fwrite，fread-fread问题

Question

I have following code:

int main()
{
    char* pedal[20];
    char* pedal2[20];
    for (int i = 0; i < 20; i++)
    {
        pedal[i] = "Pedal";
    }
    FILE* plik;
    plik = fopen("teraz.txt","wb");
    for (int i = 0; i < 20; i++)
    {
       fwrite(pedal[i],strlen(pedal[i]),1,plik);
    }
    system("pause");
    fclose(plik);
    plik = fopen("teraz.txt","rb");
    for (int i = 0; i < 20; i++)
    {
        fread(pedal2[i],5,1,plik); //I know for now that every element has 5 bytes
    }
    for (int i = 0; i < 20; i++)
    {
        std::cout << pedal2[i] << std::endl;
    }
    fclose(plik);
    system("pause");
    return 0;
}

It's crashing at reading and second question let's assume that I have structure where I keep like integers, floats and also char* array and how can I easly write whole structure to the file? Normal fwrite with sizeof structure is not working

Answer 1

Your problem that you didn't allocate buffer for reading. In fact line

fread(pedal2[i],5,1,plik)

reads to unknown place. You need allocate memory (in your case it is 5 + 1 bytes for zero terminated string).

pedal2[i] = malloc(5+1);
fread(pedal2[i],5,1,plik)

Don't forget to release it after usage.

Answer 2

You can't read into pedal2 without first having allocated space for it.

You need something like this:

for (int i = 0; i < 20; ++i) {
    pedal[i] = malloc(100); //allocate some space
}

Answer 3

Your first question seem to have already been answered by Simone & Dewfy.

For your second question about how to write structure values into the file, you will have to write member by member.

Please check fprintf . You can probably use it for writing different data types.