How to read/write a binary file?

Question

I'm trying to write to a binary file, read from it, and output to the screen. I can write to a file, but when I try to read from it, it is not outputting correctly.

Accepted Answer

Reading and writing binary files is pretty much the same as any other file, the only difference is how you open it:

```
unsigned char buffer[10];
FILE *ptr;

ptr = fopen("test.bin","rb");  // r for read, b for binary

fread(buffer,sizeof(buffer),1,ptr); // read 10 bytes to our buffer
```

You said you can read it, but it's not outputting correctly... keep in mind that when you "output" this data, you're not reading ASCII, so it's not like printing a string to the screen:

```
for(int i = 0; i<10; i++)
    printf("%u ", buffer[i]); // prints a series of bytes
```

Writing to a file is pretty much the same, with the exception that you're using `fwrite()` instead of `fread()`:

```
FILE *write_ptr;

write_ptr = fopen("test.bin","wb");  // w for write, b for binary

fwrite(buffer,sizeof(buffer),1,write_ptr); // write 10 bytes from our buffer
```

Since we're talking Linux.. there's an easy way to do a sanity check. Install `hexdump` on your system (if it's not already on there) and dump your file:

```
mike@mike-VirtualBox:~/C$ hexdump test.bin
0000000 457f 464c 0102 0001 0000 0000 0000 0000
0000010 0001 003e 0001 0000 0000 0000 0000 0000
...
```

Now compare that to your output:

```
mike@mike-VirtualBox:~/C$ ./a.out 
127 69 76 70 2 1 1 0 0 0
```

hmm, maybe change the `printf` to a `%x` to make this a little clearer:

```
mike@mike-VirtualBox:~/C$ ./a.out 
7F 45 4C 46 2 1 1 0 0 0
```

Hey, look! The data matches up now*. Awesome, we must be reading the binary file correctly!

*Note the bytes are just swapped on the output but that data is correct, you can adjust for this sort of thing

How to read/write a binary file?

Solution

Problem