C，从bin文件中读取二进制文件

Question

I'm pretty new to C, but I have come across a problem with fread...

My end goal is to read (and then printf to console) the binary from a .bin file, but for now i'm taking it one step at a time and trying to just read the first bit...

My code:

...
FILE *file = fopen("test1.bin", rb);
int i = 0;
fread(&i, 1, 1, file);
printf("%i\n", i);
...

Now i've tried this on three different .bin files, one outputted 0, other 2 and the other 12!

Why is it outputting 2/12 when I am reading in just one 1 bit from file? Shouldn't it be a 0 or a 1? what am I doing wrong? Thanks alot.

Answer 1

Change the fread() call to:

fread(&i, sizeof(int), 1, file);

The second argument is the size of an element to read, the third argument is the number of elements to read. The posted code is reading a single byte into an int .

You should also check the return values from fopen() and fread() calls to ensure they were successful.

Answer 2

Like @hmjd said, you should read the number of bytes necessary to fill an int value. So either this way:

len = fread(&i, sizeof(int), 1, file);

or this way:

len = fread(&i, 1, sizeof(int), file);

The first case reads one int value (typically as 4 bytes). After the call, len should be equal to 1 if the read succeeded.

The second case reads multiple bytes into an int value. The difference is that after this call, len should be equal to sizeof(int) .

Either way will work, the only difference being that the first way specifies reading a multiple-byte single object (an int ), whereas the second specifies the number of bytes to read into the object. It's a subtle difference, and people will disagree which is best, of course.

The advantage of the second method is that len tells you exactly how many bytes were actually read (which might be useful for debugging I/O errors). The advantage of the first method is that it's a conceptually simpler test for success (was one int read or not?).

But no matter how you do it, you should always check the return value of fread() .