEEK_END) == -1) {
        printf("Erro ao localizar o registro\n");
        return;
    }
    size = ftell(fp);
    if(size == -1) {
        printf("Não há registros\n");
        return;
    }
    if(fseek(fp, 0, SEEK_SET) == -1) {
        printf("Erro ao localizar o registro\n");
        return;
    }
    records = size / sizeof(Carros);
    for(count = 0; count < records; count++) {
        fread(&carro, sizeof(Carros), 1, fp);
        if(carro.id == id) {
            printf("Identificação: %d\n", carro.id);
            printf("Placa: %s\n", carro.placa);
            printf("Modelo: %s\n", carro.modelo);
            printf("Motor: %f\n", carro.motor);
            printf("Ar condicionado: %d\n", carro.ar);
            printf("Cor: %s\n", carro.cor);
            printf("Ano: %d\n", carro.ano);
            printf("Quilometragem: %d\n", carro.kilo);
            printf("Valor da diária: %f\n", carro.diaria);
            printf("Disponibilidade: %d\n", carro.dispo);
            break;
        }
    }
    fclose(fp);
}

Answer 1

You're writing the new struct after the one that was read with the matching ID, not in place of it. You need to seek back to the beginning of the record before writing.

    for(count = 0; count < regs; count++) {
        fread(&carro, sizeof(Carros), 1, fp);
        if(carro.id == id) {
            fseek(fp, -(int)sizeof(Carros), SEEK_CUR);
            fwrite(&carro, sizeof(Carros), 1, fp);
            break;
        }
    }

You also need to add fclose(fp); at the end of the function.