如何在C中将数据写入二进制文件

Question

I've got a problem when i try to write data to a binary file. This is the code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

typedef struct
{
char name[255];
int quantity;
float price;
} product;

int main()
{
product x;
FILE *f;
strcpy(x.name,"test");
x.quantity=10;
x.price=20.0;
f=fopen("test.txt","wb");
fwrite(&x,sizeof(x),1,f);
fclose(f);
return 0;
}

When I run the program,it only writes the x.name string,ignoring the other 2(quantity and price). I've googled it and this seems to be the correct function to write data to a binary file...but it still doesn't work for me. What should I do? Thanks in advance!

Answer 1

Your function works fine, the problem is that you write a lot of unused data, are not using the right tool to view your binary file.

You put "test" into the name, which has a size of 255 characters. This uses up the first five (four letters plus null terminator) while the remaining 250 characters are unused. They are written to the file, and their content become "junk filling" between "test" and the other data.

If you write a simple program to read your file back, you would discover that both the quantity and the price are set correctly to the values that you wrote:

int main()
{
    product x;
    FILE *f;
    f=fopen("test.txt","rb");
    fread(&x,sizeof(x),1,f);
    fclose(f);
    printf("'%s' - %d %f\n", x.name, x.quantity, x.price);
    return 0;
}

Answer 2

According to your code you are trying to write address of x. But if you want to write full object then you have to serialize the object first.